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Occult Thoughts

Week of September 7, 1998

The sky can be a crowded place. As the Moon makes its stately orbit around the Earth once a month, there are a lot of objects which it passes along the way. If you make the naive assumption that the stars are spread out all over the sky, the odds of the Moon passing directly in front of a naked eye star are pretty low. But the stars are not spread out evenly; they tend to be more dense in some spots in the sky over others. It so happens that a handful of bright stars lie near the path on the sky that the Moon takes, and so the Moon does actually pass close to or even in front of stars every now and then. When it passes close to a star we say there is a conjunction, and if it passes in front of a star we say there is an occultation. There is on every special kind of stellar occultation; when the Moon passes in front of the Sun! Remember, the Sun is a star. That kind of occultation is called an eclipse.

Planets are not spread out either. All the planets orbit the Sun more or less in one plane, and the Earth is no exception. Since we are also in that same plane, we see the planets moving across the sky in a line (called the ecliptic), and they never venture far from that line. Think of it this way: take a piece of paper and draw a bunch of concentric circles representing the planets' orbits. Now hold that paper edge on. All the circles will look like lines!

As it happens, the Moon orbits the Earth in very nearly the same plane as the Earth orbits the Sun. This means that the Moon frequently passes by planets in the sky. Just last night as I write this, I happened to see the Moon very close to Jupiter in the sky. Jupiter appears as a very bright star, and still looks bright even when near the full Moon, when lesser stars tend to fade away. This makes that kind of conjunction even more striking. People notice them more, and planetaria and newspapers get more phone calls when the Moon is near Jupiter; people wonder what it is they saw near the Moon! They aren't used to seeing bright objects so close to the Moon.

[Since the planets all orbit in about the same plane, they appear to get close to each a lot as well. Some people worry that when they do, the combined gravity will affect the Earth. Actually, the combined force from the planets is extremely weak, and can have no real effect here on Earth.]

Conjunctions always amaze me. The Moon is small, barely 3000 kilometers across. Jupiter is enormous: over 40 times the Moon's diameter! yet Jupiter is so far away that something as small as the Moon can blot it out. But then, I can hold up my thumb at arm's length and occult them both. It just goes to show you: something big can be totally overwhelmed by something small, depending on your distance from it. That holds true for a lot of situations... not just astronomical ones!

©2008 Phil Plait. All Rights Reserved.

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