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Total Solar Eclipse in July

Posted on 06/07/2009

Most people have never in their lives seen a total solar eclipse. While solar eclipses are not exceedingly rare-there is one roughly every year or so-the phenomenon of totality, with the bright surface of the sun completely occluded by the moon, is one that is limited to only a small part of the Earth's surface. You really do have to be in the right place at the right time.

Why is this? Total lunar eclipses are generally visible over entire hemispheres. The last total lunar eclipse in February 2008 was visible from anywhere in South America, anywhere in North America except the west coast, from western Europe and Africa, and even from Greenland! And lunar eclipses are relatively lengthy as well, the February eclipse having a 50-minute period of totality. A total solar eclipse that lasts as long as 5 minutes, on the other hand, is considered quite a long one.

First of all, you'll have to picture the relative positions of the sun, the Earth, and the moon during both lunar and solar eclipses. I'm going to try to help out this process with pictures, because I know if this is new to you, it's hard to get those pictures into your head without some assistance. And I'm going to move from the simplest basics of eclipses to more complex features, so jump in wherever you feel comfortable starting.

Sun, Earth, and moon: Eclipses take place when sunlight is prevented from falling on either the Earth or the moon by the other body. Total lunar eclipses take place when the Earth blocks sunlight that would otherwise fall on the moon; total solar eclipses take place when the moon blocks sunlight that would otherwise fall on the Earth.

(Click for the full-sized image)

Lunar versus solar: The earth is a lot bigger than the moon, with an 8000-mile diameter compared to the moon's 2000 miles. That makes the Earth's shadow much bigger as well. At the Earth-moon distance, the Earth's shadow is more than big enough to completely cover the moon. This makes lunar eclipses longer since it takes the moon some time to traverse the Earth's shadow, and it means that if you are anywhere on the side of the Earth facing the moon while it does so, you will see the eclipse. The moon's shadow, however, is just barely long enough to reach the Earth's surface. At any given instant, only a small spot on the Earth is completely blocked from sunlight. Viewed another way, the moon's apparent size is essentially the same as that of the sun. (The moon is both 400 times smaller and 400 times closer than the sun; an interesting coincidence.) As the moon travels across the face of the sun, it only blocks it for a few minutes at most.

Partial versus total: The sun is an extended object as opposed to a point source like a star. Stars are so far away that even though they are large objects, they only appear as points of light to us. When the moon moves in front of a star, there is no such thing as a "partial" occultation: one instant you see the star, and the next instant you don't. (And yes, you purists, I know there are complications like diffraction and the not-completely-smooth edge of the moon, but hush up for now.) But the sun can be partially eclipsed, with only part of its bright surface being obscured by the moon. Many of you probably have seen a partial solar eclipse at some point.

Why not every month? When the moon is new, the side facing the Earth is in darkness; it is the side facing away from us that is being illuminated by the sun. So why isn't there a total solar eclipse every time there is a new moon? It is because the moon's orbit around the Earth is not in the same plane as the Earth's orbit around the sun. If that failed to register with you, take a look at the diagram below (click for the full-sized version):

The blue plane is created by the path of the Earth around the sun: the Earth and the sun are always "lined up" with each other in this plane. The brown plane is created by the path of the moon around the Earth: the two of them are always "lined up" in this plane. Here's the key point: all three of these, the sun, the Earth and the moon are "lined up" only where these two planes intersect each other. This is along the "line of nodes" indicated in the diagram. The diagram below now should make more sense to you:

Why not always the same length? The last total solar eclipse was in August of last year, and at its point of maximum eclipse duration, totality was 2 minutes and 27 seconds long. This July's eclipse will have a maximum duration of 6 minutes and 39 seconds. Why the difference? It is because the moon's path around the Earth is not exactly circular, but is one that brings it sometimes closer to and sometimes farther away from the Earth. When an eclipse takes place with the moon at its closest point to the Earth, the moon's apparent size is corresponding larger, and the moon covers the sun completely for a longer period. When the moon is farther away, its apparent size drops along with the duration of totality. When the moon is at its most distant, it is too small to completely cover the sun's disk. The moon will be centered on the sun, but there will be a "ring of fire" around the edge of the dark moon. This annular eclipse (the Latin word for ring is annulus) is illustrated below:

What's so special about the July 2009 eclipse? Its duration for one thing. At 6 minutes and 39 seconds, it is close to the maximum duration possible (7 minutes and 31 seconds). Its location for another. Beginning in India, its path will carry it across central China and smack dab over Shanghai, making this possibly the most widely viewed total solar eclipse in human history. The path continues out into the Pacific south of Japan and ends just north of the Cook Islands. For eclipse chasers, totality at sea is a bonus. A ship offers easy mobility to avoid clouds and the sea gives a perfectly flat horizon to see as much of the sky as possible. And no, you won't be able to see it from Lynchburg; it will be taking place in the middle of the night here. Wrong side of the Earth!

Where is a good source of information about past and future eclipses? The NASA eclipse page is unsurpassed, I think. Here is the general page from which you can go to learn more about specific eclipses: http://eclipse.gsfc.nasa.gov/eclipse.html

What's this Saros 136 thing? Wasn't that the name of Spock's father? Actually, that was Sarek, Vulcan ambassador to the Federation. (Sorry-had to get my geek on.) A saros is an eclipse cycle of 18 years, 11 days and 8 hours, during which period of time the sun, Earth and moon return to roughly the same relative geometry and a nearly identical eclipse occurs. Because of that 8 hours, the second eclipse will occur 120° west of the first, since the Earth has rotated an extra ⅓ of a day past its original position. Interestingly, the ancient Babylonians knew of this cycle 3000 years ago, and could use it to predict future eclipses.

I caught that fudging! "Roughly" and "nearly"-what's up with that? Well, there is one more geometric complication to throw at you. You know that line of nodes we talked about earlier? It doesn't stay in the same place relative to the Earth and the sun; rather, it precesses, or wobbles like a top as it slows down. So when the saros cycle reaches its culmination, the sun, Earth and moon are almost (but not exactly) in the same relative positions. If the eclipses are taking place near a descending node (as are the eclipses of saros 136 and those of all even-numbered saros cycles), then they will begin as partial eclipses near the Earth's south pole, and gradually shift northward. When they are near the equator (as is the July 2009 eclipse), they are total eclipses and totality lasts longer. Eventually they return to partiality near the north pole and the cycle ends when the moon's shadow no longer reaches the Earth. Here is a page with information about the eclipses of saros cycle 134, which began in 1360 with a partial eclipse in the southern hemisphere, and will end 1262 years later in 2622 with a partial eclipse in the far north. The July eclipse is one of 71 total for this cycle. The animation link on this page is especially cool. http://eclipse.gsfc.nasa.gov/SEsaros/SEsaros136.html

What sorts of unique phenomena take place during a total solar eclipse? To mention a few: as the moment of totality approaches, so does the moon's shadow, rushing toward you from the horizon at 3000 km/hr (1800 mph). This is most easily seen at sea. Just before or just after totality, you may see the last bit of the sun's disc shining through a lunar valley, creating what is sometimes called a "diamond ring" effect. And during totality itself, the outer atmosphere of the sun called the corona, too dim to be seen otherwise, glows a ghostly white. The temperature will drop noticeably, and some of the brighter stars will be visible.

The closest I have ever come to seeing a total solar eclipse was in May of 1984, when an annular eclipse occurred very near Lynchburg. Some of you may remember that as well. It has been a long-time desire of mine to witness a total solar eclipse, and to do so at sea. Several years ago, I began looking at future eclipses for a likely candidate. There are tour companies devoted to taking eclipse chasers all over the world in pursuit of totality, and I contacted the best known of these in 2002. "What are your plans for the July 2009 eclipse?" I asked. I'm sure they had fun passing that e-mail around, but they very politely answered that they generally announced their plans two years in advance, and that I should check back in oh, say-five years? In May 2007, they began taking reservations, and their 320 spaces aboard the ship sold out in two weeks.

Yes, my wife and I are going. It's just a tough break that we have to go to Tahiti and the Cook Islands to do so, and that we will celebrate our 35th anniversary while we are there. But it's a sacrifice I'm willing to make for sake of science. Yes, we will take lots of pictures, and I will share as much of that experience as I can with the readers of these newsletters when we get back.

Blog and journal content is produced by an individual. All opinions are those of the individual writer and may not reflect those of Lynchburg College.