Solar eclipses have been recorded since ancient times, often misunderstood by early observers as a dragon eating the Sun or some omen of things to come. Although we have learned the true nature of eclipses in modern times, they never cease to amaze astronomers and the public alike. You can visualize solar and lunar eclipses using a Wolfram Demonstration.
Solar eclipses happen when the Moon moves between the Earth and the Sun. The Moon blocks the Sun’s light, which puts a kink into our daily expectations, making it get dark during the day. A fuzzy estimate puts the frequency of total solar eclipses at about one or two per year. This number can vary quite a bit. Often, these eclipses are only visible along narrow paths that are in out-of-the-way places that make it difficult for them to be observed (e.g. over the open ocean). Cruise ships are often booked for the sole purpose of chasing these solar eclipses for those people willing to set sail and pay the money to do so.
On May 20 of this year, people in the western United States and Pacific Ocean islands will be in a position to observe one of these solar eclipses without having to travel on the high seas.
Unfortunately for those of us in the Wolfram|Alpha headquarters in Champaign, Illinois, we will only be able to see the beginning stages of the eclipse, since the Sun sets before the show gets off to a great start. The last time the central US was graced with an eclipse of this type was in 1994. I still have fond memories of that eclipse, and even managed to get a few pictures. All of the tiny holes in the leaves acted like small pinhole cameras and projected hundreds of tiny crescents on the ground as mid-eclipse approached.
If you live in Texas, along a line to northern California, you will have a better show this time around.
The traditional view of a solar eclipse is that the Moon blocks the Sun entirely, allowing you to look at the Sun directly and see the faint glow of the Sun’s outer atmosphere, a rather beautiful sight. As usual, reality is more complicated. The Moon is not always at the same distance from Earth. On May 20 of this year, the Moon is near its farthest distance from the Earth, which means that the apparent size of the Moon is smaller than average, and so in fact will appear smaller than the Sun. So, when the Moon moves directly in front of the Sun, it won’t block all of it. A ring of the Sun’s disk will still be visible around the Moon, which is why this is called an annular eclipse.
This is enough sunlight to still make it dangerous to look directly at the Sun, so to observe this eclipse, you will need special glasses or observation methods. Check with your local astronomy clubs or observatories to see if they are hosting any events that might make use of these special tools if you don’t have any of your own.
This event is a nice warm-up to another eclipse, of sorts, in June. During that event, it’s not the Moon that will move in front of the Sun, but the planet Venus. Expect more information on this popular event soon.