The Full Moons Of Winter, 1986
Has it crossed your mind that the moon has been exceptionally noticeable this winter? It has, and there's a reason for it. We're at the peak of an 18.6 year cycle during which the moon's course rhythmically rises and drops in the sky. Ancient civilizations noticed this pattern, and the Druids living in southern England over 3000 years ago incorporated it into their alignment of the monoliths at Stonehenge.
Bear in mind that a full moon is always on the opposite side of the earth from the sun (it has to be, otherwise, it wouldn't be a full moon). But the earth's orbit around the sun is not precisely lined up--or coplanar--with that of the moon's orbit around the earth. If the orbits did lie in the same plane, we would have eclipses every month. The moon's orbit around the earth is inclined about 5 degrees from the earth's orbit around the sun, and it precesses around the earth, much as a tin plate or a quarter dropped crookedly onto a table will flutter about its axis before clattering to a halt. The most common analogy of precession is that of a spinning top as it slows down. An arrow pointing through its axis would wander around in a circle.
During the winter of 1986, we are passing through a period when the moon is at one extreme in its precession cycle. The full moon is now higher in the sky than it will be at any other time during the next 18.6 years. The new moon rises in the southeast and sets in the southwest, but as the lunar month wears on and the moon waxes, the rising point swings further and further to the left (east) and the setting point moves further and further to the right (west). Above a certain latitude, the full moon will just skim the northern horizon and never set. This means that the motion of the full moon this winter is similar to that of the sun in the summer. At high latitudes like Alaska, the winter full moon rises very high into the sky and practically never sets. In the summer the opposite is true, and the full moon practically never rises.
We're all familiar with the Arctic Circle, above which the sun never rises during the winter months and never sets during the summer. Few of us stop to consider that a similar situation prevails for the moon. However, the lunar Arctic Circle is not fixed like the solar Arctic Circle, but changes its latitude with pendulum-like regularity over the years.
The lunar Arctic Circle is now near its southernmost position, so the full moon never sets for most Alaskans. It swings down in the north (like the midnight sun), but never sets for observers living north of, say, Wasilla.
In about another nine years, or half of the moon's precession period, you won't be able to see the full moon for a full 24 hours from anyplace south of the latitude of Barrow.
