Published: 31 October, 2002

Full moon shines longest in winter; shortest in summer

Why does the moon rise and set in a few hours some nights and on other nights, the moon will rise at early darkness and still be in the sky the next morning?

The moon orbits the earth once each month. The moon's orbit lies very close to the plane of the ecliptic, which is the plane in which the earth revolves about the sun, hence the apparent path of the sun through the sky over the course of the year. Therefore, the sun and the moon follow approximately the same path on the sky. The earth's rotation axis is tilted with respect to the plane of the ecliptic, resulting in the seasons, and the fact that the length of a day varies throughout the year. Days are longest in the summer, and shortest in the winter.

Since the moon appears to follow nearly the same path as the sun in the sky, one might expect that the amount of time the moon spends above the horizon to vary as the moon orbits the earth, and indeed it does. As an example, think about the full moon. The moon is full when it is opposite the sun on the sky, so a full moon rises roughly at sunset, and sets at sunrise. Therefore, we only see full moons at night. On the summer solstice, the longest day of the year, when the sun spends the most time above the horizon, the full moon, in the night-time sky spends the least amount of time above the horizon. On the other hand, on the winter solstice, the shortest day of the year, when the sun spends the least time above the horizon, the full moon, again in the nighttime sky spends the most amount of time above the horizon. The time spent above the horizon each night for the full moon varies throughout the year about as much as the length of the day. Near the winter solstice (December 21st), the full moon appears above the horizon for almost 15 hours, near the summer solstice (June 21st), the moon will only be above the horizon for about 8 hours at the latitude of Ithaca.