The Lunar Eclipse of January 20, 2000

Week of January 17, 2000

This first Snack of the year 2000 (yes, I am one of those who says the Millennium begins in 2001) will talk a little about a wonderful event: a total lunar eclipse.

On the evening of Thursday, January 20 (or the early morning Friday, January 21 if you lives east of England) the Moon will go undergo one of the best eclipses it can. Starting at (9:03 p.m. Eastern U.S. time (2:00 a.m. Universal time on Friday morning) the full Moon enters the Earth's shadow. You probably won't see much until about an hour later, when the Moon enters the darkest part of the shadow. The reason this is such a great eclipse is that the Moon passes deeply into the shadow, so it will be eclipsed for a long time. It will be totally eclipsed for over an hour, and will be partially eclipsed for over six hours. It's quite an event!

Unlike solar eclipses, this lunar version is safe to look at and lasts much longer. It's safe because you're not looking at the Sun, which is bright enough to hurt your eye; you're looking at the Moon which is much dimmer. It lasts longer because the Earth is a lot bigger than the Moon. A solar eclipse is when the Moon passes between us and the Sun, putting us in the Moon's shadow. A lunar eclipse is when the Moon goes into the Earth's shadow. Since the Earth is much bigger than the Moon, it casts a wider shadow. The Moon stays in the Earth's shadow in a lunar eclipse longer the Earth stays in the Moon's shadow during a solar eclipse.

The two most common questions any astronomer gets about lunar eclipses (and ones I predict I will get when I'm interviewed) are: why does the Moon turn red during an eclipse, and why don't we get an eclipse every full moon? The answer to the first one is the same reason the sky is blue. Molecules and dust in the air deflect incoming sunlight, bouncing it around (the technical term is ``scattering''). This affects blue light more than red light, so we see scattered sunlight coming form all over the sky. That's why the sky is blue. When the sun sets, it passes through thicker and thicker air. By the time it's near the horizon, even the yellow and orange light can get scattered away, leaving only the red light to hit our eyes. That's why the Sun looks red near the horizon. Now imagine you are on the Moon during a lunar eclipse. To you, the Earth is blocking the Sun. You'll see all around the limb of the Earth. You'll be seeing the Sun through that thickest part of the Earth's air, making the sunlight look red. That's why the Moon looks red during a lunar eclipse. The Earth's air is the culprit!

Sometimes, after a volcanic eruption, there is extra dust in the air making the Moon look even redder. For the first time in many years there has been no major eruption (Pinatubo was the last one in 1991), so this may be one of the clearest eclipses we've had in decades.

The reason we don't get eclipses every month is because of the Moon's orbit. As the Earth orbits the Sun, it looks to us as if the Sun moves against the background stars, and makes a circle in the sky every year (note that this is different from the circle it makes every day; that's because the Earth is rotating on its own axis. If the Earth didn't rotate, we'd still see the Sun circling the sky once a year). We call that circle in the sky the ecliptic. The Moon, however, doesn't orbit the Earth in the same plane that the Earth orbits the Sun. If you draw the Moon's orbit on the sky, it's always tilted to the ecliptic by about 5 degrees. If the Moon orbited the Earth in the ecliptic, we'd get alternating lunar and solar eclipses every two weeks! But since they tilt, we only get a lunar eclipse when the full moon happens to be right where the two orbits appear to cross in the sky. And it has to be fairly exact; the Moon is only a half degree across. So if it misses by even a little we don't get an eclipse.

So, weather permitting, we'll get a fine show on Thursday night/Friday morning. I plan on watching at least some of it; the last good eclipse I saw was in 1982!

Want more info on eclipses? You're in luck! There are a lot of good websites out there.

• We can start with good old Nick Strobel's Astronomy Notes site. Click on ``Naked eye astronomy'' and from there you'll find the subheading of phases and eclipses. He has diagrams of the tilted orbits too, which should make my explanation a lot clearer.

• Fred Espenak is a leading expert (if not the expert) on eclipses. He will also be interviewed on Thursday morning with me (he's taking the early slot and I'm doing the later one). The link above will take you to his special lunar eclipse page.

• Sky and Telescope magazine has excellent eclipse pages.