http://www.space.com/15584-solar-eclipses.html
and http://www.mreclipse.com/Special/LEprimer.html
A solar eclipse occurs when the moon gets between Earth and the sun, and the moon casts a shadow over Earth. A solar eclipse can only take place at the phase of new moon, when the moon passes directly between the sun and Earth and its shadows fall upon Earth’s surface. But whether the alignment produces a total solar eclipse, a partial solar eclipse or an annular solar eclipse depends on several factors, all explained below.
The fact that an eclipse can occur at all is a fluke of celestial mechanics and time. Since the moon formed about 4.5 billion years ago, it has been gradually moving away from the Earth (by about 1.6 inches, or 4 centimeters per year). Right now the moon is at the perfect distance to appear in our sky exactly the same size as the sun, and therefore block it out. But this is not always true
Be careful: Watching a solar eclipse requires some safety measures. While it’s perfectly safe to view a total solar eclipse — when no direct sunlight is reaching your eye — you should never look directly at the sun itself. See more on this below, as well as a list of upcoming solar eclipses.
There are four types of solar eclipses: total, annular, partial and hybrid. Here’s what causes each type:
Total solar eclipses
These are a happy accident of nature. The sun's 864,000-mile diameter is fully 400 times greater than that of our puny moon, which measures just 2,160 miles. But the moon also happens to be about 400 times closer to the sun than the Earth (the ratio varies as both orbits are elliptical), and as a result , when the orbital planes intersect and the distances align favorably, the new moon can appear to completely blot out the disk of the sun.
There are actually two types of shadows: the umbra is that part of the shadow where all sunlight is completely blocked out and takes the shape of a dark, slender cone. It is surrounded by the penumbra, a lighter, funnel-shaped shadow from which sunlight is partially obscured.
During a total solar eclipse, the moon casts its umbra upon Earth's surface; that shadow can sweep a third of the way around the Earth in just a few hours. Those who are fortunate enough to be positioned in the direct path of the umbra will see the sun's disk diminish into a crescent as the moon's dark shadow rushes toward them across the landscape.
During the brief period of totality, when the sun is completely covered, the beautiful corona – the tenuous outer atmosphere of the sun – is revealed. Totality may last as long as 7 minutes 31 seconds, though most total eclipses are usually much shorter. On the average a total eclipse occurs somewhere on Earth about every 18 months.
Partial solar eclipses
On Jan. 4, 2011, the moon passed in front of the sun in a partial solar eclipse - as seen from parts of Earth. Here, the joint Japanese-American Hinode satellite captured the same breathtaking event from space. The unique view created what's called an annular solar eclipse.Credit: Hinode/XRT
A partial solar eclipse occurs when only the penumbra (the partial shadow) passes you by. In these cases, a part of the sun always remains in view during the eclipse. How much of the sun remains in view depends on the specific circumstances.
Usually the penumbra gives just a glancing blow to our planet over the Polar Regions; in such cases places far away from the poles but still within the zone of the penumbra might not see much more than a small scallop of the sun hidden by the moon. In a different scenario, those who are positioned within a couple of thousand miles of the path of a total eclipse will see a partial eclipse.
The closer you are to the path of totality, the greater the solar obscuration. If, for instance, you’re positioned just outside of the path of the total eclipse, you’ll see the sun wane to a narrow crescent, then thicken up again as the shadow passes you by.
Annular solar eclipses
An annular solar eclipse is similar to total eclipses in that the moon appears to pass centrally across the sun, but it’s too small to cover the disk of the sun completely. Because the moon circles the Earth in an elliptical orbit its distance from Earth can vary from 221,457 miles to 252,712 miles. But the dark shadow cone of the moon’s umbra can extend out for no longer than 235,700 miles; that’s less than the moon’s average distance from Earth.
So if the moon is at some greater distance, the tip of the umbra does not reach Earth. During such an eclipse, the antumbra, a theoretical continuation of the umbra, reaches the ground, and anyone situated within it can look up past either side of the umbra and see an annulus, or “ring of fire” around the Moon.
A good analogy is putting a penny atop a nickel, the penny being the moon, the nickel being the sun. An annular eclipse, though a rare and amazing sight, is far different from a total one. The sky will darken . . . somewhat; a sort of weird “counterfeit twilight” since so much of the Sun still shows. It is really more of a day, not a night sky; the eclipse is a subspecies of a partial, not total. The maximum duration for an annular eclipse is 12 minutes 30 seconds.
Hybrid solar eclipses
These are also called annular-total (“A-T”) eclipses. This special type of eclipse occurs when the moon’s distance is near its limit for the umbra to reach Earth. In most cases, an A-T eclipse starts as an annular eclipse because the tip of the umbra falls just short of making contact with the Earth; then it becomes total, because the roundness of the Earth reaches up and intercepts the shadow tip near the middle of the path, then finally it returns to annular toward the end of the path.
Because the moon appears to pass directly in front of the sun, total, annular and hybrid eclipses are also called “central” eclipses to distinguish them from eclipses that are merely partial.
Of all solar eclipses, about 28 percent are total; 35 percent are partial; 32 percent annular; and just 5 percent are hybrids.
Predictions of solar eclipses
Eclipses do not happen at every new moon, of course. This is because the moon’s orbit is tilted just over 5 degrees relative to the Earth’s orbit around the sun. For this reason, the moon’s shadow usually passes either above or below the Earth, so a solar eclipse doesn't occur.
But as a rule, at least twice each year (and sometimes as many as five times in a year), a new moon will align itself in just such a way to eclipse the sun. That alignment point is called a node. Depending on how closely the new moon approaches a node will determine whether a particular eclipse is central or partial. And of course, the moon’s distance from the Earth – and to a lesser degree the Earth’s distance from the sun – will ultimately determine whether a central eclipse is total, annular or a hybrid.
And these alignments don’t happen haphazardly, for after a specific interval of time, an eclipse will repeat itself or return. This interval is known as the Saros cycle and was known as far back as the days of the early Chaldean astronomers some 28 centuries ago. The word Saros means “repetition” and is equal to 18 years, 11 1/3 days (or a day less or more depending on the number of leap years that have intervened). After this interval the relative positions of the sun and moon relative to a node are nearly the same as before. That third of a day in the interval causes the path of each eclipse of a series to be displaced in longitude a third of the way around the Earth to the west with respect to its predecessor.
For example, on Mar. 29, 2006, a total eclipse swept across parts of western and northern Africa and then across southern Asia. One Saros later, on April 8, 2024, this eclipse will recur, except instead of Africa and Asia, it will track across northern Mexico, the central and eastern United States and the Maritime provinces of Canada.
Solar eclipse safety
You should never look directly at the sun, but there are ways to safely observe an eclipse.Credit: Karl Tate, SPACE.com Contributor
It is normally the custom prior to an impending solar eclipse that the mainstream media will provide a variety of warnings and advisories against looking at the Sun with bare eyes, as blindness could ensue. This has given most people the idea that eclipses are dangerous.
Not so!
It’s the Sun that is dangerous. All the time! The sun constantly emits invisible infrared rays that can damage your eyes. Ordinarily, we have no reason to gaze at the sun. An eclipse gives us a reason, but we shouldn't.
There are safe ways, however . . .
By far, the safest way to view a solar eclipse is to construct a “pinhole camera.” A pinhole or small opening is used to form an image of the Sun on a screen placed about three feet behind the opening. Binoculars or a small telescope mounted on a tripod can also be used to project a magnified image of the Sun onto a white card. The farther away the card, the larger you can focus the image. Look for sunspots. Notice that the Sun appears somewhat darker around its limb. This method of solar viewing is safe so long as you remember not to look through the binoculars or telescope when they are pointed toward the Sun; put another way, never look directly at the Sun when any part of its blindingly bright surface is visible.
A variation on the pinhole theme is the “pinhole mirror.” Cover a pocket-mirror with a piece of paper that has a ¼-inch hole punched in it. Open a Sun-facing window and place the covered mirror on the sunlit sill so it reflects a disk of light onto the far wall inside. The disk of light is an image of the Sun’s face. The farther away from the wall is the better; the image will be only one inch across for every 9 feet from the mirror. Modeling clay works well to hold the mirror in place. Experiment with different-sized holes in the paper. Again, a large hole makes the image bright, but fuzzy, and a small one makes it dim but sharp. Darken the room as much as possible. Be sure to try this out beforehand to make sure the mirror’s optical quality is good enough to project a clean, round image. Of course, don’t let anyone look at the Sun in the mirror.
If you’re around leafy trees, look at the shadow cast by them during the partial phases. What do you see? Is it worth a photograph? You will see scores of partially eclipsed suns projected through pinhole gaps between the leaves. If a significant fraction of the sun is covered, notice that it gets cool as the eclipse deepens. Does the wind begin to pick up?
Acceptable filters for unaided visual solar observations include aluminized Mylar. Some astronomy dealers carry Mylar filter material specially designed for solar observing. Also acceptable is shade 14 arc-welder’s glass, available for just a few dollars at welding supply shops. Of course, it is always a good idea to test your filters and/or observing techniques before eclipse day.
Unacceptable filters include sunglasses, old color film negatives, black-and-white film that contains no silver, photographic neutral-density filters, and polarizing filters. Although these materials have very low visible-light transmittance levels, they transmit an unacceptably high level of near-infrared radiation that can cause a thermal retinal burn. The fact that the Sun appears dim, or that you feel no discomfort when looking at the Sun through the filter, is no guarantee that your eyes are safe.
Of course during a total eclipse, you can safely look directly at the sun when its disk is entirely covered. During those few precious seconds or minutes, the magnificent corona shines forth in all its glory surrounding the darkened sun; a marvelous fringe of pearly white light. It differs in size, in tints and patterns from eclipse to eclipse. It is always faint and delicate, with a sheen like a pale aurora. It has a variable appearance. Sometimes it has a soft continuous look; at other times, long rays of it shoot out in three or four directions. It may stand out from the disk in filmy petals and streamers. But when the sun begins to again emerge into view, the corona quickly disappears and you’ll need to protect your eyes once again.
Eclipses in ancient history
As best as we can determine, the earliest record of a solar eclipse occurred over four millennia ago. In China, it was believed that the gradual blotting out of the sun was caused by a dragon who was attempting to devour the sun and it was the duty of the court astronomers to shoot arrows, beat drums and raise whatever cacophony they could to frighten the dragon away.
In the ancient Chinese classic Shu Ching is the account of Hsi and Ho, two court astronomers who were caught completely unaware by a solar eclipse, having gotten drunk just before the event began. In the aftermath, Chung K’ang, the fourth emperor of the Hsai dynasty ordered that Hsi and Ho be punished by having their heads chopped off. The eclipse in question was that of October 22 in the year 2134 B.C.
In the Bible, in the book of Amos 8:9, are the words, “I will cause the sun to go down at noon, and I will darken the Earth in the clear day.” Biblical scholars believe this is a reference to a celebrated eclipse observed at Nineveh in ancient Assyria on June 15, 763 B.C. An Assyrian tablet also attests to the event.
A solar eclipse even stopped a war.
According to the historian Herodotus, there was a five-year war that raged between the Lydians and the Medes. As the war was about to move into its sixth year, a Greek sage, Thales of Miletus foretold to the Ionians that the time was soon approaching when day would turn to night. On May 17, 603 B.C. the sun faded away just as Thales had alluded that it would. So believing that it was a sign from above, the combatants called a truce which was cemented by a double marriage, for, as Herodotus wrote: “Without some strong bond, there is little of security to be found in men’s covenants.”
And giving new meaning to the term, “Scared to death,” is the timid emperor Louis of Bavaria, the son of Charlemagne, who witnessed an unusually long total eclipse of the sun on May 5, 840 A.D., which lasted for over five minutes. But no sooner had the sun begun to emerge back into view, Louis was so overwhelmed by what he had just seen that he died of fright!
Modern study of eclipses
Astronomers have learned much by studying eclipses and by the 18th century, observations of solar eclipses were recognized as providing veritable treasure troves of astronomical information, though sometimes getting that information wasn't easy.
Samuel Williams, a professor at Harvard, led an expedition to Penobscot Bay, Maine to observe the total solar eclipse of Oct. 27, 1780. As it turned out, this eclipse took place during the Revolutionary War and Penobscot Bay lay behind enemy lines. Fortunately, the British granted the expedition safe passage, citing the interest of science above political differences.
And yet in the end, it was all for naught.
Williams apparently made a fatal error in his computations and inadvertently positioned his men at Islesboro — just outside the path of totality — likely finding this out with a heavy heart when the narrowing crescent of sunlight slid completely around the dark edge of the Moon and then started to thicken!
During a total solar eclipse, a few ruby-red spots may seem to hover around the jet-black disk of the Moon. Those are solar prominences, tongues of incandescent hydrogen gas rising above the surface of the Sun. During the total eclipse of August 18, 1868, the French astronomer Pierre Janssen trained his spectroscope on the prominences and discovered a new chemical element. Two English astronomers, J. Norman Lockyer and Edward Frankland, later named it “helium,” from the Greek helios (the Sun). The gas was not identified on Earth until 1895.
And because sunlight is blocked during a total eclipse, some of the brighter stars and planets can be observed in the darkened sky. Under such conditions astronomers were able to test part of Einstein’s now-celebrated general theory of relativity. That theory predicted that light from stars beyond the Sun would bend from a straight path in a certain way as it passed the Sun. The positions of stars photographed near the Sun’s edge during a total eclipse on May 29, 1919, were compared with photographs of the same region of the sky taken at night; the results strongly supported Einstein’s theory.
Our modern technology now allows astronomers to make most of the observations that once had to await an eclipse. But a total eclipse of the sun will always remain among the most impressive of natural spectacles and is a sight that will always be remembered. Be sure to put it on your bucket list; you will not be disappointed.
Lunar Eclipses
The Moon is a cold, rocky body about 2,160 miles (3,476 km) in diameter. It has no light of its own but shines by sunlight reflected from its surface. The Moon orbits Earth about once every 29 and a half days. As it circles our planet, the changing position of the Moon with respect to the Sun causes our natural satellite to cycle through a series of phases:
- New Moon > New Crescent > First Quarter > Waxing Gibbous> Full Moon >
Waning Gibbous > Last Quarter > Old Crescent > New Moon (again)
Phases of the Moon.
The phase known as New Moon can not actually be seen because the illuminated side of the Moon is then pointed away from Earth. The rest of the phases are familiar to all of us as the Moon cycles through them month after month. Did you realize that the word month is derived from the Moon's 29.5 day period?
To many of us, Full Moon is the phase of love and romance.When the Moon is Full, it rises at sunset and is visible all night long. At the end of the night, the Full Moon sets just as the Sun rises.None of the Moon's other phases have this unique characteristic.It happens because the Moon is directly opposite the Sun in the sky when the Moon is Full.Full Moon also has special significance with regard to eclipses.
Geometry of the Sun, Earth and Moon During an Eclipse of the Moon
Earth's two shadows are the penumbra and the umbra.
(Sizes and distances not to scale)
Types of Lunar Eclipses
An eclipse of the Moon (or lunar eclipse) can only occur at Full Moon, and only if the Moon passes through some portion of Earth's shadow. That shadow is actually composed of two cone-shaped components, one nested inside the other. The outer or penumbral shadow is a zone where the Earth blocks part but not all of the Sun's rays from reaching the Moon.In contrast, the inner or umbral shadow is a region where the Earth blocks all direct sunlight from reaching the Moon.
Astronomers recognize three basic types of lunar eclipses:
- The Moon passes through Earth's penumbral shadow.
- These events are of only academic interest because they are subtle and hard to observe.
- A portion of the Moon passes through Earth's umbral shadow.
- These events are easy to see, even with the unaided eye.
- The entire Moon passes through Earth's umbral shadow.
- These events are quite striking due to the Moon's vibrant red color during the total phase (totality).
1. Penumbral Lunar Eclipse
2. Partial Lunar Eclipse
3. Total Lunar Eclipse
Now you might be wondering "If the Moon orbits Earth every 29.5 days and lunar eclipses only occur at Full Moon, then why don't we have an eclipse once a month during Full Moon?". I'm glad you asked!You see, the Moon's orbit around Earth is actually tipped about 5 degrees to Earth's orbit around the Sun.This means that the Moon spends most of the time either above or below the plane of Earth's orbit.And the plane of Earth's orbit around the Sun is important because Earth's shadows lie exactly in the same plane.During Full Moon, our natural satellite usually passes above or below Earth's shadows and misses them entirely.No eclipse takes place. But two to four times each year, the Moon passes through some portion of the Earth's penumbral or umbral shadows and one of the above three types of eclipses occurs.
When an eclipse of the Moon takes place, everyone on the night side of Earth can see it. About 35% of all eclipses are of the penumbral type which are very difficult to detect, even with a telescope.Another 30% are partial eclipses which are easy to see with the unaided eye.The final 35% or so are total eclipses, and these are quite extrordinary events to behold.
What is the difference between a lunar eclipse and a solar eclipse? A solar eclipse is an eclipse of the Sun. It happens when the Moon passes between the Earth and the Sun. This is only possible when the Moon is in the New Moon phase. For more information, see Solar Eclipses for Beginners.
Total Lunar Eclipse of 2004 Oct 27-28
Beginning (right), middle (center) and end (left) of totality
(click to see photo gallery)
Why is the Moon Red During a Total Lunar Eclipse?
During a total lunar eclipse, the Earth blocks the Sun's light from reaching the Moon. Astronauts on the Moon would then see the Earth completely eclipse the Sun. (They would see a bright red ring around the Earth as they watched all the sunrises and sunsets happening simultaneousely around the world!) While the Moon remains completely within Earth's umbral shadow, indirect sunlight still manages to reach and illuminate it.However, this sunlight must first pass deep through the Earth's atmosphere which filters out most of the blue colored light. The remaining light is a deep red or orange in color and is much dimmer than pure white sunlight. Earth's atmosphere also bends or refracts some of this light so that a small fraction of it can reach and illuminate the Moon.
The total phase of a lunar eclipse is so interesting and beautiful precisely because of the filtering and refracting effect of Earth's atmosphere.If the Earth had no atmosphere, then the Moon would be completely black during a total eclipse. Instead, the Moon can take on a range of colors from dark brown and red to bright orange and yellow. The exact appearance depends on how much dust and clouds are present in Earth's atmosphere.Total eclipses tend to be very dark after major volcanic eruptions since these events dump large amounts of volcanic ash into Earth's atmosphere. During the total lunar eclipse of December 1992, dust from Mount Pinatubo rendered the Moon nearly invisible.
All total eclipses start with a penumbral followed by a partial eclipse, and end with a partial followed by a penumbral eclipse (the total eclipse is sandwiched in the middle). The penumbral phases of the eclipse are quite difficult to see, even with a telescope. However, partial and total eclipses are easy to observe, even with the naked eye.
Total Lunar Eclipse of 2000 Jan 20-21
Beginning (right), middle (center) and end (left) of totality
(click to see more photos)
Observing Lunar Eclipses
Unlike solar eclipses, lunar eclipses are completely safe to watch. You don't need any kind of protective filters.It isn't even necessary to use a telescope. You can watch the lunar eclipse with nothing more than your own two eyes.If you have a pair of binoculars, they will help magnify the view and will make the red coloration brighter and easier to see.A standard pair of 7x35 or 7x50 binoculars work fine.Remember to dress warmly and enjoy the spectacle!
Amateur astronomers can actually make some useful observations during total eclipses.It's impossible to predict exactly how dark the Moon will appear during totality. The color can also vary from dark gray or brown, through a range of shades of red and bright orange. The color and brightness depend on the amount of dust in Earth's atmosphere during the eclipse. Using the Danjon Brightness Scale for lunar eclipses, amateurs can categorize the Moon's color and brightness during totality.
Another useful amateur activity requires a telescope.Using a standard list lunar craters, one can careful measure the exact time when each crater enters and leaves the umbral shadow. These crater timings can be used to estimate the enlargement of Earth's atmosphere due to airborne dust and volcanic ash.
Of course, an eclipse of the Moon also presents a tempting target to photograph. Fortunately, lunar eclipse photography is easy provided that you have the right equipment and use it correctly.See MrEclipse's Picks for camera, lens and tripod recommendations.For more photographs taken during previous lunar eclipses, be sure to visit Lunar Eclipse Photo Gallery.
Lunar Eclipse Frequency and Future Eclipses
Penumbral eclipses are of little interest because they are hard to see. If we consider only partial and total lunar eclipses, how often do they occur?
During the five thousand year period from 2000 BCE through 3000 CE, there are 7,718 eclipses of the Moon (partial and total). This averages out to about one and a half eclipses each year. Actually, the number of lunar eclipses in a single year can range from 0 to 3. The last time that 3 total lunar eclipses occurred in one calendar year was in 1982. Partial eclipses slightly outnumber total eclipses by 7 to 6.
The table below lists every lunar eclipse from 2009 through 2015. Click on the eclipse Date to see a diagram of the eclipse and a world map showing where it is visible from. Although penumbral lunar eclipses are included in this list, they are usually hard to see because they are faint.
The Umbral Magnitude is the fraction on the Moon's diameter immersed in the umbra at maximum eclipse. For values greater than 1.0, it is a total eclipse. For negative values, it is a penumbral eclipse. The Eclipse Durationcolumn lists the length of the partial eclipse in hours and minutes. If it is a total eclipse, two values are given. The first is the amount of time between the start and end of the partial phases while the second (in bold) is the length of the total eclipse.
| Eclipses of the Moon: 2009 - 2015 | ||||
| Date | Eclipse Type | Umbral Magnitude | Eclipse Duration | Geographic Region of Eclipse Visibility |
| 2009 Feb09 | Penumbral | -0.088 | - | e Europe, Asia, Aus., Pacific, w N.A. |
| 2009 Jul07 | Penumbral | -0.913 | - | Aus., Pacific, Americas |
| 2009 Aug06 | Penumbral | -0.666 | - | Americas, Europe, Africa, w Asia |
| 2009 Dec31 | Partial | 0.076 | 01h02m | Europe, Africa, Asia, Aus. |
| 2010 Jun26 | Partial | 0.537 | 02h43m | e Asia, Aus., Pacific, w Americas |
| 2010 Dec21 | Total | 1.256 | 03h29m 01h12m | e Asia, Aus., Pacific, Americas, Europe |
| 2011 Jun 15 | Total | 1.700 | 03h40m 01h40m | S.America, Europe, Africa, Asia, Aus. |
| 2011 Dec 10 | Total | 1.106 | 03h32m 00h51m | Europe, e Africa, Asia, Aus., Pacific, N.A. |
| 2012 Jun 04 | Partial | 0.370 | 02h07m | Asia, Aus., Pacific, Americas |
| 2012 Nov 28 | Penumbral | -0.187 | - | Europe, e Africa, Asia, Aus., Pacific, N.A. |
| 2013 Apr 25 | Partial | 0.015 | 00h27m | Europe, Africa, Asia, Aus. |
| 2013 May 25 | Penumbral | -0.934 | - | Americas, Africa |
| 2013 Oct 18 | Penumbral | -0.272 | - | Americas, Europe, Africa, Asia |
| 2014 Apr 15 | Total | 1.291 | 03h35m 01h18m | Aus., Pacific, Americas |
| 2014 Oct 08 | Total | 1.166 | 03h20m 00h59m | Asia, Aus., Pacific, Americas |
| 2015 Apr 04 | Total | 1.001 | 03h29m 00h05m | Asia, Aus., Pacific, Americas |
| 2015 Sep 28 | Total | 1.276 | 03h20m 01h12m | e Pacific, Americas, Europe, Africa, w Asia |
Geographic abreviations (used above): n =north, s = south, e = east, w = west, c = central
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