Sections
The obvious impact crater in the south is Tycho, the dark areas are the maria (the "seas", but actually basalt plains from long-ago lava flows), and the Mare Tranquillitatis (Sea of Tranquility)---where Apollo 11 landed---is right of the center line at a mid-latitude.
Can you see the Leaping Rabbit?
There are two Moon rabbits actually---I call them Bugs and Peter.
Credit/Permission: T.A.Rector, I.P. Dell'Antonio, NOAO/AURA/NSF, © NOAO/AURA, before or circa 2003 / NOAO/AURA Image Library Conditions of Use.
Download site: NOAO Image Gallery: Moon and Stars.
I.P. Dell'Antonio is my old friend Ian from my days at Harvard in 1991--1993.
He rode up a sliver of moonlight
and over the crest of a hill,
turned and backed into black night,
whither and whither still.
---The Highwayman
So old astronomy mostly, but with a few new astronomy touches.
The new astronomy, which is mostly lunar geology and lunar evolution, we leave to IAL 12: The Moon and Mercury.
And what of the old Moon?
As the Sun is KING of the day, the Moon has always been QUEEN of the night---or vice versa depending on whose culture is counting. Of course, the Moon is often seen in the day. Anyhow, it's always been with us.
Caption: "The Wolves Pursuing Sol and Mani (the title given to the work in the list of illustrations on page vii of the source)".
Mani is the Moon goddess and Sol is the Sun god in Norse mythology.
Credit/Permission: John Charles Dollman (1851--1934) in H. A. Guerber's (1859--1929) Myths of the Norsemen from the Eddas and Sagas, 1909, London George G. Harrap and Co. (uploaded to Wikipedia by Haukur Thorgeirsson (AKA User:Haukurth), 2008) / Public domain.
Image linked to Wikipedia.
Caption: "Artemis with a hind, better known as 'Diana of Versailles'. Marble, Roman artwork, Imperial Era (1st--2nd century CE). Found in Italy." Possibly a copy of a Greek statue by Leochares (4th century BCE).
Artemis (equated to the Roman goddess Diana) was the Greek Moon goddess, the twin sister of Apollo, the Greek Sun god.
As I know from days in Tennessee, deer are still hunted by the light of the Moon.
Actually, the Greeks had two other Moon goddesses: the Titan Selene and Hecate---of whom Hesiod (circa late 8th century BCE) thought well of.
Credit/Permission: Marie-Lan Nguyen (AKA User:Jastrow), 2005 / Public domain.
Image linked to Wikipedia.
Although much fainter than the Sun, moonlight, particularly of a full or nearly full Moon is a significant resource in the absence of modern lighting---it's the brightest object in the sky after the Sun.
In fact, astronomers, when NOT looking at the Moon itself, consider moonlight a source of light pollution and are always desirous of dark time when the Moon's away.
Of course, other traditional problems come with a full moon.
But some find a happy Moon.
Caption: "Geraldine Ulmar (1862--1932) in The Mikado, 1885". An object of "modified rapture".
The The Mikado is an operetta by W.S. Gilbert (1836--1911) and Arthur Sullivan (1842--1900).
Credit/Permission: B. J. Falk, 1885 (uploaded to Wikipedia by User:Ssilvers, 2008) / Public domain.
Image linked to Wikipedia.
Caption: "Screenshot from Le Voyage dans la lune (A Trip to the Moon) (1902)."
An early Moon shot.
Credit/Permission: Georges Melies (1861--1938), a early French film maker, 1902 (uploaded to Wikipedia by Magnus Manske, 2008) / Public domain.
Image linked to Wikipedia.
Caption: "Lunar base concept drawing from NASA."
The large field with "furrows" is probably an array of solar panels. But on the other hand, someone seems to be turning sod.
The long linear structure is a mass driver is an electromagnetic launch device. A few experimental mass drivers have existed since 1976.
Credit/Permission: NASA, 1977 (uploaded to Wikipedia by Square87, 2006) / Public domain.
Image linked to Wikipedia.
In fact, the use of the lunar month (about 29.5 days) both for secular TIME-KEEPING and RELIGIOUS PURPOSES goes back to prehistory in many societies---probably in all societies to prehistory.
The origin of the seven-day week is lost in prehistory, but it may trace back to the fact that 7 days is approximately a quarter of the lunar month and is marked clearly by the phases: new moon, 1st quarter moon (which is a half moon), full moon, and 3rd quarter moon (which is also a half moon). Some think this idea is doubtful.
Caption: A diagram illustrating the phases of the Moon in their order starting from the new moon and progressing through waxing crescent moon, 1st quarter moon, and waxing gibbous moon, full moon, waning gibbous moon, 3rd quarter moon, and waning crescent moon and completing the full cycle at the new moon again.
"The dotted circle around the Earth shows the Moon's orbit. The dotted line illustrates moon's trajectory. The solid ivory line passing through the Earth is indicative of Earth's orbit around the Sun."
This diagram is NOT to scale---the Earth-Moon distance is about 60 Earth radii recall. The bend in the Earth's orbit is unreal.
Credit/Permission: © User:Miljoshi, User:Fresheneesz, 2006 / Creative Commons CC BY-SA 3.0.
Image linked to Wikipedia.
The lunar month is not, of course, the modern calendar month of the modern civil calendar: the calendar month is divorced from the lunar month only retaining the family name month---this divorce began with the Julian calendar (see below).
If you count the lunar month as starting from the first visible crescent after new moon as was often done, then the lunar month alternates between 29 and 30 days. And if you had to wait out cloudy evenings to see a crescent for the first time in a lunar month, then the old month could be longer than 30 days and the new month shorter than 29 days.
The mean lunar month---the cruelest month---is, in fact, 29.53059 DAYS.
This creates a problem since the (mean) lunar month is not made of an INTEGRAL NUMBER OF DAYS (nor weeks), nor is the year made of an INTEGRAL NUMBER of lunar months.
Many societies tried to keep a 12 lunar month calendar and a solar calendar at the same time: i.e., a lunisolar calendar.
Since 12 lunar months (which is the lunar year) is about 354.37 DAYS and the solar year (or tropical year) is about 365.2421897 DAYS (Cox-15), the time discrepancy between a count of lunar years and count of solar years increases by 11 days per solar year.
After 3 solar years, the discrepancy is about 33 days or a bit more than a lunar month.
In order keep counting time in lunar months and solar years on average, a 13th lunar month (an intercalary month) has to be inserted into a calendar year a bit more frequently than every 3 years.
A pretty accurate way of inserting lunar months is to use the 19-year Metonic cycle named after the Meton of Athens (late 5th century BCE), who may have been the first to discover it (Ne-7; No-65).
To illustrate the Metonic cycle,
consider 19 years:
Let 12 years consist of 12 lunar months.
Let 7 years consist of 13 lunar months.
In the 19 years counted thusly, there are 235 lunar months
which equal 6939.69 days.
19 solar years equal 6939.60 days.
Thus, there is a discrepancy of only about 0.09 days.
19 years counted in lunar months by the Metonic cycle end 0.09 days
after 19 solar years.
From time zero, it takes about 190 years of using Metonic cycle
for the calendar year based on the
Metonic cycle.
to end about about 0.9 days after the end of the
190th solar year
also counting from time zero.
For a trivial procedure, this isn't bad.
Most societies using a lunisolar calendar used more elementary means than the Metonic cycle for calculating when to insert the intercalary months.
Often it seems responsible officials in each state or city inserted intercalary month just when the ``year was not good'' (i.e., the climate season was in the wrong lunar month) as far as they could tell.
The result was calendrical chaos for anyone (like a modern historian) who is trying to figure out how to correlate events in ancient times.
Julius Caesar (100--44 BCE) in his calendar reform of 46--45 BCE (which established the Julian calendar) dispensed with lunar months altogether replacing them with the 12 arbitrary months that divide up the solar year.
Caption: Bust of Julius Caesar (100--44 BCE) in the Naples National Archaeological Museum.
I've always liked his hair style.
Arguably, Caesar's best achievement was his calendrical reform of 46--45 BCE which established the Julian calendar.
The modern Gregorian calendar is only a small reform of the the Julian calendar.
Credit: Andreas Wahra, 1997 (uploaded Wikipedia by User:Saperaud, 2005) / Public domain.
Image linked to Wikipedia.
By the by, Julius Caesar actually undertook the calendar reform in his role as Pontifex Maximus (or chief priest of Rome): as such, he was the legitimate responsible official for the calendar.
Julius Alien.
After his death in 44 BCE, a month was name for him: Iulius which is the modern July. This change was proposed by Mark Antony (83--30 BCE) (see Calendar, Julius).
Caption: Les Tres Riches Heures du Duc de Berry, Juillet (i.e., July), Musee Conde, Chateau de Chantilly. The Palace of Poitiers is in the background.
The month is July as lunette shows.
July was named for Julius Caesar (100--44 BCE)---fair enough since he instituted the Julian calendar in 46--45 BCE.
Credit/Permission: Brothers Limbourg (fl. 1385--1416), 1412--1416, source/photographer: R.M.N./R.-G. Ojeda (uploaded to Wikipedia by User:Petrusbarbygere, 2005) / Public domain.
Image linked to Wikipedia.
In this section, we look at a few Moon facts, especially those pertaining to the Moon's orbit.
The table below summarizes some of the facts.
Remember, we don't memorize at lot of numbers, but we do think about their signficance when we are looking at them.
_______________________________________________________________________________________________________________ Table of Moon Facts ________________________________________________________________________________________________________________ mean lunar month 29.53059 days (new moon to new moon) mean sidereal month 27.321661 days (orbital period relative to fixed stars) mean anomalistic month 27.554551 days (apogee to apogee) which differs from the lunar orbital period because of the lunar precession of the Moon's orbit due gravitational perturbations. mean orbital radius 384,400 km = 0.00257 AU = 60.27 Earth equatorial radii mean angular diameter 0.5178 degrees which is almost the same as the Sun angular diameter 0.5178 degrees eccentricity 0.0549 (or 5.49 % variation from mean distance) inclination to ecliptic 5.145 degrees axial tilt to eclipitc 1.5424 degrees which is much less than Earth's 23.4 degrees equatorial radius 1738.14 km = approximately (1/4) Earth's equatorial radius mass 7.3477*10**22 kg = 0.0012300034 Earth masses = 1/81.30059 Earth masses _______________________________________________________________________________________________________________Source: Cox-16,303,305; Wikipedia: Moon; Wikipedia: Earth.
_______________________________________________________________________________________________________________The following figure illustrates a few more Moon facts.
Caption: Moon numbers (Cox-16,240,303,305).
Let's expand a bit on some of the facts.
This is why even though the Moon has about a quarter of the Earth's diameter its angular diameter on the sky is only about 0.5 degrees.
Thus, the center of mass of the Earth-Moon system is actually inside the Earth and this is the relatively unaccelerated point that both bodies orbit in elliptical orbits.
"Physical" orbital motion is always accelerated motion in physics since the motion is not in a straight line.
This is because the lunar month is new moon to new moon and the Earth moves along its orbit in this time. The Moon must travel more than one complete orbit relative to the fixed stars to return to the Earth-Sun line: i.e., to new moon.
Caption: Lunar month and lunar sidereal month.
This means the Earth-Moon distance varies up and down from the mean Earth-Moon distance by about 5.5 %. The total range of variation is 11 %.
The 11 % variation in DISTANCE causes Moon's angular diameter to vary by 11 % too.
The variation in angular diameter is probably too small ever to be noticed by casual observation since we usually see the Moon at perigee and apogee without a convenient sufficiently accurate natural STANDARD OF COMPARISON.
But if you compare the Moon at perigee and apogee with the same magnification as in the figure below, the difference is striking.
Caption: "Lunar perigee and apogee apparent size comparison (from April and October 2007, when events occurred near full-phases)."
Credit/Permission: © Tom Ruen (AKA User:Tomruen), 2007 / Creative Commons CC BY-SA 3.0.
Image linked to Wikipedia.
Answers 1 and 2 are right.
In a total solar eclipse, you know by direct observation that Moon's angular diameter is larger than the Sun's.
In annular solar eclipses, the Moon's angular diameter is just smaller than the Sun's, and one sees a bright annulus (or ring) of the Sun (or to be more precise the solar photosphere: see section Solar Eclipses below) around the Moon.
You should NEVER look at an annular eclipse with the naked eye.
I suppose in partial solar eclipses also provide a STANDARD OF COMPARISON, but you should never look at them either with the naked eye, and, in any case, it would be hard to tell whether Sun or Moon had the larger angular diameter.
We consider solar eclipses below in the section Solar Eclipses.
This inclination to ecliptic too has an important eclipse consequence.
Caption: Tilt of the Moon's orbit.
Answer 1 is right.
The inclination to ecliptic of the Moon's orbit badly complicates eclipse phenomena.
Of course, if one had zero inclination to ecliptic for the Moon's orbit, then total/annular solar eclipses would only occur in the tropics.
Caption: The Moon's orbit and the line of nodes.
Now I know what you are thinking.
Why, why must the line of nodes ROTATE?
In an exact gravitational two-body system, the line of nodes wouldn't rotate. But the Earth-Moon system is a two-body system only to first order. So the orbit is simple only to first order.
The Sun and to a much lesser degree the planets add complicated gravitational perturbations to the Earth-Moon system. This results in subtler, complex motions (Se-48).
Answer 1 is right.
Eclipses can happen because the Moon can be very close to the ecliptic plane and be on the Earth-Sun line (as in seen in projection on the ecliptic plane) at the SAME TIME.
If the line of nodes is NOT closely aligned with the Earth-Sun line, the Moon will be well above or below the ecliptic plane when it is on the Earth-Sun line.
Because the Earth, Moon, and Sun have finite sizes, exact alignment of the line of nodes (i.e., an exact nodal alignment) and the Earth, Moon, and Sun are NOT needed for an eclipse.
Just good enough alignment. Thus, about every nodal alignment is an eclipse season when an eclipse of some type is possible.
Now if the line of nodes were fixed in space, alignment would happen every 6 months or a bit more precisely about every 182.6 DAYS.
Since the line of nodes itself rotates westward as the Earth revolves eastward, and it turns out that there is an alignment and and eclipse season every 173.31 DAYS
Rotating line of nodes.
Because of the 173.31 DAYS alignment period, the eclipse season in time migrates through the entire calendar year.
Thus, eclipses can occur at any time of the calendar year.
We discuss eclipses, nodal alignments, and eclipse seasons further below in sections Eclipses, Lunar Eclipses, and Solar Eclipses.
There are more below, of course.
Night on the Moon's near side is not extremely dark because it is illuminated by Earthshine. Thus the night side as seen from Earth is not totally dark, but it often appears that way since our eyes are adjusted to looking at the bright day side.
Caption: "A waning crescent moon above Earth's horizon in an image by an Expedition 24 crew member on the International Space Station (ISS)."
The phases of the Moon have a haunting beauty.
Credit/Permission: NASA, 2010 (uploaded to by User:Originalwana, 2010) / Public domain.
Image linked to Wikipedia.
And the names of the phases of the Moon in time order are:
And one also has the abbreviated expressions:
To "wax" (NOT used in the sense of coating in wax) is a nearly obsolete verb in English now used almost exclusively for the waxing of the Moon.
You see English and German are mutually intelligible---and you can understand Swedish too---if you really, really try.
The diagram below illustrates the lunar phases.
Caption: A diagram illustrating the phases of the Moon in their order starting from the new moon and progressing through waxing crescent moon, 1st quarter moon, and waxing gibbous moon, full moon, waning gibbous moon, 3rd quarter moon, and waning crescent moon and completing the full cycle at the new moon again.
"The dotted circle around the Earth shows the Moon's orbit. The dotted line illustrates moon's trajectory. The solid ivory line passing through the Earth is indicative of Earth's orbit around the Sun."
This diagram is NOT to scale---the Earth-Moon distance is about 60 Earth radii recall. The bend in the Earth's orbit is unreal.
Credit/Permission: © User:Miljoshi, User:Fresheneesz, 2006 / Creative Commons CC BY-SA 3.0.
Image linked to Wikipedia.
Here's an animation showing the lunar phases---and the lunar libration too which we will briefly consider below in section Lunar Rotation and Tidal Locking.
Caption: An animation of lunar phases and lunar libration. It is animation since it is constructed from still images, but one could call it a very time-lapsed film with some enhancements. The time epoch is about 2007 October.
The apparent growing and shrinking of the Moon is due to the 11 % difference between distance to perigee and distance to apogee.
The anomalistic month is also the period of what is probably the main component of the lunar libration.
The anomalistic month's mean value is 27.554551 days.
So the anomalistic month is not the lunar month (mean length 29.53059 days), nor the sidereal month (mean lengh 27.321661 days).
We only see the near side because the Moon is tidally locked to the Earth.
So we never see the far side of the Moon from the Earth.
But the tidally locking does NOT completely limit us to just one hemisphere of the Moon. Various small viewing effects cause the lunar libration which is a bit of rotational oscillation from the perspective of the Earth. The explanation by viewing effects is as much as we will give here.
Because of the lunar libration, we do see a 59 % of the Moon's surface from the Earth (see Wikipedia: Tidal locking: Occurrence: Earth's Moon), but only 50 % at one time, of course.
Credit/Permission: Tom Ruen (AKA User:Tomruen), 2007 / Public domain.
Image linked to Wikipedia.
Answer 2 is right.
The Moon is always approximately along the ecliptic as we discussed in IAL 2: The Sky.
But once you get the hang of them, they are easy.
They are sort of analogous to a problem in algebra with one equation and THREE VARIABLES.
You can solve for any ONE variable if you know the other TWO.
The three "variables" are:
Remember the Moon is always near the ecliptic: i.e., in a day it will be carried around with the celestial sphere on almost the same arc on the sky as the Sun.
----One Ring, etc.
Caption: Lunar phases.
Note the diagram is NOT to scale.
In particular, the Earth is actually small compared to the Earth-Moon distance, but "you" are actually a pinprink on the Earth which looks like an infinite plane to you.
For example, at face-value the diagram shows "you" CANNOT see the full moon at exact sunset since it is below your horizon at that moment.
And this is actually true for exact lunar opposition. You cannot see the center of the Moon rise at exact sunset when the center of the Sun sets.
But the Earth is relatively small, and Moon and Sun have finite sizes, and so you see the Moon rise as the Sun sets or you see that so nearly as to make no difference to casual description.
We do NOT usually worry about finicky effects due to the finite sizes of Earth, Moon, and Sun.
Thus to first order, you can take the Moon as fixed on the rotating celestial sphere and fixed in phase for any single day.
Let's do three examples of lunar phase problems.
Phase and time are the knowns. Location on the sky is the unknown.
Glance at the lunar phases diagram to find the answer.
Caption: Lunar phases.
The Moon must be on the eastern horizon. It is just rising. It is in opposition to the Sun as it must be when it is full.
If the time were midnight, then the Moon would be transiting meridian.
Time and location on the sky are knowns. Phase is the unknown.
Glance back lunar phases diagram and find the time location on Earth and identify the eastern direction.
The Moon must be a waning crescent.
Location in sky and phase are knowns. Time of day is the unknown.
Glance back at the lunar phases diagram.
It must be sunset.
If the Moon was on the eastern horizon, it would be noon.
Caption: Hey Diddle Diddle
---Mother Goose.
Is the image astronomically possible?
See the video: Hey Diddle Diddle - Nursery Rhyme - With Text.
Credit/Permission: William Wallace Denslow (1856--1915), 1902 (uploaded to by User:Tagishsimon, 2006) / Public domain.
Image linked to Wikipedia.
Say you are at the sunset location:
Actually the Moon does move a finite distance on the celestial sphere during a day. A simple angular velocity calculation shows this.
Relative to the Sun, the Moon moves
360 degrees / 29.53059 days = 12.19 degrees/day .
Relative to the fixed stars the Moon moves
360 degrees / 27.321661 days = 13.17 degrees/day .
Either way the Moon moves about 0.5 degrees per hour.
Since the Moon itself subtends about 0.5 degree, it moves about its own angular diameter every hour.
If one checks the Moon against the fixed stars during a night, the Moon's motion can be easily seen.
Not that yours truly has ever done such a thing.
Form groups of 2 or 3---NOT more---and tackle, homework 3 problems 12--17 on lunar phases.
Discuss each problem and come to group answer.
Oh, 5--10 minutes.
See solutions 3.
The winners get chocolates.
Caption: "Bars of dark Swiss chocolate. From left to right: a) about 75% cocoa; b) contains chili; c) dark chocolate." (Slightly edited.)
Did you know that cocoa may be the great brain food---see Daisy Yuhas, 2013, Is Cocoa the Brain Drug of the Future?---it makes mice smarter---but maybe only in unprocessed form---just when you thought it was safe to scarf.
Credit/Permission: © Simon A. Eugster (AKA User:LivingShadow), 2010 / Creative Commons CC BY-SA 3.0.
Image linked to Wikipedia.
This behavior is the tidal locking.
We discuss the Moon's tidal locking and tidal locking in the subsections below.
Since the Moon's axial rotation rate is on average equal to its orbital rotation rate, it always turns the same side to us.
We call this side the near side of the Moon and, until recent history, it was the only side we ever saw.
Caption: The near side of the Moon with the major maria and lunar craters identified.
The near side of the Moon which is the only one we see from Earth.
It's actually the most interesting side to look at and probably for lunar geology because of the large maria: i.e., the "seas" which are actually basalt rock from long-ago lava flows. The Mare Tranquillitatis (i.e., Sea of Tranquility) is right of the center line at a mid-latitude.
The obvious lunar crater in the south is Tycho---which is the only one I ever remember.
The names of the large features were given long ago before 1881 anyway: see Map of the Moon, Andrees Allgemeiner Handatlas, 1st Edition, Leipzig (Germany) 1881, p. 4, but note that the south is at the top in that map.
Credit/Permission: © User:Tau`olunga, 2006 (uploaded to Wikipedia by User:MachoCarioca, 2008) / Creative Commons CC BY-SA 3.0.
Image linked to Wikipedia.
Throughout human history until 1959, the far side of the Moon was a mystery.
Caption: The far side of the Moon---which is NOT always dark: it has as much daytime and nighttime as the near side of the Moon.
North is at the top; south is at the bottom. The west is at the left and the east at the right.
Because of the lunar libration, we do see a 59 % of the Moon's surface from the Earth (see Wikipedia: Tidal locking: Occurrence: Earth's Moon), but only 50 % at one time, of course.
So a sliver of the far side is obliquely seen from the the Earth.
The far side lacks the large maria which make the near side more interesting to look at and probably for lunar geology.
The far side was first seen by the Soviet probe Luna 3, 1959 Oct07.
Since the Soviets first saw the far side they gave lots of Russian names.
For example, the largest of the small far side maria is Mare Moscoviense. It's in the upper left quadrant of the image.
The Mare Moscoviense is at 147.9 degrees E longitude in selenographic coordinates which have their zero at the center of the near side of the Moon. This verifies---when you think about it---that the lunar west is the at the left in the image.
Credit/Permission: NASA, 1998 (uploaded to Wikipedia by User:Pringles, 2006) / Public domain.
Image linked to Wikipedia.
Answer 3 is right.
The tidal locking is not perfect, however, due to other small effects which cause the lunar libration.
Caption: The anomalistic month (apogee to apogee) for 2007 April. No lunar phase shown.
The author doesn't explain how he has removed the lunar phase. Maybe he just artificially brightened the night side of the Moon (which does glow from earthshine) to give an approximately uniform surface brightness. Or he did some other trick.
The apparent growing and shrinking of the Moon is due to the 11 % difference between distance to perigee and distance to apogee.
The anomalistic month is also the period of what is probably the main component of the lunar libration.
The anomalistic month's mean value is 27.554551 days.
So the anomalistic month is not the lunar month (mean length 29.53059 days), nor the sidereal month (mean lengh 27.321661 days).
We only see the near side because the Moon is tidally locked to the Earth.
So we never see the far side of the Moon from the Earth.
But the tidally locking does NOT completely limit us to just one hemisphere of the Moon: various small viewing effects cause the lunar libration which is a bit of rotational oscillation from the perspective of the Earth. The explanation by viewing effects is as much as we will give here.
Because of the lunar libration, we do see a 59 % of the Moon's surface from the Earth (see Wikipedia: Tidal locking: Occurrence: Earth's Moon), but only 50 % at one time, of course.
Credit/Permission: Tom Ruen (AKA User:Tomruen), 2007 / Public domain.
Image linked to Wikipedia.
The animation below further illustrates the tidal locking of the Moon to the Earth
Caption: An animation of the tidal locking of the Moon to the Earth on the left-hand side and the counterfactual case of a non-rotating Moon orbiting the Earth on the right-hand side. The objects are NOT to scale.
Tidal locking causes the Moon's axial rotation rate to equal its orbital rotation on average.
Thus, the Moon turns one side to the Earth always. This side is the near side of the Moon. The side we do not see from the Earth is the far side of the Moon.
Because of the lunar libration, we do see a 59 % of the Moon's surface from the Earth (see Wikipedia: Tidal locking: Occurrence: Earth's Moon), but only 50 % at one time, of course.
Credit/Permission: © User:Stigmatella aurantiaca, 2013 / Creative Commons CC BY-SA 3.0.
Image linked to Wikipedia.
As mentioned above, tidal locking is a gravitational effect.
A mutually orbiting pair of astro-bodies tend to become tidal locked to each other (i.e., always face to each other with the same sides) because of the tidal force they exert on each other.
Typically, pair are NOT formed in a tidally locked configuration.
Whether tidal locking goes to completion for either of the astro-bodies depends on the strength of the tidal force, the resistance of the astro-bodies to being tidal locked, and the complicating gravitational effects of other astro-bodies:
The more massive the astro-body of any pair resists tidal locking more strongly because it usually has larger rotational inertia (which is a measure of the resitance to changing its axial rotation rate) simply by having more mass. Everyday experience shows that it is harder to change the rotation of a massive body than a less mass of body all other things being equal.
The complicating effects of other are various. But one thing is clear, an astro-body cannot be tidal locked to two other astro-bodies (except maybe in some very unusual cases). For example, the Moon cannot become tidal locked to the Sun without stopping being tidal locked to the Earth---it cannot serve two masters.
Caption: A figure illustrating the tidal force and tidal locking.
We have a small astro-body orbiting a much larger astro-body.
The small astro-body has orbital angular velocity ω and axial rotational angular velocity Ω. The units of the angular velocities could be, e.g., degrees per day.
The tidal force is the differential gravitational force on the small astro-body exerted by the large astro-body.
So the near side of the small astro-body is pulled on more strongly than than the far side.
This difference in forces---which is the tidal force---stretches the small astro-body to some degree along the line between the two astro-bodies.
The bulges that appear on the body are called tidal bulges and are shown clearly in the upper right small astro-body image in the figure.
This upper right image is what one would see if the small astro-body were already tidally locked to the large astro-body.
When tidally locked, the small astro-body has Ω = ω, and so always turns the same side facing the large astro-body.
The axial rotational angular velocity is perpetually rotating the tidal bulges away from alignment with the line between the astro-bodies and the tidal bulges keep trying to reform or migrate back into aligment.
The tidal force is now NOT only stretching the small astro-body, but also acting to decelerate its axial rotational acting on the tidal bulges: the gravitational force pulls more strongly one way on the near bulge than it does on the far bulge in the other way.
In time, the tidal force will reduce to Ω to ω, and then one has tidal locking (i.e., Ω = ω) and the tidal bulges will be permanently aligned and there is no more deceleration.
If the system started with Ω < ω, then the tidal force would have accelerated the axial rotational angular velocity until tidal locking was achieved again.
Actually, there are always perturbations that act to desynchronize the two angular velocities of the small astro-body. But the tidal force acts as a restoring force to damp out the effects of the perturbations and drive the system back to being tidally locked.
We would call the tidally locked a stable state since perturbations cannot change it.
Actually, virtually all real continually static systems are stable. Perturbations try to move the system, but a restoring force damps them out.
For example, tall buildings sway with wind perturbations, but keep returning to being nearly exactly upright.
But large astro-bodies in mutually orbiting pairs because of their larger mass almost always initially has more angular momentum (resistance to change of axial rotation angular velocity) than small astro-bodies. Thus, it takes longer, often much longer, for the a large astro-body to become tidally locked to a small astro-body, than vice versa.
The axial rotation angular velocity of many minor moons are not perfectly known.
Some are known NOT to be tidally locked because they have been only recently been captured by their planets and tidal locking has not yet been established or perturbations may be so strong that tidal locking cannot be established (see Wikipedia: Tidal locking: Moons).
Among the planets, only ex-planet Pluto is tidally locked (see Wikipedia: Tidal locking: List of known tidally locked bodies). Pluto and its largest moon Charon are mutually tidally locked. If you were on the Charon-facing side of Pluto, you would always see Charon in the sky at the same location relative to the ground and with its Pluto-facing side turned toward you.
The Moon's tidal force on the Earth is slowing down the Earth's rotation, and thus increasing the length of the day.
However, the slowing rate is so slow that the Earth will probably not become tidally locked to the Moon before the Sun's red giant in about 4.5 Gyr when the Sun may well vaporize Earth and Moon (see Wikipedia: Tidal acceleration: Effects of the Moon's gravity)---lucky us.
If the orbit of astro-body about a second astro-body is NOT circular (i.e., has non-zero eccentricity), the tidal force varies with the orbital radius: stronger when orbital radius is smaller, weaker when orbital radius is larger.
The varying tidal force perpetually flexes the astro-body which results in resistive forces inside the astro-body to turn macroscopic mechanical energy from the two astro-bodies's motions into heat energy.
The heat energy can cause some degree of geologically activity.
In the Solar System, Jupiter's moon Io has the strongest tidal force heating of any astro-body. As a result, Io exhibits continual volcanism. Io is the most geologically active astro-body. Earth is a distant second.
Credit/Permission: © User:Sghvd, 2010 / Creative Commons CC BY-SA 3.0.
Image linked to Wikipedia.
As explained with the figure above, moons tend to get tidally locked to their parent planets during the course of solar system evolution, but the reverse process has generally not happened.
Among the planets, only ex-planet Pluto and its biggest moon Charon are mutually tidally locked (see Wikipedia: Tidal locking: List of known tidally locked bodies).
Caption: The Pluto system with ex-planet Pluto near the center of mass and 3 of its 5 currently known moons:
Credit/Permission: NASA, 2006 (uploaded to Wikipedia by Magnus Manske, 2008) / Public domain.
Image linked to Wikipedia.
The specifications of the Pluto system are given in the
table below.
The Pluto system --------------------------------------------------------------------------------------- Astro-Body Discovery Year Radius Mass Orbital Radius Orbital Period (km) (10**18 kg) (km) (days) --------------------------------------------------------------------------------------- Pluto 1930 1153 13050 2040 6.3872 70 % Moon 18 % Moon Charon 1978 602 1520 17530 6.3872 35 % Moon 2 % Moon 15.20 Pluto radii Nix 2005 44 1 48708 24.9 Hydra 2005 36 .391 64749 38 S/2011 P4 2011 20 approx ? 59000 32.1 S/2012 P5 2012 10--25 ? 42000+/-2000 20.2+/-0.1 ---------------------------------------------------------------------------------------
Because Pluto and Charon are mutually tidally locked, their orbital periods and rotation periods are all the same (i.e., 6.3872 days).
This means, among other things, on one side of Pluto/Charon you always see Charon/Pluto sitting in the sky at the same orientation to the surface of Pluto/Charon.
On the other side of Pluto/Charon, you never see Charon/Pluto.
The rotation periods of the other moons are unknown, but they may be tidally locked to Pluto, and so have rotation periods equal to their orbital periods---on the other hand, they might have some strange relation between rotation period and the other periodicities of the system because of the multiple sources of gravity.
Pluto videos:
The Earth has tidally locked the Moon.
The reverse has NOT happened, but the Moon's working on it.
The great angular momentum of the Earth greatly slows the process and the competing effect of the Sun's tidal locking effect complicates things---actually, I'd guess the Sun helps slow the Earth's rotation, and so at present is helping toward tidal locking of the Earth to the Moon.
Geological evidence suggests that 620 megayears ago (0.62 gigayears), the solar day was 21.9(4) hours (i.e., 21.9(4) modern standard hours) (Wikipedia: Tidal accelration: Historical evidence).
Historical records for the past 2700 years suggests that the solar day is increasing by 1.70(5)*10**(-3) seconds per century (Wikipedia: Earth-Moon case).
This is an amount-rate-time problem.
t = A/R = 1 second / ( 1.70(5)*10**(-3) seconds per century )
= approximately 600 centuries
= 60 millennia
So we'd have to wait 60 millennia for even ONE more second in the day.
I spent most of my life waiting for it to end---and now I'm nostalgic for the good old days.
All things considered from the Dark Ages to the World Wide Web, it wasn't so bad.
Caption: "Bayeux Tapestry (Normandy) with Halley's comet. Text reads ISTI MIRANT STELLA: These (people) are looking in wonder at the star."
The Bayeux Tapestry is sort of medieval graphic novel.
It's all about the Norman conquest of England---1066 and All That.
Credit: Medieval artists, circa 1070 (uploaded to Wikipedia by User:Urban, 2005) / Public domain.
Image linked to Wikipedia.
There are all kinds of complicating small effects---like the shifting of material in the Earth's interior.
But without even without an exact prediction, it seems that the slowing rate of the Earth's rotation is so slow that the Earth will probably not become tidally locked to the Moon before the Sun's red giant in about 4.5 Gyr when the Sun may well vaporize Earth and Moon (see Wikipedia: Tidal acceleration: Effects of the Moon's gravity)---lucky us.
In principle, planets can be tidally locked to the Sun.
None are.
Mercury and Venus are the likest cases one would think a priori since they are closest to the Sun and have no moons that can out-compete the Sun.
It can't serve two masters.
In the case of Mercury, the large eccentricity of its orbit (e=0.205630 or about 20 %) has it seemed caused it to settle into a stable Mercury's 3:2 spin-orbit resonance.
The Mercury's 3:2 spin-orbit resonance situation has 3 rotation periods (each 58.646 days) for every 2 revolution periods (each 87.9691 days) which is also one Mercurian day of 175.9382 days) (see Wikipedia: Mercury: Spin-orbit resonance).
In this case, the oscillations are the rotations and revolutions.
The figure below illustrates Mecury's 3:2 spin-orbit resonance.
Caption: Mecury's 3:2 spin-orbit resonance.
Old scifi stories (pre-1965) often make a point of the supposed tidal locking and sometimes give Mercury a habitable zone at its fixed terminator (line between day and night).
Well we've digressed from the Moon.
Let's get back on topic in the sections below.
Generally speaking, an eclipse is when one astro-body moves into the shadow of another.
But there is also the transitive verb eclipse which in astronomy means when one astro-body (the subject) blocks the view of another astro-body (the object).
Caption: Phobos Transit of the Sun.
Phobos is the larger of the two moons of Mars.
It's not round, but has a mean radius of 11.1 km---you can even see it's not round.
It's mean orbit radius is 9377.2 km, but Phobos still is a finite, if tiny, disk on the sky as seen from the Martian surface. Mars's equatorial radius is 3396.2(1) km by the way.
Here little Phobos makes a valiant effort to totally eclipse the Sun, but it fails.
It's too small. All it can do is an annular eclipse: it leaves a annulus or ring of the Sun uncovered.
The transit lasts only about 30 seconds.
This transit was observed by Opportunity rover 2004 Mar10.
By the way, a transit is when one astro-body passes in front of another one from the point of view of an observer---or crosses meridian---which is like passing in front of an imaginary astro-body.
There is only an eclipse if the observer or thing observed is in the shadow of the transiting body.
But because the Sun is so bright, it's hard to see an astro-body that transits the Sun by its reflected light against the Sun.
So usually to see an astro-body transit the Sun, it has to at least partially eclipse the Sun from the observer's perspective.
Credit/Permission: NASA, 2004 (uploaded to Wikipedia by User:Yaohua2000, 2005) / Public domain.
Image linked to Wikipedia.
There's a bit of inconsistency in our terminology for eclipses seen from Earth.
A solar eclipse is when the Moon eclipses the Sun from the point of view of the Earth.
A lunar eclipse is when the Earth eclipses Sun from the point of view of the Moon.
In both cases, the ECLIPSED object is the Sun.
If we wanted consistency---which we don't---we could call a lunar eclipse a solar eclipse as seen from the Moon.
In any case, there's nothing to be done about the inconsistency now.
We could be very clear if we always specified the three astro-bodies: the eclipsed, the eclipser, and the observer.
Usually when discuss eclipses without qualification, we mean eclipses as seen from the Earth: i.e., lunar eclipses and solar eclipses.
These eclipses occur near the time of nodal alignment which happens every 173.31 days as discussed above in the section Moon Facts.
The eclipse season is the time period around nodal alignment when eclipses are possible.
The eclipse season is finite since the Earth, Moon, and Sun are all finite bodies and do not require exact nodal alignment for eclipses.
The eclipse season considering all types of eclipses collectively varies between 31 and 37 days, and is on average 34 days (see Wikipedia: Eclipse season: Details).
Since the lunar month is mean length 29.53059 days, there will always be at least 2 eclipses and never more than 3 eclipses in an eclipse season.
Three occur if the Moon is just before full moon or new moon when an eclipse season starts. The Moon can then race through 3 eclipse positions before the eclipse season ends.
If the Moon starts an eclipse season in any other position, then it passes through 2 eclipse positions in all cases before the eclipse season ends.
Any body illuminiated by a finite source of light has two kinds of shadow: umbra where the source is totally covered (or occulted or eclipsed) and penumbra where the source is only partially covered (or occulted or eclipsed).
Penumbra is Latin for almost shadow.
A point source can only cause umbras.
Of course, when other sources of light are around (including reflecting sources), an umbra won't be totally dark and a penumbra not as dark as otherwise.
The Earth has an umbra and penumbra.
They both stretch away from the Earth in the anti-solar direction.
There three main lunar eclipse types: total lunar eclipse, partial lunar eclipse, and penumbral lunar eclipse.
Every total lunar eclipse includes a partial lunar eclipse and penumbral lunar eclipse.
But when we say partial lunar eclipse without further qualification, we usually mean one that does not include a total lunar eclipse.
Every partial lunar eclipse includes a penumbral lunar eclipse.
But when we say penumbral lunar eclipse, without further qualification, we usually mean one that does not include partial lunar eclipse
Sometimes context implies the "qualifications".
A total lunar eclipse including penumbral stage (see below) can last up to 6 hours; totality (when the Moon is entirely within the Earth's umbra) lasts at most 1 hour 40 minutes (Se-41).
The eclipse season for a total lunar eclipse is only 9 days??? around exact nodal alignment (with the Sun-Earth line): i.e., for about 4.5 days before and 4.5 days after.
Caption: Rotating line of nodes.
There is a finite-time eclipse season because the Sun, Moon, and Earth are finite bodies and do NOT require exact nodal alignment for eclipses.
Whether an eclipse happens depends on whether the Moon is in the right place at any time in an eclipse season.
If the Moon is not close enough to the full moon when the eclipse season begins, the eclipse season will end before the Moon reaches full moon and the nodal alignment passes without a total lunar eclipse.
At only about 35 % of nodal alignments is there total lunar eclipse (Lunar Eclipses for Beginners).
The eclipse season for a partial lunar eclipse is 22 days??? around exact nodal alignment: i.e., for about 11 days before and 11 days after.
Because the eclipse season is shorter than the lunar month, a partial lunar eclipse is NOT always possible.
At only about 30 % of nodal alignments is there a partial lunar eclipse without a total lunar eclipse (Fred Espenak: MrEclipse.com: I assume a good source since he works for NASA).
No one gets too excited about partial lunar eclipses without total lunar eclipse, but they are noticeable.
Answers 1 and 2 are right.
Caption: An illustration of 1988 Mar03 total penumbral eclipse.
There are, in fact, total penumbral eclipses. These are when the Moon goes completely into the penumbra without going into the umbra at all.
These are rather rare---and boring---events occur between 0 and 9 times per century.
Credit/Permission: Tom Ruen (AKA User:SockPuppetForTomruen and User:Tomruen), 2009 (uploaded to Wikipedia by Magnus Manske, 2008) / Public domain.
Image linked to Wikipedia.
The eclipse season for penumbral lunar eclipse is 32 days??? around exact nodal alignment: i.e., for 16 days before and 16 days after.
Now 32 days is longer than a lunar month, and so at every nodal alignment, there is at least a penumbral lunar eclipse.
At about 35 % of nodal alignments is there a penumbral lunar eclipse (but usually not total penumbral eclipse) without a partial or total lunar eclipse (Lunar Eclipses for Beginners).
No one gets excited about penumbral lunar eclipses.
The Moon just looks a little diminished in brightness in an uneven way. A layer of cloud could have almost the same effect. So penumbral lunar eclipses usually go unnoticed and unannounced.
In the period, 2000 BCE--3000 CE, there are 12188 lunar eclipses.
See the table below for the frequency of lunar eclipse types.
The 3 lunar eclipse types occur with approximately equal frequency which on average is 33.3... %, of course.
------------------------------------------------------- Table: Frequency of Lunar Eclipse Types for 2000 BCE--3000 CE -------------------------------------------------------- Type Number Percentage -------------------------------------------------------- total 3502 28.7 partial 4207 34.5 penumbral 4479 36.7 all types 12188 100.0 --------------------------------------------------------
The penumbral lunar eclipses are those without partial or total lunar eclipses and the partial lunar eclipses are those without total lunar eclipses.
The 4479 penumbral lunar eclipse include 191 total penumbral eclipses.
Of course, total lunar eclipses arn't as awe-inspiring as total solar eclipses.
The two kinds of total eclipses occur with the same order of frequency, but there is a major distinction in how many people can see them.
Total lunar eclipses can be seen from the entire night side of the Earth---except where there is cloud cover, of course.
Total solar eclipses can be seen only from a restricted geographic area: see the section Solar Eclipses below.
Thus, everyone will likely see a few total lunar eclipses in their lives---or at least sleep through a few---but to see a total solar eclipse you must travel to an eclipse path (the region of total eclipse) or be lucky enough to live on one---and be lucky enough not to be clouded out.
Now for total lunar eclipse images and videos.
Caption: "A series of images taken aboard the amphibious assault ship USS Boxer (LHD 4) shows the Moon during a full lunar eclipse." Date: 2007 Mar03.
Click on image and on the next image for the high-resolution view.
This is a coppery total lunar eclipse.
The coppery color is due to refraction of sunlight by the Earth's atmosphere.
The prominent crater with rays in the south is Tycho.
Credit/Permission: U.S. Navy photos by Mass Communication Specialist Seaman Joshua Valcarcel, 2007 / Public domain.
Image linked to Wikipedia.
At totality of a total lunar eclipses the Moon can take on a coppery color as we see in the US Navy lunar eclipse image above. This is due to refraction of sunlight by the Earth's atmosphere (Se-41).
The Earth's atmosphere has a continuous variation in properties, and so give a continuous bending or refraction effect.
The reddening of the Moon in a
total lunar eclipse.
The out-scattering of bluish light is, of course, the reason why sunrise and sunset are red. At sunrise and sunset, sunlight takes a long tangential path through the Sun to the observer.
Reddened color of the Moon in a total lunar eclipse depends on the atmospheric conditions and may be more or less.
The color will also be uneven as the image shows because of uneven conditions around the Earth's terminator.
Also the closer the Moon is to the center of the umbra, the less light will be refracted to it on average.
A dark total lunar eclipse can occur if the atmosphere is particularly opaque on the Earth's terminator.
Caption: "No sudden, sharp boundary marks the passage of day into night in this gorgeous view of ocean and clouds over our fair planet Earth. Instead, the shadow line or terminator is diffuse and shows the gradual transition to darkness we experience as twilight. With the Sun illuminating the scene from the right, the cloud tops reflect gently reddened sunlight filtered through the dusty troposphere, the lowest layer of the planet's nurturing atmosphere. A clear high altitude layer, visible along the dayside's upper edge, scatters blue sunlight and fades into the blackness of space. This picture actually is a single digital photograph taken 2001 June from the International Space Station (ISS) orbiting at an altitude of 211 nautical miles."
The Other Side of the Sky as Arthur C. Clarke (1917--2008) would say.
Credit/Permission: ISS Crew, Earth Sciences and Image Analysis Lab, Johnson Space Center, NASA, 2001 (uploaded to Wikipedia by Andrew Dunn (AKA User:Solipsist), 2006) / Public domain.
Image linked to Wikipedia.
No one has been on the Moon for a lunar eclipse which, of course, from the Selenite perspective is a solar eclipse.
However, some approximations to have have been seen as the figure and caption below show.
Image from Apollo 12, 1969 Nov24, passing into or out of the Earth's umbra on its homeward journey.
No one has been on the Moon for a lunar eclipse which, of course, from the Selenite perspective is a solar eclipse.
But this Apollo 12 is sort of like a solar eclipse from Selenite perspective.
The image shows the Sun just vanishing or emerging.
You can see the bright edge of the Sun peeping over the disk of the Earth and a partial ring illumination of refracted light around the Earth's terminator.
Note the Earth's angular diameter is 4 times that of the Sun's as seen from the Moon.
Night on Earth in a sense is an eclipse of the Sun by the Earth from a on-the-ground human perspective.
Credit/Permission: NASA, 1969, download site: Johnson Space Center, Digital Image Collection / Public domain.
Lunar eclipses nowadays are of no special scientific value. They are just spectacles.
In the past, lunar eclipse were scientifically interesting.
For example, they were interesting for themselves if you didn't understand how they worked or how to predict them.
Lunar eclipses also provided one the earliest pieces of evidence for a spherical Earth.
The shadow of the umbra of the Earth on the Moon is always round.
This would be hard to arrange without having a spherical Earth.
A flux of light from the Sun encountering the a spherical Earth would create a cylindrical occulted area.
The shadow of this formed on any object would look round if the object can be taken for a flat screen as the Moon can at it's distance from the Earth.
The round umbra argument was given by Aristotle (384--322 BCE), but may have been known earlier.
Parmenides of Elea (early 5th century BCE), who may have been the first proponent of the spherical Earth, may have known the argument.
Answer 3 is right.
I believe the ancient Greeks were the first to realize that solar time varied with locality and that west is earlier, east is later in the solar day. But I can't find a reference at the moment.
A partial lunar eclipse is "total eclipse" of the Sun as seen from the part of the Moon in the Earth's umbra.
Thus a total solar eclipse is a "partial eclipse" using the "lunar" sense of the word "partial" since the whole Earth isn't in the Moon's umbra.
A "total" eclipse of the Sun in the "lunar" sense never happens on the Earth because the Moon's umbra can only cover a small part of the Earth at most.
There are three main solar eclipse types: total solar eclipse, annular solar eclipse, and partial solar eclipse.
A fourth non-main type is the hybrid solar eclipse which is one that transitions between being a total solar eclipse and a annular solar eclipse.
Every total solar eclipse/annular solar eclipse includes a partial solar eclipse.
Context usually decides when we say partial solar eclipse whether we mean a solar eclipse without a total solar eclipse/annular solar eclipse or one with a total solar eclipse/annular solar eclipse.
Sometimes we have to be explicit about what we mean by partial solar eclipse.
Because the Moon's umbra is very small at the distance of the Earth, only a small part of the Earth experiences totality (when the solar photosphere is completely covered) or none at all.
Remember the Moon is only about a quarter of Earth in diameter.
So the lunar umbra scales down from the Earth's umbra by a factor of 4.
But this means at 1/4 of the distance from the Moon to the Earth, the lunar umbra is 1/4 of the Earth's lunar umbra at the Moon.
By the distance from the Moon to the Earth, the lunar umbra is small or zero.
The two possibilities are because the Moon's distance from the Earth varies because of the Moon's orbital eccentricity.
Solar eclipses can happen when the Moon is at any of its distances: but total solar eclipses only when the Moon is relatively near and its umbra touches down on the Earth.
The diameter of the lunar umbra on the Earth (i.e., the totality region) is 269 km at most??? and the umbra remains over any one point on the Earth for 7.5 minutes at most.
But if the surface is NOT perpendicular to the umbra axis, the umbra tip could be stretched out as shadows are at sunset.
Presumably, this stretching out effect is accounted for in the 269 km at most value reported above. But I cannot find a discussion of this fine point.
Answer 1 is right.
When the Moon is closest (i.e., at perigee), its umbra on the Earth is biggest.
Caption: The total solar eclipse of 2002 Dec04.
This image from the International Space Station (ISS) shows the lunar umbra on the Indian Ocean.
The crew were standing on their heads.
Credit/Permission: ISS, NASA, 2002 / Public domain.
Download site: Marshall Space Flight Center, Marshall Image Exchange (MiX) and search for 0300612 (just number no leading space) for image 0300612 information or just click on 0300612 for the image itself.
A qualification is needed to the statement that the Moon covers the whole Sun during totality.
The Sun has no hard surface actually: it is a gas ball that just gets less dense going outward.
But there is a layer from which most of the light we see comes. The layer is called the solar photosphere or more loosely the solar "surface" as we ordinarily think of it and observe it---but only in glimpses with the naked eye.
It is the photosphere that is covered in a total solar eclipse.
There are outer parts of the Sun can be seen around the Moon during totality that can never been seen by the naked eye otherwise.
Only during totality and ONLY THEN is it safe to look at the Sun with the naked eye.
Eye safety during solar observations is discussed below.
We'll look at some total solar eclipse images in the subsection The Main Event: The Total Solar Eclipse below.
The uncovered photosphere appears as a bright ring around the black Moon. Annulus is just Latin for ring.
Another perspective on annular solar eclipses is to say the Moon's umbra doesn't reach the Earth.
Caption: The annular solar eclipse of 2005 Oct03.
A typical annular solar eclipse.
Credit/Permission: © User:sancho_panza, 2005 (uploaded to Wikipedia by User:ComputerHotline, 2007) / Creative Commons CC BY-SA 2.0.
Image linked to Wikipedia.
Annular solar eclipses are somewhat more frequent than total solar eclipses.
Thus, when the tip of umbra of the Moon passes in front of the Earth, slightly more than half of the time the umbra doesn't touch down on the Earth's surface.
One can also have hybrid eclipses (also called annular/total solar eclipses), where the eclipse shifts between total and annular as the umbra moves across the Earth.
But since there is a total solar eclipse somewhere during a hybrid eclipse, hybrid eclipses are often just counted as total solar eclipses---except by the pedantic---but with Wikipedia, we're all pedantic now.
Annular solar eclipses arn't nearly as popular as total solar eclipses. They are spectacular, but you can't look at them with the naked eye and everything doesn't get nighttime dark.
From the observer's location the Sun is a crescent.
But do NEVER look at any part of the solar photosphere with the naked eye.
Partial solar eclipses don't attract much attention usually.
The day gets a little darker, but often no more so than if there was some haze.
Bright patches of sunlight filtered through trees can become crescent-shaped due to the pinhole projection effect discussed below.
People often pass through partial solar eclipses without noticing a thing.
Partial solar eclipses without total and/or annular solar eclipses happen about 35.3 % of the time. But they cause no great popular interest.
Just as with lunar eclipses, solar eclipses can happen only near a nodal alignment which happens 173.31 days.
Will there be at least a partial solar eclipse every nodal alignment?
Remember the mean lunar month is about 29.5 days.
Answer 1 is right.
Because the lunar month is shorter than the eclipse season, the Moon will be at new moon at some time during the eclipse season.
Even if the eclipse season started just after new moon, the Moon still has enough time to race around the Earth and reach new moon again before the eclipse season ends.
So some kind of annular solar eclipse must occur every eclipse season.
In fact, two partial solar eclipses in a single eclipse season can occur if one happens right at the beginning of the eclipse season. The Moon can race around the Earth and gets back to new moon before the eclipse season is over.
Two partial solar eclipses in a single eclipse season is a rare event, but one such event will happen in 2036: the dates are 2036 Jul23 and 2036 Aug21 (Fred Espenak: MrEclipse.com which I assume a good source since he works for NASA: Solar Eclipses: 2031 - 2040
Two total solar eclipses in one eclipse season seems to be impossible---at least there is no mention of such a thing that I can find.
Total and annular solar eclipses combined are more frequent than just partial solar eclipses.
Thus, in reality total and annular solar eclipses are not all that uncommon.
But annular solar eclipses don't usually cause great interest. Recall also that they are slightly more common than total solar eclipses.
Also total and annular solar eclipses are geographically limited to tight eclipse paths.
Thus, only a lucky few will ever see one without traveling.
Below we have a table illustrating the fequency of solar eclipse types.
One sees that hybrid solar eclipse are rarest by far (only about 5 %) and the other solar eclipse types occur with approximately the same frequency of about 30 % each.
------------------------------------------------------- Table: Frequency of Solar Eclipse Types for 2000 BCE--3000 CE ------------------------------------------------------- Type Number Percentage ------------------------------------------------------- total 3173 26.7 annular 3956 33.2 hybrid 569 4.8 partial 4200 35.3 all types 11898 100.0 -------------------------------------------------------
In the context of tables like this, the partial lunar eclipses are NOT included in the amounts for the other solar eclipse types though, of course, those types include partial lunar eclipses of course.
You MUST NOT look a the Sun directly with the naked eye whenever any of the photosphere is visible.
Of course, we're always catching small glimpses without disaster---but one should minimize those glimpses.
Only during totality of a total solar eclipse is it safe to look at the Sun with the naked eye---because the photosphere is totally covered.
The ONLY way to look at the photosphere of the Sun safely is with a proper astronomical solar filter either just for viewing or on a telescope.
Other kinds of filters and old photograph negatives are NOT guaranteed to be adequate and very often are NOT adequate.
Even at sunrise and sunset or through a thick haze, the Sun is still NOT safe to view with the naked eye. We've all, of course, had glimpses, but again one should minimize those.
For more on safety during solar eclipses, see the NASA: Eye Safety During Solar Eclipses.
If you don't have a proper astronomical solar filter, you can use pinhole projection to look at the Sun at any time.
Pinhole projection.
Actually this figure is incorrect for
partial solar eclipse
since the Moon really gives
convex bite, not
a concave
bite as in the figure.
The next to images illustrate pinhole projection during solar eclipses.
Caption: The pinhole projection method for observing a solar eclipse. In the insert in the upper left corner of the image, you can see the partially eclipsed Sun that was photographed with a white solar filter. In the main image you can see multiple projections of the partially eclipsed Sun.
The image is a bit of fancy work to get multiple crescent images.
Light filtering through leafy trees can give multiple crescent images during a partial solar eclipse.
The solar eclipse was total solar eclipse of 2006 Mar29.
Credit/Permission: © User:Brocken Inaglory, 2006 (uploaded to Wikipedia by User:Mbz1, 2009) / Creative Commons CC BY-SA 3.0.
Image linked to Wikipedia.
Caption: The image shows multiple pinhole projections caused by leafy trees of the partial solar eclipse that accompanied the annular solar eclipse of 2005 Oct03. The image was taken at St. Julian's, Malta.
The holes in the tree canopy provide rough pinholes---the holes do not need to be round.
Partial solar eclipses are often NOT spectacular events.
The Sun disk has convex bite taken out of it, but without specially viewing equipment you cannot see that.
The sky will just look a bit dimmer than usual as if there were some extra cloud cover and if there is cloud cover the dimming is really hard to notice.
But one thing that can be seen without special equipment is the multiple pinhole projections caused by leafy trees. The spectacle is better the closer to totality you are.
If you were unaware of the partial solar eclipse, you might wonder "what the heck".
Credit/Permission: © Elly Waterman (AKA User:Ellywa), 2005 / Creative Commons CC BY-SA 3.0.
Image linked to Wikipedia.
A simple pinhole projector can be made just with a sheet of cardboard with a hole punched in it and some surface to project the image on.
The image with pinhole projection is better focussed the smaller the hole, but less bright---there is a trade-off.
The image is also point inverted as the diagram above shows.
This is like a Keplerian telescope for similar reasons.
There are some sites that show how to build a good pinhole projecter:
You can also try using a hand mirror to reflect the Sun's image onto a piece of paper: cover all but about a dime's worth of the mirror to create a pinhead mirror (FMW-79).
Apparently, you can see sunspots with pinhole projection---but at least some say that it is marginal: e.g., Cloudy Nights Telescope Review.
Can you see sunspots? Probably not.
Incidentally, can you see narrow dark and bright fringes just near the edges of shadows (e.g., of a pencil)? You need to look really closely. A magnifying glass might help.
In the classroom, a darkened room and single bright light can be used for pinhole projection experiments. Ask students to put pinholes in sheets of paper and ask them to find the pinhole projection image.
The Sun and Moon have both almost the same angular diameter on the sky: i.e., about 0.5 degrees.
Caption:
di_moon=0.5178 degrees
and
di_sun=0.5327 degrees.
This was allows us to have just barely
total solar eclipses and just
barely annular solar eclipses.
Before the modern age, it is no wonder that people regarded the angular size symmetry between Sun and Moon as having a deep significance.
The Sun and Moon could be viewed as or as representing twin gods: Apollo and Artemis in Greek mythology.
But the angular size symmetry doesn't have any deep significance.
It is just a coincidence.
In fact, in the distant past the Moon was closer to the Earth than at present and in the distant future it will be farther away.
Thus the size angular diameter equality is just a coincidence of our era on Earth.
The Moon's umbra follows an eclipse path on or over the Earth.
Two motions are compounded to make the umbra move.
The second motion somewhat compensates for the first.
Here is a cute animation.
Caption: An animation of the Solar eclipse of August 11, 1999.
First, note that the Earth is NOT rotating in the animation. So the observers---us that is---are NOT rotating either.
Second, note that the lunar umbra starts and ends its eclipse path on the Earth's terminator. It always must do this in total solar eclipses since as lunar umbra travels through space it touches down and leaves at the Earth's terminator
Now eclipse paths (the paths of the lunar umbra) are always followed east by the lunar umbra because the Moon moves eastward in space at an average speed of 1.022 km/s.
Over the time of solar eclipse, the Moon is moving nearly in a straight line through space: the Moon is only moving a little along its curved orbital path.
The upper limit on the Earth's speed eastward on a parallel path is the Earth's equatorial rotational speed of 0.4651 km/s.
So no place on Earth can keep up with with the Moon's shadow: the shadow must move east.
An upper limit on how long the umbra can stay on Earth is about:
amount diameter 2*6378.1 km
t = -------- = ------------ = -------------- = 12480 s = 3.467 hours = 3.5 hours .
rate Moon speed 1.022 km/s
Most Earth-contact times for the umbra will be less than 3.5 hours since the umbra will move along a chord that is not a diameter on the disk of the Earth.
The Earth-contact time for the Solar eclipse of August 11, 1999 was 3.1 hours which is close to maximal which is consistent with the eclipse path being close to a diameter as the animation shows.
Credit/Permission: © Andrew T. Sinclair, 2000 / The permission for this animation is unclear. It is posted on a NASA Eclipse Web Site: Google Maps and Solar Eclipse Paths: 1981 - 2000, but NOT all items on NASA web sites are Public domain. Right on the animation, it says © A.T. Sinclair. Being cautious, we take that as being decisive and conclude there is no obvious permisssion to display it even though many sites display animations by Andrew T. Sinclair.
Image linked to Wikipedia.
We can do a nifty approximate calculation of the speed of umbra on or over the Earth.
Eclipse paths are always followed east because the Moon moves eastward in space at an average speed of 1.022 km/s.
Over the time of solar eclipse, the Moon is moving nearly in a straight line through space: the Moon is only moving a little along its curved orbital path.
The upper limit on the Earth's speed eastward on a parallel path is the Earth's equatorial rotational speed of 0.4651 km/s.
All other speeds of the Earth's surface in one direction in space are less since the rotation speed decreases with latitude north and south and rotation of the Earth means that the direction of motion is not in a straight line, but in circular path that is also not in the same plane as the Moon's motion.
So only a componet of the velocity is along a path parallel to the Moon's nearly straight line path in space.
Since 0.4651 km/s is the maximum velocity of the Earth's surface parallel to the Moon in space, the minimum eclipse velocity relative to the ground eastward is v_rel = 1.022 - 0.4651 = 0.557 km/s = 2000 km/h .A more exact calculation shows that the minimum umbra speed is about 1700 km/h (Se-43).
Because the Moon goes well above and below the ecliptic plane, the Moon's umbra and penumbra can be at any latitude.
At higher latitudes, the Earth speed is lower---going to zero at the poles---and so the umbra speed is greater with an upper bound of about 3700 km/h.
Given these high speeds and the fact that diameter of the lunar umbra on the Earth (i.e., the totality region) is 269 km at most???, it's not surprising that the umbra remains over any one point on the Earth for 7.5 minutes at most.
Here is an nice umbra from a eclipse path that crossed the eclipse path of the animation above.
Caption: "Solar eclipse, as seen from the International Space Station over Turkey and Cyprus".
This is the total solar eclipse of 2006mar29.
The north half of Cyprus is at the bottom of the image.
Credit: NASA.
Image linked to Wikipedia.
Permission: Public domain at least in USA.
In predicting solar eclipses, a lot of complications have to be accounted for.
But, fortunately, some people have done that for us and provided eclipse predictions for centuries in advance. Let us just consider eclipse paths for the 2001--2025 period.
Caption:
Eclipse paths
for total solar eclipses
for 2001--2015.
An eclipse path
is the path of the
umbra on the
Earth's surface during
a total solar eclipse.
Mainly because of the tilt of the Earth's axis from the
ecliptic pole,
the eclipse paths
are curved as the image show:
i.e., curved with respect to straight lines of a latitude
on a Mercator projection map.
A solar eclipse region
(umbra and
penumbra)
sweeps basically eastward in
space.
The Earth also turns
eastward.
In this horse race,
the solar eclipse region
is faster, and so it
moves eastward on the
surface of the Earth.
So the umbra on an
eclipse path moves
eastward on the
surface of the Earth.
Recall that the diameter of the umbra on the
Earth (i.e., the
totality region)
is 269 km at most??? and the umbra remains over any one point
on the Earth for 7.5 minutes at most.
The minimum umbra speed is
about 1700 km/hr (Se-43).
Note that the
eclipse paths
are very wide in
Arctic
and Antarctic.
This is probably for two reasons.
First and certainly, Mercator projection
stetches out all lengths as one moves away from the
equator.
Second and probably, the umbra
gets stretched out when landing on the more titled surfaces of the
far from the equator.
Credit/Permission:
Fred Espenak (1953--),
2002 /
Courtesy of
Fred Espenak, NASA/Goddard Space Flight Center.
Download site: NASA Eclipse Web Site,
scroll down ∼ 60 % and click on
World Map
of Total Solar Eclipses: 2001-2025.
As the above figure shows, there will good total solar eclipses
whipping through or near Indiana on
2017aug21 and 2024apr08---homebodies take note.
The explanation of this map is essentially the same as for
the Total Solar Eclipse Eclipse Path Map 2001--2025,
mutatis mutandis.
Since the umbra
does not touch the
Earth's surface in
annular solar eclipse,
the eclipse paths
do not mark out the umbra,
except for the
hybrid solar eclipses
when they are in their total solar eclipse
phase.
I'd guess the path region is the region in which the
annular solar eclipse
looks like an annular solar eclipse
and not just like a partial solar eclipse.
Credit/Permission:
Fred Espenak (1953--),
2002 /
Courtesy of
Fred Espenak, NASA/Goddard Space Flight Center.
Download site: NASA Eclipse Web Site,
scroll down ∼ 60 % and click on
World Map
of Annular Solar Eclipses: 2001-2025.
As the above figure shows, there will a good
annular eclipse
whipping through the western US
on 2012may20.
Because of the rotation of the line of nodes,
solar eclipses can happen at any time
of the year.
But where can they happen?
The above eclipse-path figures suggest that
total solar eclipses can occur
anywhere on Earth.
This is true.
The orbital inclination
of the Moon takes the
Moon above and below the
ecliptic plane by
an amount greater than the Earth's radius.
If it didn't, there would be
solar eclipses every
lunar month.
But those same swings above the
ecliptic plane
mean that
solar eclipses
will happen at any latitude.
The lunar umbra
can touch down anywhere from the
equator to the
poles.
Also because the periods of the
solar day,
lunar month,
and
line of nodes
are effectively NOT
commensurable
eventually every longitude can get hit by
the lunar umbra too.
Recall longitude
along any line from the
Earth to the
Sun is a function of
time of solar day.
Because the time periods are
commensurable,
there will be an exact repeating cycle with the all-events event happening every repeat time
of 2*3*5=30 time units.
But if event B happens every π time units, then
after the first all-events event there will never be another exact all-events event.
To prove this, let's assume the contrary and show that there is a contradiction.
Say the all-events event repeats.
Then it must happen at a time 2*f=&pi*g=5*h, where f, h and g are
integers.
But this means that π=2*f/g which implies that π is a
rational number.
But π is a known to be an
irrational number.
Caption: Pi constant (AKA π) illustrated.
π is the ratio of the
circumference of a
circle to its
diameter (which is 2 times its
radius): π=C/D=C/(2R).
π is an
irrational number---it's
mad, bad, and dangerous to know---like
Lord Byron (1788--1824).
Which means it is NOT a rational numbers:
it cannot be expressed as a ratio of integers.
Credit/Permission: User:German,
2007 /
Public domain.
Image linked to Wikipedia.
So we have a contradiction.
Thus, there is no repeat time for an all-events event.
But we don't know the periods exactly and, in fact, the periods
change slowly with time.
For example, recall that the solar day is
currently increasing is increasing by 1.70(5)*10**(-3) seconds per century
(Wikipedia:
Earth-Moon case).
So the periods are effectively NOT
commensurable.
Actually a detailed proof of the statement that eventually every longitude can get hit by
the lunar umbra is needed, but it seems
likely on the face of it.
But there is a near repeating cycle of all
eclipse phenomena: solar and lunar.
It is called the Saros cycle
and lasts 6585.3213 days
(see Wikipedia: Saros cycle) or
18 Julian years (of 365.25 days each exactly)
plus 10.8213 days.
Note the period is an empirical quantity and has some uncertainty.
It was later known to the
ancient Greek astronomers, who
probably got from the Babylonian astronomers.
Edmond Halley (1656--1742) (of
Halley's comet) gave the cycle the name "Saros"
based on a mistaken reading or interpetation of old records.
The Saros cycle only works approximately recall
and the Ancients could never
predict solar eclipses
and, in particular, total solar eclipses
with much accuracy.
Oh they could name the day for a
total solar eclipse, but they
could only say where it would happen to within maybe 2000 km---just guessing on that.
A total solar eclipse
is what people travel to see---and
with any luck they arn't clouded out.
It's what people want to see.
It's dark as night in the
day, animals get confused,
Sun gets eaten.
Total solar eclipses
are so rare in any locality on Earth (only once every
370 years on average it is estimated:
Wikipedia:
Solar eclipse: Occurrence and cycles), that they must have been unprecedented
and terrifying events for most pre-literate or low-literate societies.
The eclipse path map above
(see Total Solar Eclipse Path Map 2001--2025)
shows the opportunities for year 2001--2025 period.
The U.S. will get
total solar eclipses
in 2017 Aug21
and 2024 Apr08.
The 2017 Aug21
total solar eclipse
will pass near Topeka, Kansas---but why should you
care about Topeka, Kansas.
Here's a set From Russia
with Love.
Caption: A collage of the Total solar
eclipse of 2008 Aug01 taken in Novosibirsk,
Siberia,
Russia.
Click on image and then again to see the high resolution version.
The image is based on 38 photos.
In all except the central photo, you are seeing the
photosphere with
Moon biting it: i.e.,
partial solar eclipse photos.
In the central photo, one has totality: i.e.,
the solar photosphere is completely covered.
ONLY during totality
is safe to view the Sun
with the naked eye:
see NASA: Eye Safety During Solar Eclipses.
The exposure time
for the central photo may have been much longer than the others---but I don't know
for sure.
The long exposure time may
be needed to bring out the outer layer of the Sun
called the corona---not a beer, but a wispy white
halo only visible to the
naked eye during
totality.
The animation shows
File:Solar eclipse
animate (2008-Aug-01).gif by A.T. Sinclair shows the eclipse.
Credit/Permission:
© User:Kalan,
2008 /
Creative Commons
CC BY-SA 3.0.
Image linked to Wikipedia.
Caption: A partial solar eclipse image
of the Total solar
eclipse of 2008 Aug01 taken in
Moscow---the one in
Russia---not my old home
Moscow, Idaho.
There was no totality in
Moscow---just a
partial solar eclipse there.
If you didn't know there was going to be
partial solar eclipse and it was cloudy,
you may never notice that there was one.
If it wasn't cloudy, you might think it odd that sunlight seems a little dim without obvious clouds.
You might guess there was some haze.
Actually, sunlight filtering through leaf cover might give rise to odd crescent shape patches of light.
The holes in the leaf cover giving rise to crude
pinhole projections.
Credit/Permission:
© Pavel Leman,
2008 /
Creative Commons
CC BY-SA 3.0.
Image linked to Wikipedia.
Caption: A partial solar eclipse image
of the Total solar
eclipse of 2008 Aug01 taken in
Jinta County (postal code 735300),
Gansu Province,
China.
You are NOT seeing
the corona since its only
a partial solar eclipse at this location:
it's not dark.
I think the image is over-exposed for the Sun
in order not to be solar-filtered to blackness for everything else.
Stretching away from the Sun
to the upper left is the ecliptic.
On which one
sees Mercury
(always within about 28 degrees of the Sun on the sky because of
its inner planet orbit).
Then one sees Venus
(always within about 47 degrees of the Sun on the sky
because of its inner planet orbit).
Finally, at the about the distance from Venus
as Venus is from
Mercury
one does NOT (but the author claims one should)
see Saturn as a faint red dot.
As Saturn is an
outer planet
can have any angle with respect to the Sun on the sky.
It probably has to be farther away than the
Sun in spatial distatance to be as close to the
Sun in angle as it is claimed to be in this image.
Credit/Permission:
© User:Ivanip,
2008 /
Creative Commons
CC BY-SA 3.0.
Image linked to Wikipedia.
Here are some other images.
There seems to be a sunspot
in the upper part near the Moon's limb.
In astro-jargon, a LIMB is the edge of an
astro-body's projection on the
sky.
Credit/Permission:
Bill Livingston, NSO/AURA/NSF,
© NOAO/AURA /
NOAO/AURA Image Library Conditions of Use.
Credit: NSO/AURA/NSF.
In the diamond ring effect,
the solar photosphere just peeps through
a single notch (e.g., valley) at the edge of the lunar disk.
Credit: Bill Livingston/NSO/AURA/NSF.
Caption: A set of images of the
Solar eclipse of August 11, 1999.
I think the images are a sequence.
The end ones are
partial solar eclipse images.
The middle one shows totality with
a clear corona.
The others may be just different exposures times for
totality, but I think they
might be just on the
verge of totality.
The 2nd to last image on the right seems to be showing the
diamond ring effect.
The solar prominences (the reddish features) seen.
Credit/Permission: Luc Viatour AKA User:Lviatour /
Creative Commons
CC BY-SA 3.0.
Image linked to Wikipedia.
We will discuss the corona
and solar wind later
in IAL 8: The Sun.
But we can give brief discussion here.
The obvious surface of the Sun---the thing that the
Moon just covers in a total solar eclipse---is the
solar photosphere as discussed above.
This is the surface of
the Sun from which most of the light travels to
us without further scattering by solar matter.
But there are very rarefied layers of the
Sun above the photosphere.
The corona is the most obvious outer layer though it
is only visible to the
naked eye during a
total solar eclipse.
The corona is a very tenuous, but very
hot, gas of
solar composition (hydrogen and helium mainly).
It's temperature is of order 10**6 K which is
much hotter than the photosphere which is about 6000 K.
The corona's low density causes it's low emission even
though it is extremely hot.
To the eye the corona is a milky white.
The corona varies in time, and so looks a bit different in
all images.
It's part of solar weather.
Of course, the images themselves are taken with
different exposure times,
and so all images look different for that reason too.
Because of its high temperature all the gas
in the corona is IONIZED: the atoms are
split into positively charged particles atoms---which are called
ions---lacking some or all of their electrons and
free electrons.
The corona really has no sharp outer edge.
From high-altitude balloons or aircraft it
can be traced out to 30 solar radii
(Se-151).
The corona just gradually changes into being the
solar wind: a
stream of solar gas that is being blown out into
interstellar space from the Sun.
Caption: "Coronal mass ejection
hits Comet Encke and rips off the tail".
A coronal mass ejection is a very
strong blast of the solar wind.
Credit: NASA
Image linked to Wikipedia.
Permission: Public domain at least in USA.
The mechanism causing the solar wind is not entirely
understood, but it is a small loss and has
no great effect on the Sun's overall properties
at present.
The particles in the corona
spiral away from the Sun along
magnetic field lines
(see below).
This is what gives the corona a wispy or haired appearance.
The image below shows the wispy appearance more clearly.
Recall the structure of the corona is time-varying, and so
the image is just a typical appearance for some exposure time.
Caption: "Here is a view of the solar disk in 195 Angstroms ultraviolet light
(colored green in this movie) and the Sun's extended atmosphere,
or corona, (blue and white in this movie).
The corona is visible to the
SOHO/LASCO
coronagraph instruments, which block the bright disk of the
Sun so the significantly fainter
corona can be seen. In this movie,
the inner coronagraph (designated C2) is combined with the outer coronagraph (C3).
This movie covers a two week period in October and November
2003 which exhibited some of the largest solar activity events
since the advent of space-based solar observing.
As the movie plays, we can observe a number of features of the active
Sun. Long streamers radiate outward from
the Sun and wave gently due to their
interaction with the solar wind.
The bright white regions are visible due to their high density of free electrons which scatter
the light from the photosphere towards the observer.
Protons and other ionized atoms are there as well, but are not as visible since they do not interact
with photons as strongly as electrons.
Coronal mass ejection (CMEs)
are occasionally observed launching from the
Sun.
Some of these launch particle events can saturate the cameras with snow-like artifacts.
Also visible in the coronagraphs are stars and planets.
Stars are seen to drift slowly to the right, carried by the relative motion of the
Sun and the
Earth.
The planet Mercury
is visible as the bright point moving left of the
Sun.
The horizontal "extension" in the image is called blooming
and is due to a charge leakage along the readout wires in the CCD imager in the camera."
Yours truly is just interested in this as a good image of the
corona---not in the movie at the moment.
The corona is a hot
(of order 1,000,000 K), very lower density
layer of gas surrounding the Sun.
It is milky white and appears wispy because the
ionsionized atoms
(charged atoms)
stream along
magnetic field lines.
Recall the structure of the corona is time-varying, and so
the image is just a typical appearance for some time and
exposure time.
Credit/Permission:
NASA/Goddard Space Flight Center,
Scientific Visualization Studio, 2008 /
Public domain.
Image linked to Wikipedia.
The Sun
is surrounded by a complex and time-varying magnetic field.
The magnetic force on free particles
caused by this field
partially traps the charged particles in the direction
perpendicular to the field lines (which we'll discuss
later, but you've probably heard of them before).
The particles tend to helix around the field lines.
As a result charged particles of the solar wind
tend to helix outward along field lines.
When the solar wind particles interact with
the Earth's magnetic field, they can also go into
spiral motion as the figure illustrates.
Another feature of the Sun easily visible from the
Earth during total solar eclipses
are solar prominences.
We will discuss them in more detail later in
IAL 8: The Sun.
These are vast eruptions of material that can shoot
up from the Sun in a few hours and last weeks or
months. They are also controlled by magnetic fields
it seems.
The solar prominences are part of solar weather.
Solar weather is magnetic phenomenon among other things.
The prominences can be seen as little tongues of fire
in solar eclipse images.
The red color comes from the emission of strongest visible line of the hydrogen
atom at temperatures of order 10000 K
(Se-150,160).
Note 10000 K is hotter than the photosphere's 6000 K, but
much colder than the 10**6 K that is characteristic of the corona.
Note the Sun's diameter is just about 100 times that of the
Earth.
Consequently, the prominences are huge---they can be bigger than
the Earth.
Credit: ?: This image was downloaded long ago before I became careful
about images. It's such a nice image, I like to show it.
I assume it falls in the no-one-is-agrieved category.
However, if anyone queries its use, I will satisfy them.
Form groups of 2 or 3---NOT more---and tackle,
homework 3
problems 25--28 on
solar eclipses.
Discuss each problem and come to group answer.
Oh, 5--10 minutes.
See solutions 3.
The winners get chocolates.
Caption: "Bars of dark
Swiss chocolate.
From left to right:
a) about 75% cocoa;
b) contains chili;
c) dark chocolate."
(Slightly edited.)
Did you know that cocoa may be the
great brain food---see
Daisy Yuhas, 2013,
Is Cocoa the Brain Drug of the Future?---it makes mice
smarter---but maybe only in unprocessed form---just when you thought it was safe to
scarf.
Credit/Permission: ©
Simon A. Eugster (AKA User:LivingShadow),
2010 /
Creative Commons
CC BY-SA 3.0.
Image linked to Wikipedia.
Caption:
Eclipse paths
for annular solar eclipse
and hybrid solar eclipse
for 2001--2015.
NOT commensurable
means that there is no set of integer
multiples of the periods
of the
solar day,
lunar month,
and
line of nodes
that is the same length of time---which
is the same as saying the ratios of the periods are NOT
rational numbers.
It is estimated that on average every place on Earth
gets a totality every
370 years (Wikipedia:
Solar eclipse: Occurrence and cycles).
For example, say event A happens every 2 time units, B every three time units,
and C every 5 time units, and all the events happen simultaneously a time zero.
Since we only know the periods of the
solar day,
lunar month,
and
line of nodes
to finite number of digit places, those periods taken as exact
are commensurable for
some very large multiples.
The Saros cycle was discovered by the
Babylonian astronomers in the
2nd half of the 1st millennium BCE,
it was not called that by them.
The trailing approximate 1/3 of day of the
Saros cycle period
means that for the Earth
to be in approximately the same rotational orientation as in a previous cycle for a given
eclipse phenomenon, one has to wait
3 Saros cycle periods or about 19756 days (about 56
Julian years and 32.5 days).
It will probably be a cloudy day there then---maybe with tornados.
There are great solar eclipse images
on the web---and nowadays at last some can be used---with proper credit.
Three quarters partial solar eclipse.
total solar eclipse with
corona.
The diamond ring effect.
Solar eclipse videos:
Earth magnetic field.
(Correct Charge particle to Charged particles.)
We will discuss lines in
IAL 7: Spectra
but for now they are just wavelength bands in which atoms
emit light.
Solar eclipse with prominences.
Group Activity:
We can go there for a moment to look up information about upcoming
eclipse season: for
lunar eclipses; for
solar eclipses.
