This page calculates the times of moonrise and moonset on any date, accurate to the minute within several centuries of the present. It correctly describes what happens in the Arctic and Antarctic regions, where the Moon may not rise or set on a given date. The program was adapted from a BASIC program in Sky & Telescope magazine, July, 1989, page 78.

Moonrise occurs when the rotation of the Earth causes the upper limb of the Moon to move above the visible horizon as seen by an observer on the Earth's surface. Moonset is similar, occurring when the upper limb of the Moon moves below the visible horizon. The mathematical model of the Moon's apparent motion in the sky gives the position of the center of the Moon as it would be seen from the center of the Earth. Because the Moon is relatively close to the Earth, it is necessary to correct for parallax, the angular offset due observation from the Earth's surface. Also, it is necessary to account for the visual radius of the Moon since the position of the upper limb determines moonset and moonrise. Additionally, the Earth's atmosphere refracts light noticeably for light source near the horizon. This calculator includes corrections that account for all of these under average conditions. Actual observations of moonset and moonrise may differ from the predicted times due to variations in atmospheric conditions and due to local topography.

The predicted times are given in local time, standard or daylight saving, obtained from your computer's operating system. The azimuth of moonrise and moonset is given, measured in degrees from true North. You must determine your correct latitude and longitude to obtain valid predictions for your location. A good on-line source for this information is Sky & Telescope

Copyright © 2004, Stephen R. Schmitt