Solar eclipses are caused when the moon comes between the Sun and the Earth; basically we see the moon's shadow. The moon's orbit is tilted with respect to the Earth, so most months the moon passes well north or south of the sun in the sky, but every 6 months it has a chance of passing over part or all of the sun.
The moon and the sun are almost exactly the same size, as seen from the Earth. But the moon's orbit is not a circle, it is elliptical (oval), so sometimes it is a little closer to the Earth and sometimes a little further away. Remember a few weeks ago when the "Supermoon" was big news? I was in an ice cream shop when one of the other patrons saw the full moon rising and shouted "It's the supermoon! Look how big it is! Let's all go look! Supermoon!". You can see some pictures of the Supermoon here. The reason for the "supermoon" was that the full moon was almost exactly coincident with the moon's closest approach to Earth, so it was fully lit at the same time it appeared largest in the sky.
We are now 2 weeks after "Supermoon". The moon takes about 4 weeks do a complete orbit around the Earth, and if it was at its closest two weeks ago, it is at its furthest point from the sun this Sunday. This means the moon will appear the smallest it can in the sky, and, in fact, it will appear slightly smaller than the sun.
So, during this weekend's solar eclipse, the moon will try to fully cover the sun, but it will be too small to do so, and we Earthlings can see what is called an annular eclipse, where the sun will be visible as a ring around the dark shadow of the moon. Because the sun is not fully blocked, viewers must take care to protect their eyes. It may not hurt to look at the eclipsed sun, but harmful radiation from the sun will be very quick to permanently damage your eyes.
Solar eclipse aficionados are aware of something called the Saros cycle. Every 18 years and 10 or 11 days the Sun, moon, and Earth are in almost exactly the same geometry, so a very similar eclipse can be seen. 18 years and 10 days ago was May 10, 1994. That day there was another annular eclipse of the sun, this one visible from much of the United States as well (a bit of a coincidence, since the eclipse path moves 120 degrees west each 18 years). And on that day, the moon's shadow passed straight over my grandmother's house in northern Indiana. Since I had a good excuse to visit my grandmother, I did so. (I had just finished my sophomore year of college a few days previously).
We had some fantastic weather. I owned a small telescope but only a point and click camera, so my pictures aren't professional quality. But we still had a great view (click on the image for a larger view):
|This view is of the sun during the annular portion of the eclipse. You can see the sun fully surrounds the moon.|
If you live in the western U.S. and have clear skies on Sunday, go take a look! Just be safe - never look at the sun without protected eyes. Sunglasses aren't good enough! If you don't have mylar eclipse glasses, welder's glasses #13 or #14 will work, otherwise project the sun's image by putting a pinhole in a piece of paper and holding it above another surface (or just use your fingers). Again, don't look at the sun through the pinhole! Details on where to see the eclipse are in the Sky and Telescope article. The eclipse path goes through several national parks, and many of them will have special viewing events. If it is cloudy or you live too far east, you can also watch the eclipse online.
If you miss this eclipse, there will be more. Looming on the horizon is a total solar eclipse in August 2017, which crosses the entire continental U.S. I also saw portions of the last total eclipse of that Saros cycle from a hill on the French-German border in August 1999. Hopefully in 2017 I won't end up quite as wet as in 1999.