Lunar Eclipse Demystified

Tomorrow night the Moon will once again plunge into the shadow of the Earth, marking the third lunar eclipse in the past 12 months. Despite the recent frequency of such events, your next chance to see a total lunar eclipse won't come for nearly three years, in December 2010. While not as spectacular (or rare) as a solar eclipse, watching the Moon turn dark red (but not invisible) has its own rewards. And this time, it will be joined by Saturn and the bright star Regulus -- both within a few degrees on the sky.

People often wonder why lunar eclipses are so much more common than solar eclipses, and why the Moon doesn't pass through the shadow of the Earth every month. Since both events involve the alignment of the Sun, Earth, and Moon it seems like they would both occur with similar frequency. But the dark shadow of the Earth (the umbra) is roughly three times the diameter of the full Moon, so the alignment doesn't have to be perfect as with a solar eclipse -- where the Sun and Moon appear almost the same size in the sky. This also makes lunar eclipses last much longer (about an hour, compared to just a few minutes) and allows half of the planet to see them each time (since you just need to be on the night side of the planet when they happen, not a special location where the alignment is perfect). Despite these advantages, the Moon doesn't pass through the Earth's shadow every month because its orbit is actually inclined by 11 degrees relative to the plane of the Earth's orbit around the Sun. So we only see a lunar eclipse when the full Moon occurs near the intersection of these two orbital planes (the so-called "line of nodes").

One of the interesting features of a lunar eclipse is that the Moon doesn't go completely black. Instead, it usually turns some shade of red when it passes through the darkest part of the Earth's shadow. We know that the Moon normally just reflects the light of the Sun -- so where does this red light come from? During a lunar eclipse, the light that reaches the Moon is just sunlight that has been diverted towards the Moon through the Earth's atmosphere. Why is it red? The answer turns out to be the same as "why is the sky blue?". The molecules in the Earth's atmosphere are similar in size to the wavelength of blue light, so they interact and scatter the blue light (making the sky blue) while letting most of the red light through (making the Moon look red). This is also why sunsets often look red.

If you've never seen Saturn through a telescope, this event will offer a good chance, since the ringed planet will appear within a few degrees (5 or 6 Moon diameters) of the lunar eclipse. The bright blue star Regulus (in the constellation Leo) will also appear nearby -- but it should be easy to distinguish from yellowish Saturn. Viewed with high power binoculars or a small telescope, the rings of Saturn are fairly easy to see, even for a pea-sized image.