Predicting moonrise

Many people get confused when it comes to predicting when and where the Moon will appear in the sky. Unlike the Sun, the Moon doesn't rise and set daily, nor does it appear in the same part of the sky on a nightly basis. Moreover, sometimes it appears in the the evening, while at other times it's in the morning. The Moon just seems all over the sky. Once you learn a few lunar basics, though, it is not difficult to master the comings and goings of our nearest neighbor in space.

First, the Moon moves eastward in the sky, despite the fact that we see it carried along with the stars toward the west. The overt westward motion, like the rising and setting of the stars, is due to Earth's rotation. The Moon's more subtle eastward movement, however, is a result of its orbit around Earth. This eastward motion can be seen if you carefully note the Moon's position with respect to bright background stars over the course of a few nights. This task is easier when you observe the Moon between crescent and first-quarter phase. If you observe the Moon at the same time every night, you'll see that it has a daily motion of about 15┬░ eastward - that's about one and a half times the width of your fist held at arm's length.

Second, since Earth rotates toward the east and the Moon orbits Earth toward the east, it takes a bit longer each day for any location on Earth to rotate around so that the Moon has the same altitude above the horizon that it had the previous night. On average, this delay is about 50 minutes; thus the Moon rises later each day by roughly 50 minutes. Bear in mind, however, that this period is abbreviated in the autumn at high northerly latitudes. (See the section on the Harvest Moon for more on this exception.)

Finally, the Moon displays a phase because it shines by reflected sunlight. As the Moon orbits Earth, the lighting angle changes in predictable ways, moving from crescent to quarter to full and back again. If you know the lunar phase, you can predict with some accuracy when the Moon will rise and set.

For example, when the Moon is at quarter phase (or half lit), it must be 90┬░ from the Sun in our sky. In other words, the Moon is either on the meridian as the Sun sets, in which case the Moon sets about midnight, or is on the meridian as the Sun rises, in which case it sets around noon. You'll never see a half-lit Moon in the west just after sunset or setting in the west just before sunrise. It's geometrically impossible.

When the Moon is full, it must always appear 180┬░opposite the Sun in the sky. In other words, a full Moon rises at sunset, is on the meridian at midnight, and sets when the Sun rises in the morning. You'll never see a full Moon on the meridian at sunset. If, however, you see a half-lit Moon rising in the east at sunset, then you must assume that it is undergoing a lunar eclipse.

Knowing that the Moon rises about 50 minutes later each night allows you to make a rough prediction for moonrise simply by knowing which phase the Moon is in. You need only one additional fact: the period from new Moon to new Moon (called a synodic month) is 29.5 days, so the interval of days between the four main phases of the Moon is approximately 7 days. If the Moon is 3 days past full - when it rose at sunset - then it will rise about 150 minutes after sunset (3X50), or about two and a half hours later. That also means that the Moon will set about this much time after sunrise.

Once you can relate lunar phases to lunar position, the Moon's schedule becomes almost second nature, making moonrise a more predictable monthly event.

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