Expert: Jesse Martinez - 1/10/2010

Question
We live in south-central Alaska around Anchorage.  In January we noticed the orbit of the moon upon setting was more northernly and then within five days it was more southernly.  Why such a huge veriation in it's path in such a short time?  Scale of 5

Answer
Hi Joe,

This is a subject which can be confusing, so try to hang in there. At your latitude in the winter, the angles at which the sun and the moon rise and travel across the sky are low, that is, they don't rise very high above the horizon. I believe your location is below the arctic circle. You known that the sun rises and sets in strange directions. I believe that in summer, the sun rises and sets in the same general direction, north. East of north at sunrise and west of north at sunset. The moon does not follow the path of the sun. It varies about 6 degrees + and - from the sun's path every month. In the winter the same effect happens, but the days are shorter and the nights are longer and the sun rises and sets near the south, as does the moon. Now, take into account the low angles at which these objects travel across the sky.

I tried to construct a drawing to illustrate this and explain the following, but I failed. I have little control over my hands at this time and cannot use a mouse. So maybe you can imagine this. The sun can be counted on to appear at a certain location at a certain day when it rises or sets. This defines the direction. It is the same year after year. The moon's location at moonrise and moonset is not. It varies from the sun's fixed location for that day, since the moon does not follow the sun's path, but is close. At higher latitudes like yours, the angle can cause exaggerated rising and setting directions. Closer to the north pole, you could see a full moon one night and then not see it at all the next night, because the angle of the moon's orbit relative to sun. Since it varies, the place where the moon appears to rise and set will vary. The higher your latitude the higher the difference as the days go by. I will get an illustration to you as soon as I can.