Why Does The Moon Rise Later Each Day?

Why does the Moon rise 30 to 70 mins later each day than the previous day?
The Galileo spacecraft took this 1992 shot showing the Moon in orbit about Earth. With only a third of the brightness of Earth, the Moon has been digitally enhanced to improve visibility. Image credit: NASA

The Galileo spacecraft took this 1992 shot showing the Moon in orbit about Earth. With only a third of the brightness of Earth, the Moon has been digitally enhanced to improve visibility. Image credit: NASA

Originally posted on Forbes!

The Moon does indeed rise on average 50 minutes later each day in our skies, which may come as a surprisingly large daily change, particularly if you’re used to the much more gradual changes of sunrise and sunset times. To explore why the Moon’s arrival in the sky changes so much, let’s look into the Moon’s orbit around our Earth.

The Moon orbits our home planet once every 28 days or so, and as it orbits our planet, the angle between the Sun, the Earth and the Moon changes. If we think about looking down on the solar system from above, the Earth spins like a top below us. On a slower cadence, the Moon drifts in a wide ellipse around the Earth. The Sun itself is a distant, stable source of light. When the Moon is to the left or the right of the Earth (if we place the Sun at a distant 12 o'clock), an observer on the portion of the Earth facing the Moon would see the sunward half of the Moon illuminated by daylight. The other half of the Moon, which falls in shadow, would appear dark. This gives us the half-moon phase.

This is an image of Earth and the moon, acquired on October 3, 2007, by the HiRISE camera on NASA's Mars Reconnaissance Orbiter. At the time the image was taken, Earth was 142 million kilometers (88 million miles) from Mars, giving the HiRISE image a scale of 142 kilometers (88 miles) per pixel. The moon image is brightened relative to Earth for this composite. Image credit: NASA/JPL-Caltech/University of Arizona

This is an image of Earth and the moon, acquired on October 3, 2007, by the HiRISE camera on NASA's Mars Reconnaissance Orbiter. At the time the image was taken, Earth was 142 million kilometers (88 million miles) from Mars, giving the HiRISE image a scale of 142 kilometers (88 miles) per pixel. The moon image is brightened relative to Earth for this composite. Image credit: NASA/JPL-Caltech/University of Arizona

If the Moon is between the Sun and the Earth, an earthbound observer is stuck looking at the shadowed half of the Moon - the daylit side is facing away from us. And when the Moon is on the other side of the Earth, so that the Earth is between the Sun and Moon, the half of the Moon which is illuminated is also the half of the Moon which faces the surface of the Earth, giving us a full moon. So it is this changing alignment between the Earth, where we are looking from, the Sun, which provides the light to the surface of the Moon, and the Moon’s position itself, which gives us these changing phases of the Moon. Along with the Moon’s orbit, these phases cycle every 28 days.

This physical motion of the Moon around around the Earth every 28 days is half the puzzle of the changing Moon rising times. The other half is that the Earth is spinning much faster than the Moon is orbiting. The Earth rotates once every 24 hours, so if you’re standing motionless on the surface of the Earth, looking up, any object in space which isn’t also orbiting our planet every 24 hours will appear to traverse the skies. This is true of the Sun, which crosses the skies every day (by definition), and is also true of the Moon.

Taken June 21, 2016 by Commander Jeff Williams of NASA during Expedition 48 on the International Space Station. Image credit: NASA

Taken June 21, 2016 by Commander Jeff Williams of NASA during Expedition 48 on the International Space Station. Image credit: NASA

Moonrise happens when the Earth has rotated enough on its own axis that the Moon has appeared in your personal sky - your horizon has caught up with the Moon. When exactly that happens is a combination of the Earth’s rapid rotation, and the Moon’s continual motion through the skies. The Moon has to make it through an entire loop of the Earth in about 28 days, which means it’s moving by about 13 degrees in the sky every day. The Earth, meanwhile, rotates through 15 degrees every hour, in order to rotate 360 degrees every 24 hours.

The Moon is continually moving on ahead in its orbit while the Earth rotates. So 24 hours later, the Earth has rotated back around to the same place it was the night before, but the Moon has gone on ahead. Think of it like the second hand on an analog watch; it’s going around the face of the clock much faster than the minute hand, but each time the second hand goes around, the minute hand has moved, and so it takes an extra second to line back up with the minute hand. Because the Moon has moved 13 degrees or so since its last moonrise, it’s going to take another hour or so for the Earth to catch back up to the Moon’s new location, delaying the Moon's rising above your horizons by ~50 minutes each day.

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