On a sliver of moonlight he rode
over the crest of the hill
---``The Highwayman'' ???
Sections
Although much fainter than the Sun, moonlight of a full Moon is a significant resource in the absence of modern lighting.
In fact astronomers, not looking at the Moon itself, consider it a source of light pollution and are always desirous of DARK TIME when the Moon's away.
Of course, other traditional problems come with the FULL MOON.
Alien werewolf.
The Moon since prehistoric times has been used CALENDRICALLY: e.g., ``it was many moons ago.''
In fact the use of the LUNAR MONTH both for secular TIME-KEEPING and RELIGIOUS PURPOSES goes back to prehistory in many societies.
The origin of the 7 DAY WEEK is lost in prehistory (I think???), but probably it goes back to the fact that 7 days is approximately a quarter of the lunar month and is marked clearly by the phases: new moon, half moon, full moon, and three quarters moon.
The LUNAR MONTH is not, of course, the CALENDAR MONTH of the modern civil calendar: the calendar month is divorced from the lunar month only retaining the family name MONTH.
If you count the LUNAR MONTH as starting from the first visible crescent after NEW MOON as was and is often done, then the lunar month alternates between 29 and 30 days---it could be longer if you had to wait out cloudy evenings to see a crescent for the first time in a lunar month.
In fact the MEAN LUNAR MONTH---the cruelest month---is in fact 29.53059 DAYS (Cox-16).
This creates a problem since the lunar month is not made of an INTEGRAL NUMBER OF DAYS (nor weeks), nor is the year made of 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 LUNI-SOLAR CALENDAR.
Since 12 lunar months is about 354.36 DAYS and the solar (i.e., tropical) year is about 365.2421897 DAYS (Cox-15), a lunar time year count would be ahead of a solar time year count by about 33 days every 3 years.
In order to keep the 12 regular lunar months from moving out of the solar-determined seasons, a 13th lunar month (an INTERCALARY MONTH) had to be inserted into a calendar year a bit more frequently than every 3 years.
A pretty precise way of inserting INTERCALARY MONTHS is to use the 19-year cycle or METONIC CYCLE named after the Athenian astronomer Meton (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 thus, there are 235 lunar months
which equal 6939.69 days.
19 tropical years equal 6939.60 days.
Thus, there is a discrepancy of only about 1/10 of day.
It takes about 190 years of using METONIC CYCLE for
the calendar year based on the METONIC CYCLE to
be 1 day off from a true count of tropical years.
For a trivial procedure, this isn't bad.
Most societies using a luni-solar 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 MONTHS just when the ``year was not good'' as far as they good 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 (102?--44 BCE) in his calendar reform of 46 BCE dispensed with lunar months altogether replacing them with the 12 arbitrary months that divide up the tropical year.
This broke the connection between the physical Moon and time-keeping. The JULIAN CALENDAR is of course the ancestor of the modern civil calendar.
By the by, Julius Caesar actually undertook the calendar reform in his role as Pontifex Maximus (or chief priest of Rome): he was the legitimate responsible official for the calendar. And he had a month named for himself: JULY.
Julius Alien.
Moon numbers
(Cox-16, 240, 303,305).
You note that the DIAMETERS of both Earth and Moon are pretty small compared to the distance separating them.
This is why even though the Moon has a quarter of the Earth's diameter its angular diameter on the sky is only about 0.5 degrees.
Also striking is that the Moon is much less massive than the Earth: only about 1/80 of the Earth's mass.
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.
The Earth's orbit is so small that we can say physically as well as just geometrically that the Moon orbits the Earth---for many purposes, but not for Earth's TIDES as we'll see in IAWL Lecture 5: Newtonian Physics, Gravity, Orbits, Energy, Tides.
___________________________________________________________________________ Table of Moon Orbital Facts ___________________________________________________________________________ mean Lunar Month 29.53059 days mean Sidereal Period 27.321661 days (orbit relative to fixed stars) eccentricity 0.0549 (or 5.49 % variation from mean distance) ecliptic angle 5.145 degrees ___________________________________________________________________________(Cox-16,303,305).
___________________________________________________________________________We see that the Moon's SIDEREAL PERIOD is less than the LUNAR MONTH.
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.
Lunar month and lunar sidereal period.
The Moon's orbit is ECCENTRIC by 0.0549.
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 SIZE ON THE SKY vary by 11 % too.
The variation in angular size is probably too small ever to be noticed by casual observation since we usually see the Moon at perigee and apogee without a convenient 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.
The angular diameter of the Moon at perigee and apogee compared.
Credit: John Walker's Inconstant Moon page. John Walker has declared this page and its images public domain.
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 around the Moon. Of course, you should never look at an annular solar eclipse with the unaided eye.
I suppose in partial solar eclipses also provide a STANDARD OF COMPARISON, but you should never look at them either with the unaided eye, and, in any case, it would be hard to tell whether Sun or Moon had the larger angular diameter.
We consider solar eclipses in the section Solar Eclipses.
The ecliptic angle of the Moon's orbit is 5.145 degrees: i.e., the tilt of the Moon's orbit from the Ecliptic Plane defined by the Earth's orbit around the Sun.
This tilt too has an important eclipse consequence.
Tilt of the Moon's orbit.
Answer 1 is right.
The tilt of the Moon's axis badly complicates eclipse phenomena.
The two points where the Moon's orbit crosses the Ecliptic Plane are called the NODES. The line that connects the nodes passes through the Earth. This line is called---very imaginatively---the LINE OF NODES. The line of nodes rotates westward 19.4 degrees per year.
The Moon's orbit and the line of nodes.
Now I know what you are thinking.
Why, why must the NODES ROTATE?
In an exact TWO-BODY GRAVITATIONAL SYSTEM the nodes wouldn't rotate. 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 and the Moon. 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 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 nodes 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 is possible. We discuss ECLIPSE SEASONS when we discuss eclipses in the Lunar Eclipses and Solar Eclipses. below
Now if the line of nodes were fixed in space, alignment would happen every 6 months or a bit more exactly about every 183 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 calendar year.
Thus eclipses can occur at any time of the calendar year.
Some Moon phases.
Credit: NASA; download source NASA's Applied Engineering Competition.
Answer 2 is right.
The Moon is always approximately along the Ecliptic as we discussed in the celestial sphere lecture.
Simple phases of the Moon questions often seem very difficult to people.
But once you get the hang of them, they are easy.
They are sort of analogous to an algebra problem 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.
Moon phases.
Remember the Moon takes a lunar month to go eastward from new moon to new moon (the mean lunar month is 29.53059 days [Cox-16]), and the Earth rotates eastward in one day.
Thus to first order, you can take the Moon as fixed on the rotating CELESTIAL SPHERE and fixed in PHASE for any day.
Let's do three examples of phases of the Moon problems.
Time and location on the sky are knowns. Phase is the unknown.
Glance back Moon 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 Moon phases diagram.
It must be sunset.
If the Moon was on the eastern horizon, it would be noon.
Phase and time are the knowns. Location on the sky is the unknown.
Glance back at the Moon phases diagram.
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 the meridian.
Say you are at the SUNSET location:
Actually the Moon does move a finite distance in the sky 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 1 degree per two hours.
The Moon itself subtends about 0.5 degrees. Thus every hour it moves about its own angular diameter.
If one checks the Moon agains the fixed stars during a night, the Moon's motion can be easily seen.
In a lunar eclipse we talk of the Moon as being eclipsed when its in the Earth's shadow.
In a solar eclipse we say the Sun is eclipsed when the Moon covers it.
If we wanted consistency (which we don't), we should say a lunar eclipse is really a solar eclipse as seen from the Moon: i.e., the Earth is eclipsing the Sun as seen from the Moon.
Henry David Thoreau, or so I seem to recall. Like most aphorisms, its only true when you want it to be.]
ECLIPSES REQUIRE SHADOWS
The Earth has two kinds of shadow: UMBRA and PENUMBRA.
They both stretch away from the Earth in the anti-solar direction.
Earth umbra and penumbra.
THREE KINDS OF LUNAR ECLIPSES
A total lunar eclipse including penumbral stage can last up to 6 hours; totality 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: i.e., for about 4.5 days before and 4.5 days after.
Because the eclipse season is shorter than the lunar month, a total lunar eclipse is not always possible.
If the Moon is not close enough to the full Moon phase when the eclipse season, the eclipse will end before the reaches full Moon phase and the nodal alignment passes without a total lunar eclipse.
Only at about 35 % of nodal alignments there is 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.
About 30 % of nodal alignments there is partial lunar eclipse without a total lunar eclipse ( Lunar Eclipses for Beginners).
No one gets too excited about partial lunar eclipses without total lunar eclipses, but they are noticeable.
Answers 1 and 2 are right.
The eclipse season for penumbral eclipses is 32 days??? about exact nodal alignment: i.e., for 16 days before and 16 days after.
About 35 % of nodal alignments there is penumbral lunar eclipse without a partial or total lunar eclipse ( Lunar Eclipses for Beginners).
But it may be that a ``total'' penumbral eclipse must occur every nodal alignment and a second eclipse that is only a ``partial'' penumbral eclipse can occur.
I havn't been able to find any information on this fine, but not very interesting, point.]
The Moon just looks a little diminished in brightness. A layer of cloud could have almost the same effect. So penumbral eclipses go unnoticed.
TOTAL LUNAR AND SOLAR ECLIPSES AS SPECTACLES
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.
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, but to see a total solar eclipse you must travel to an ECLIPSE PATH or be lucky enough to live on one.
A TOTAL LUNAR ECLIPSE
The images below show the total lunar eclipse of 2000jan20.
The visible crater to the left side is again Tycho: Tycho is actually close to the south end of the Moon: i.e., south is more or less to the right.
1. The Moon just moving into the umbra.
2. The Moon half into the umbra.
3. The Moon about three quarters into the umbra.
4. The Moon during totality in color.
Credit: image 1, NASA; image 2, NASA; image 3, NASA; image 4, NASA.
THE COPPERY COLORED MOON
At totality the Moon can take on a coppery color as we see in the 4th NASA lunar eclipse image above. This is due to sunlight refracted through the Earth's atmosphere (Se-41).
The bluish light of the Sun is more strongly scattered out of travel to the Sun by the atmosphere, and so it is the reddish light that reaches the Moon and then is reflected back to observers on Earth.
The reddening of the Moon in a total lunar eclipse.
(Internet Explorer does NOT show this image!!!)
The out-scattering of bluish light is, of course, the reason why sunrise and sunsets 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 lunar eclipse can occur if the atmosphere is particularly opaque on the Earth's terminator.
A LUNAR ECLIPSE AS SEEN FROM THE MOON?
Apollo 12, 1969nov, passing into or out of the Earth's umbra.
(Internet Explorer will NOT show this image!!!)
No one has been on the Moon for a lunar eclipse which, of course, from the Selenite perspective is a solar eclipse.
But Apollo 12 in 1969nov passed into the Earth's umbra on the return leg from the Moon and obtained images of this event.
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.
But Earth's angular diameter is larger the closer the spacecraft get to Earth.
Night on Earth in a sense is an eclipse of the Sun by the Earth.
Credit: NASA. The credit has the date wrong: the Apollo 12 mission was from 1969nov14 to 1969nov24.
SCIENTIFIC VALUE
Lunar eclipses nowadays are of no special scientific value. They are just spectacles.
In the past lunar eclipses were scientifically interesting.
For example, they were interesting for themselves if you didn't understand how they worked or how to predict them.
They could also be used in determining the distance to the Moon.
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 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.
Because the Moon's umbra is very small at the distance of the Earth, only a small part of the Earth experiences totality.
Remember the Moon is only about a quarter of Earth in diameter and so its umbra at the Earth is only about a quarter of the Earth's umbra at the Moon.
And, of course, the Earth is much larger than the Moon and so the Moon's umbra doesn't cover much of the Earth.
Also remember 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 NOT always total solar eclipses.
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.
Answer 1 is right.
When the Moon is closest, its umbra on the Earth is biggest.
The umbra of the Moon on the Earth.
This image from the International Space Station shows the umbra on the Indian Ocean on 2002dec04.
Credit NASA.
A qualification is needed to the statement that the Moon covers the whole Sun at 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 PHOTOSPHERE or more loosely the ``surface.''
It is the photosphere that is covered in a total solar eclipse.
Parts the Sun can be seen around the Moon during totality that can never been seen by the unaided eye otherwise.
Only during totality and ONLY THEN is it safe to look at the Sun with the unaided eye.
Eye safety during solar observations is discussed below.
The 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.
A annular solar eclipse.
Annular solar eclipses are slightly more frequent than total 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 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.
Annular solar eclipses arn't nearly as popular as total solar eclipses. They are spectacular, but you can't look at them with the unaided eye and everything doesn't get nighttime dark.
From the observer's location the Sun is a crescent.
But do NOT look at the Sun with the unaided 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 solar eclipses happen about ??? 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 new at some time during the eclipse season.
Two partial solar eclipses can occur if one happens right at the beginning of the eclipse season. The Moon can races around the Earth and gets back to new Moon before the eclipse season is over.
Two partial solar eclipses is a rare event, but one such event will happen in 2036: the dates are 2036jul23 and 2036aug21. See Solar Eclipses: 2031 - 2040 by Fred Espenak.
Thus, in reality total and annular solar eclipses are not all that uncommon.
But annular solar eclipses don't usually cause excessive interest and there 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.
You cannot---MUST NOT---look a the Sun directly with the unaided 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 unaided 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 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 unaided eye. We've all, of course, had glimpses, but again one should minimize those.
For more on safety see the NASA eye safety page
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.
The image is fuzzy (and upside down), but it's better than nothing. I think you can see sunspots???.
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 (FMW-79).
THE GREAT COINCIDENCE
The Sun and Moon have both almost the same angular diameter on the sky: i.e., about 0.5 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 symmetry is just a coincidence of our era on Earth.
THE MOVING UMBRA
The Moon's umbra follows an ECLIPSE PATH on or over the Earth.
Two motions are compounded to make the umbra move.
(1) The Moon is moving east on the sky causing it's shadow to move east; (2) the Earth is spinning east as well.
The second motion somewhat compensates for the first.
We can do a nifty approximate calculation of the speed of umbra on or over the Earth.
The umbra speed (the speed of the Moon in space relative to the Sun) and the speed of the Earth at the Equator are:
v=(2*pi*D_Moon)/(29.5*24 hours) = 6 * 4*10**5 km /(30*24 hr) = 3000 km/hr
and
v=(2*pi*R_Earth_equatorial)/(24 hours) = 6*6*10**3 km/24 hr = 1500 km/hr .
Both speeds are eastward, and so the approximate umbra speed
on the ground at the equator is about 3000-1500=1500 km/hr.
A more exact calculation shows that the minimum umbra speed is about 1700 km/hr (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 3000 km/hr.
To 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.
Total Solar Eclipse Paths 2001--2025.
Note the Mercator projection: thus the path width is latitude dependent. Thus Arctic and Antarctic paths look very large even though they are not really any larger than tropical paths.
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).
The eclipse sweeps basically eastward: both umbra and earth are moving east, but umbra is faster.
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 map.
Credit: courtesy of Fred Espenak, NASA/Goddard Space Flight Center, Eclipse Home Page.
Annular and Hybrid Solar Eclipse Paths 2001--2025.
Note the Mercator projection: thus the path width is latitude dependent. Thus Arctic and Antarctic paths look very large even though they are not really any larger than tropical paths.
Recall the umbra does not reach the ground during an annular solar eclipse. The path shown must be that of annularity defined somehow.
The minimum umbra speed is about 1700 km/hr (Se-43).
The eclipse sweeps basically eastward: both umbra and earth are moving east, but umbra is faster.
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 map.
Credit: courtesy of Fred Espenak, NASA/Goddard Space Flight Center, Eclipse Home Page.
THE MAIN EVENT: THE TOTAL SOLAR ECLIPSE
A total solar eclipse is what people travel to see---and with any luck they arn't clouded out.
The eclipse path map above shows the opportunities for year 2001--2025 period.
The U.S. will get total solar eclipses in 2017aug21 and 2024apr08.
The 2017aug21 total solar eclipse will be near Topeka.
There are great eclipse images sequence on the web, but invariably they are copyrighted and unavailable for use. See the NASA eclipse links given below.
The best images I'm free to show are below.
Three quarters partial solar eclipse.
There seems to be a sunspot in the upper part near the Moon's limb.
Credit: Bill Livingston, NSO/AURA/NSF.
Total solar eclipse with corona.
Credit: NSO/AURA/NSF.
The diamond ring effect.
In the diamond ring effect, the Sun just peeps through a valley at the edge of the lunar disk.
Credit: Bill Livingston/NSO/AURA/NSF.
CORONA AND SOLAR WIND
We will discuss the CORONA and SOLAR WIND later in IAWL Lecture 8: The Sun.
But we can give brief discussion here.
The obvious disk 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 though it is only visible to the unaided 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 is varies in time, and so looks a bit different in all images. Of course, the images themselves are taken with different exposures, and so all 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 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.
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 figure below shows the wispy appearance more clearly.
The structure of the CORONA is time-varying.
Solar eclipse with corona.
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 ionized atoms stream along magnetic field lines.
Credit: ?
MAGNETIC FIELDS: JUST A BRIEF WORD
The Sun is surrounded by a complex and time-varying MAGNETIC FIELD.
The magnetic force this field exerts on charged particles partially traps them in the direction perpendicular to the field lines (which we'll discuss later, but you've probably heard of them before.)
The field causes them to move in a circular motion about the field lines.
But the particles are much more free to move along the field lines.
As a result charged particles tend to spiral 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.
Earth's magnetic field.
(Correct Charge particle to Charged particles.)
SOLAR PROMINENCES
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 IAWL Lecture 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 prominences can be seen as little tongues of fire.
The red color comes from the emission of strongest line of the hydrogen atom at temperatures of order 10,000 K (Se-150,160).
Note 10,000 K is hotter than the PHOTOSPHERE'S 6000 K, but much colder than the 10**6 K that is characteristic of the CORONA.
Solar eclipse with prominences.
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: ?
We can go there for a moment to look up information about upcoming eclipse seasons: for lunar eclipses; for solar eclipses.