Of Crescents and Moonsets

Callihoo Publishing author Brook West gives us a fascinating and informative article about phases of the Earth's Moon--or any moon throughout space.

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"As the thin, pre-dawn crescent moon sets, the old man returns to the lighthouse."

I recently ran across this sentence opening a story. Seems nicely evocative and all, but there's something wrong with it. Crescent moons do not set at dawn, they rise.

Why do moons show crescents? It's because we are looking mostly at the dark side of the moon. The bright side of the Moon is bright because, duh, it is facing the Sun. The dark side is in shadow because it is facing away from the sun.

Note that there is a difference between "dark side" and "far side." Because our Moon is tide-locked to the Earth, we always see one side of it, the near side, and never see the far side. The light and dark sides, on the other hand, progress around the Moon. When we see the dark side, we are looking at lunar night on the near side of the Moon.  Each day our Moon rises about an hour later than the day before (actually about 50 minutes) so the phase of the moon changes a little with each day.

So during a full moon, and on either side of full, when the Moon is gibbous (i.e. mostly full, with a thin dark crescent along one edge) the Moon will be more or less opposite the Sun in the sky--sunrise and moonset happen more or less together, as do moonrise and sunset.

On the other hand, when the Moon is in the same part of the sky as the Sun, we will see mostly the dark side of the Moon.

When the Moon is nearest to the Sun in our sky, we are seeing just the dark side--that is a new moon, and it rises and sets with the Sun. As the Moon approaches the Sun we see less and less of the light side, so the crescent we see wanes--that is, it gets thinner and thinner until new moon. After new moon, the thin crescent again appears and slowly waxes or thickens. Before new moon, the Moon rises before the Sun does; after new moon, the Moon follows the Sun, rising after sunrise. See figure 1.

Figure 1

The crescent of light will always face the Sun because that's what is illuminating it. The horns of a crescent will always point away from the Sun.

Twice in each lunar month, midway between new and full moons,  we will see a half moon rising and setting about half a day before or after the sun. Again, the light side will always face the sun.

All this is illustrated in figure 2, which shows the various phases of the Moon, both in the Moon's relation to the Sun and the Earth, and as we see the Moon in each phase. Note that what is changing is our point of view.

Figure 2

Even when we can't see the Sun, at night, the Moon is not in the Earth's shadow, so the bright side is illuminated by the Sun. The only exception to this is during a lunar eclipse, which can only occur at a full moon, on those rare occasions when the Moon actually passes through the Earth's shadow. Solar eclipses happen at new moon, when the Moon passes between the Earth and the Sun.

Since the Moon's orbit is tilted in relation to the plane of the earth's orbit, we only get an eclipse when the Moon's orbit crosses that plane during a new or full moon. If the Moon crosses just a bit to one side or the other, we get a partial eclipse of the Moon or the Sun. Otherwise the Moon passes completely to one side or the other of the Sun or of the Earth's shadow and there is no eclipse.

The same principles will hold true for any star/planet/moons situation in the universe. The bright side of a moon always faces the star and the phase of each moon as seen from the planet will depend on where that moon is in its orbit. See figure 3.

Figure 3

In a stellar system consisting of binary stars, each moon will be illuminated from two directions. In that case, you will have shadow where neither star is illuminating the moon, bright areas where one or the other of the stars is illuminating the moon, and an even brighter area where both stars are illuminating the moon. See figure 4.

Figure 4

So, circling back to the beginning, to be correct the above sentence should read: "As the thin, pre-dawn crescent moon rises, the old man returns to the lighthouse."

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((Paragraph 9 and Figure 4 corrected))

All illustrations created by, and copyright by, Brook West, 2014.

Brook's story "A Portion for Foxes," and his collaborations with Julia West, "The Peachwood Flute" and "Weeds," are available from Callihoo Publishing.