January 2000

Lunar Eclipse!

I love the Moon. It's one of my favorite objects to observe, for a number of reasons. First, it is so dynamic; from night to night the shadows move, and that ridge or crater you thought you knew now looks completely different (pictured is the crater Copernicus, two images a few days apart).
Distant galaxies will look the same to your grandchildren as they do to you, unless you see them in a bigger telescope. The moon, on the other hand, is spectacular in even a small scope: with no expensive equipment needed, it is something everyone can enjoy, even from the big city (rather than be affected by light pollution, the Moon is a source of light pollution! Now that's clout).

It seems, at first glance, to be a simple object. Yet the Moon, for all its simplicity, requires real thought to fully understand. Let's set the geology aside for now, and consider the Moon's phases. We'll get to eclipses later.

In a way, the Moon's phases are simple: the Moon is a ball that is lit up on one side. That's it. The shadow of the Earth is not involved. Light from the Sun illuminates half of the Moon, and the other half is dark. As the Moon orbits around the Earth we see these light and dark halves at different angles.

Naturally, the illuminated (or daylit) side of the Moon always faces towards the Sun (this is true for every planet, moon, and asteroid in the solar system). If the Moon is in the half of its orbit where it is closer to the Sun than Earth, we'll mostly see its night side, and the Moon will appear as a crescent.

In the other half of its orbit, where the Moon is further from the Sun than Earth, we can see more of its daylit side, and it appears more than half full (the word we use is "gibbous").

Each orbit (each month), the Moon passes between the Earth and Sun, more or less (Sun, Moon, Earth). Things don't line up perfectly, and from our perspective, the Moon would appear to pass by above or below the Sun. Its day side is away from us at this time, of course, so the Moon is downright invisible. The Sun's glare doesn't help, either.

(Occasionally, things will line up just right and the Moon will pass directly in front of the Sun, as seen from a particular location on Earth. The Moon blocks the Sun's light, and we call this rare event a solar eclipse.)

Two weeks later, the Moon is on the other side of its orbit, opposite the Sun (Sun, Earth, Moon). At this time, the day side of the Moon faces toward us, and the Moon looks full. In other words, light from the Sun streams past the Earth, and strikes the Moon's near side. Full Moon!

Now, the Earth has a shadow (so does everything else in the solar system, and, of course, all these shadows point away from the Sun). Sometimes, when things are lined up just right (Sun, Earth, Moon... in a perfect line), the Moon can pass through the Earth's shadow.
The Moon is always full at this stage in its orbit, but as the Earth blocks sunlight from reaching the Moon, the Moon goes mysteriously dark! It doesn't happen every month because, like solar eclipses, the alignment must be perfect.

So if you're not lucky enough to be at Kitt Peak on January 20th, step outside around sunset, face east (away from the sunset), and enjoy the Moonrise. Then, keep an eye on the Moon for the next few hours as it eases its way through the shadow of your beloved homeworld!

Steve White
Nightly Observing Program
Kitt Peak Visitor Center

PS: Click here to see why the Moon looks so big when it rises, and here for detailed information about all aspects of our natural satellite!

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Telescope Operators/Guides for Nightly Program:
Adam Block (ablock@noao.edu) Lead Observer
Steve White (swhite@noao.edu)
Flynn Haase (fhaase@noao.edu)
Richard Barchfield (richardb@noao.edu)

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Updated: 1/28/2000