Many people think of the planets as orbiting the Sun in a flat plane, like a CD. If that were true, then we'd see Mercury and Venus cross the face of the Sun -- what astronomers call a "transit" -- every few months! But the orbits of all the planets are tilted a bit. Not a whole lot, really, but enough to prevent the tiny eclipses from happening all the time.
So they're actually rather rare. Venus transits happen very rarely, and are spectacular! Mercury transits the Sun about 13 times per century... and the next time is Wednesday, November 8 (or the 9th depending on your time zone). Here in California we get to see the whole thing (it starts at 11:12 local time and ends a little under five hours later). I just hope it's clear!
Wanna watch? It's not too hard.
First, duh, don't look at the Sun! It's very bright. You may have noticed. Anyway, Mercury is too small to see with your unaided eye. You'll need binoculars at least... and if you look at the Sun through binoculars you'll cook your retinae for sure. I plan on mounting my binocs on a tripod and projecting the Sun's image on paper. That's a safe way, though you have to be careful not to cook your optics.
If it's cloudy where you are, or this happens at night where you are, or you're just lazy, about a million sites are going to webcast the hours-long event, and many more have general info. Here's a list:
- Sky and Telescope has lots of info. This is your first stop.
- NASA's eclipse page has tons more info, including maps and figures that can help you observe it.
- Another NASA page has a huge list of webcam sites covering the transit.
- GONG is an international solar observing cooperative, and they will have awesome images.
Have fun! This is not as spectacular an event as some, but it's rare, interesting, and pretty cool anyway.