On May 20, 2012, a rare and spectacular annular solar eclipse will take place for the Western half of the United States. Sadly, this Astropotamus is in the Eastern half of the United States and won’t see it. If you are interested in this annular eclipse, lots of information is available on the Internet. If you want to know more about what makes an annular eclipse special (and not an annual eclipse), then read on.
Most people know that an eclipse is when one object passes in front of another object, thus “eclipsing” it and blocking it from a third object. When the eclipsing object is the Moon and the eclipsed object is the Sun, we see the Sun disappear and be replaced with a black circle. That’s because our Moon is almost the same angular size as our Sun, which means that they are about the same width as seen from the Earth. Even though the Sun is thousands of times bigger, its huge distance makes it look much smaller – about the same size as the moon.
Our Moon does not have a circular orbit, though. It’s more of an oval. And sometimes, when the eclipse occurs, the moon is closer to Earth and sometimes it is farther away. When it is farther away, it appears smaller. Even though the Moon appears smaller, the Sun does not, so the Moon covers less of the apparent surface of the Sun. This is called an annular eclipse, not to be confused with the word annual, and not to be confused with a partial eclipse.
In a partial eclipse, the Moon only partially covers the sun – the centers of their two discs never quite line up as seen from Earth. An annular eclipse is a true full solar eclipse where the centers of the Moon and Sun do line up, but the smaller apparent size of the Moon means the Sun will be seen as an annulus, or “ring of fire” around a dark circle in the middle.
If you live in the path of the event, you are in for a treat. Take proper precautions and be sure to look at the sun. If you do not live in the path of the event, try to find an online astronomy web site or group that will be broadcasting a live feed of the event. You will not want to miss it!