June 2010: I feel the need to update this. Below I mention two lunar and two solar eclipses each year. Poking through some predictions of future eclipses I’ve found four lunars in 2009 and four solars in 2011. Other than that, there do seem to be two a year. Now I have to figure out exactly why, but my suspicion is that it has to do with perfecting the math, and that a lot of those “extra” eclipses are not total.
Here’s some news ahead of time for you. On March 3, 2007 there will be a total lunar eclipse. Unfortunately, it will be best viewed from the middle of Africa. Here in Seattle, we’ll see just the tail end of the eclipse which will appear as a dark shadow covering a small section near edge of the moon as it rises at 6:00 p.m.
Luckily for us, there are two lunar eclipses a year! On August 28, 2007 we’ll get to see the whole show – a total lunar eclipse best viewed from the West Coast of North America.
Before you read the next part, I have a caveat to mention: everything depends on where you’re standing. If you were standing above our solar system, you would see that the Earth goes around the Sun, the Moon pretty much goes around the Earth, and the Sun doesn’t go anywhere. If you were standing in the middle of the galaxy, you’d see all that, but the Sun would be traveling around the galaxy also. From standing on Earth, it looks like the Sun moves around us, even though it doesn’t. Since we’re talking about observing eclipses, what it looks like from Earth is more important than what it looks like from outer space.
Why are there Two Lunar Eclipses and Two Solar Eclipses Each Year?
Update, according to Mr. Eclipse (who I trust, since NASA does) there are between 2 and 5 solar eclipses each year. I ASSUME this means there are between 2 and 5 lunar eclipses as well.
Take any two rings of approximately the same size: two hula hoops, two key rings, the two rings from an embroidery hoop, or two bangle bracelets. I’m going to use hula hoops.
If you stand inside the hula hoop, you’re the Earth, and the hoop is the path the Sun follows as it seems to travel through your sky, the ecliptic. The Sun takes one year to trace this path. Now, stand inside the other hula hoop. This is the path the Moon travels through your sky. It’s very close to the ecliptic, but isn’t quite the same.
Time to use your dexterity! Using both hula hoops, hold them together, one on top of the other (Image 1). They should be touching all the way around. Now, tilt one a little. The one you’ve tilted should go down through the middle of the other (Image 2). These are the Sun’s path and the Moon’s path. How many places are they touching?
The Sun’s traveling along one hoop, crawling 1/12th of the way around the hoop every month. The Moon is zooming around the other hoop, covering the entire distance in one month. When the Sun finally arrives at one of the intersections (a node) it will still be there when the Moon gets to the same intersection, since it’s moving so slowly. This is a Solar Eclipse. Two weeks later, the Sun hasn’t moved much, and the Moon is at the opposite intersection (node) forming a Lunar Eclipse!
There’s a lot more to tell about eclipses, I’ve simplified a little, but that’s the basic reason we have two of each per year.
Where’d I Get My Info?
Eclipse Predictions by Fred Espenak, NASA/GSFC (Mr. Eclipse himself)