Dear Straight Dope: My friend recently visited Costa Rica. While she was there she noticed that the shadow of the earth on the moon was in a different spot than it is in North America. She said it was like a dish, the shadow being on the bottom. What causes this? If I went to the north or south pole would the shadow be on top or perfectly down the middle or what? Also, which way does the shadow move across at the equator, up or down? C. Dufek, Vallejo, CA
There’s a common misconception that the phases of the moon are caused by the earth’s shadow falling on the moon. This does happen occasionally, but the result is a lunar eclipse, not phases, and those are rare enough that that’s probably not what your friend was talking about. The lunar phases are actually caused by the shadow of the moon itself. At any given time, half of the moon is facing the sun, and looks bright, and the other half is facing away, and looks dark. To see how this causes the phases, you’ll need a baseball and a bright light bulb. If you hold the ball out at arm’s length almost directly between your head and the bulb, the part of the ball you’re seeing will be almost entirely the side facing away from the bulb, so the whole thing will look dark: This is what’s happening at new moon. Now, turn 90 degrees to your left. The bulb is to your right now, and the right face of the ball is lit, while the left is dark, just as with the first quarter phase. If you turn another 90 degrees, then you’ll be at full moon, and the entire portion you see will be lit, so long as it’s not in your shadow. Since the plane of the moon’s orbit doesn’t quite line up with the earth’s orbit around the sun, we usually don’t have to worry about the moon falling exactly into the earth’s shadow, but when it does (that is, when an eclipse occurs), it’s always at full moon. Similarly, a solar eclipse–that is, when the moon blocks the sun and casts a shadow on the earth–always occurs during a new moon.
Now it’s easy to understand why the moon looks the way it does in the sky. The moon appears to be a crescent just before and after the new moon. During the waxing phase, after the new moon, it’s visible immediately after sunset above the western horizon. (It may be visible before sunset too, but it’s harder to spot because sunlight washes it out.) The moon is between us and the sun at this point, so the side we see is mostly dark except for a lit crescent. The crescent is part of the side facing the sun, so it points to the sun in the sky, i.e., down, since the sun has just set. If you’re near the equator, the sun sets more or less straight down, so the crescent does the same–that is, it looks like a dish. The angle varies quite a bit over the course of the year due to the earth’s axial tilt and so on, but on average the crescent is centered at the 6 o’clock position. In more temperate latitudes, the sun sinks at an angle, so the crescent points in that direction. At 30 degrees north latitude (about New Orleans), the crescent on average is at the 5 o’clock position on the lunar face; at 45 degrees north (Minneapolis), it’s at 4:30. At the north pole, where in the summer the sun doesn’t set but instead makes a circuit of the horizon, the moon’s crescent on average is at the 3 o’clock position in the waxing phase. All of this is reversed when the moon is waning. In the final quarter, the crescent moon can be seen above the eastern horizon at sunrise–on average, in the 6 o’clock position at the equator, 7 o’clock in New Orleans, 7:30 in Minneapolis, and so on. You can figure out for yourself where the crescent is at different times in the southern hemisphere.
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