How does the direction of the moonrise change?

Moonrise over Mather Point, Arizona. Credit: James Marvin Phelps (Flickr source, license: CC-BY NC)

I see the Sunrise most mornings, and I notice and understand how over the course of a year it rises in a different area on the horizon. These position changes having to do with the Earth’s tilt and rotation around the Sun.

I also get to see the moonrise and its position change on the horizon, much like the Sun. What I have trouble grasping is how this takes place in only a months time? Does the Earth have a month-long wobble, or what?

Thanks for the great question, and it’s excellent that you were observant enough to notice that change in the rising and setting position of the Sun and Moon over the course of the year. While in the question you mention that you understand why the Sunrise and Sunset positions change, let me start there and then move onto the Moon, because the explanation is somewhat connected, and is related to the geometry of the orbits and rotations in the Earth-Moon-Sun system.

The Earth rotates counter-clockwise on its axis (picture a spinning top). Because of this motion, celestial bodies such as the Sun, Moon and stars appear to rise in the eastern sky and set in the western sky. In addition, the Earth is tilted on its axis by 23.5 degrees, relative to the plane of the orbit of the Earth around the Sun. Because of this, the Sun’s rise and set positions vary by up to 23.5 degrees north or south of due east or west throughout the course of a year (the orbital period of the Earth around the Sun).

The Moon’s pattern is close to that of the Sun’s. It orbits the Earth on a plane that is approximately 5.1 degrees offset from the orbital plane of the Earth around the Sun. This causes the position of the moonrise and Moonset to vary up to 28.6 degrees north or south (that’s 23.5+5.1 degrees).

The Moon orbits completely around the Earth in 28.5 days, about once a month. This causes the Moon to move through its 28.6 degree range of variation much quicker than the Sun appears to, creating a noticeable position change against the horizon each night. The Moon also doesn’t rise at the same time each night. Due to the speed of Earth’s rotation and the Moon’s orbit, the Moon rises about 50 minutes later each day. Interestingly, all these changes in relative position to the Sun make the Moon appear to go through its waxing and waning phases.

There is a nice animation here that shows the rotation of the Earth, and the relative timescales of the Earth and Moon orbits.

I hope this answers your question, and clarifies why the Moon rises and sets in different positions over the course of a month. Please do not hesitate to contact me directly or the Ask an Astronomer page with follow-up question!

Dr. Aaron Rizzuto
UT Austin