The most relevant factor in determining the times of moonrise and moonset is the phase of the moon.  Here is a first approximation based on moon phase (mean moon phase, corresponding to mean elongation of the moon, also the moonstick definition of moon phase).

At this point you should be aware that the time you keep on your watch may not be the actual time where you are.  This can be for several reasons.  The biggest reason is probably "Daylight Saving Time".  In the summer, many countries set their clocks one hour ahead of what they would otherwise be set at.  Weather or not your country does this is up to you to find out.  Another reason your watch may differ from actual time is if you set it to a particular time zone.  Take me for example; I live at 85°W longitude, but I set my watch to United States Eastern Standard Time, which is reckoned from 75°W longitude.  This 10° difference means that I am actually 40 minutes behind the longitude from which my watch is set.  So when my watch says it is 12:40PM, it is 12:00PM locally.  (A 15° change in longitude changes local time by one hour.)

The times in the above table indicate actual time, so you will need to determine the difference between your watch time and the actual time.  Let me give you an example.  There is a person that lives near Chicago, Illinois at 88°W longitude.  He (like most other people in that area) sets his watch to United States Central Time, which is reckoned from 90°W longitude (in the winter anyway).  So he sets his watch 2° behind his local time.  (Behind means to the west, of course, because the Earth turns to the east.)  As stated earlier, a 2° difference in longitude corresponds to an 8 minute difference in local time.  So if moonrise is determined to be at 6:29PM local time; his watch will read 6:21PM when the moon rises.  (As we said earlier, he sets his watch 8 minutes behind local time.)  If it were summertime, he (like most of the other people in that area) would have his watch set an additional one hour ahead, thus his watch would read 7:21PM when the moon rises. 

Now that that is out of the way, we can get back to the actual predicting.  Step one, determine the current moon phase precisely.  Step two, make a first approximation of the times of moonrise and moonset from the above table.  (You must, of course, interpolate where necessary.)  Step three, adjust these times for the particular way that you set your watch.  You now have the times of moonrise and moonset within about an hour.  (If you are near the equator, you probably have it better than an hour.  If you are near the poles, you probably have it a lot worse than one hour.)

To get better, we must incorporate the season and latitude of observation into our prediction.  To do this, we will first use the season (mean season, corresponding to the mean ecliptic longitude of the sun) together with the moon phase to determine the mean location angle of the moon (corresponds to the mean zodiacal sign of the moon).

Now use the mean location angle of the moon together with the latitude of observation (your latitude) to determine the amount (h:mm) by which moonrise should be corrected.  (The word "up/down" indicates that the moon does not rise or set but is up/down continuously.)

To determine the correction for moonset, reverse the latitude of observation (swap north and south).  You now have the correct times for moonrise and moonset within about 30 minutes.  To do better, we will have to correct for the eccentricity of the moons orbit.

This page is part of the Moonstick Information Site.
copyright © 2000 Sean Barton, all rights reserved