NavList:

Watch the Moon rise tonight if your skies are clear. For me tonight, on Narragansett Bay in New England, the azimuth at moonrise will be about 50° true. This is about as far northeast as possible from my latitude. The Moon just looks wrong rising so far north of east! And tomorrow it will set almost as far north of west, after spending over sixteen hours in the sky.

We're generally accustomed to the range of the sunrise and sunset azimuths. They repeat from year to year. On the summer solstice in the northern hemisphere the Sun rises in the northeast... 23.5° north of east close to the equator, sliding out to 27° north of east at 30°N latitude and 34° north of east at 45°N. The pattern is reversed on the winter solstice with the Sun rising the same number of degrees south of east. This pattern is essentially identical from one year to the next. And generally, on average, the rising and setting of the Moon has similar ranges of azimuth, but the Moon's motion does not exactly follow the ecliptic.

The Moon's orbit is tilted relative to the mean plane of the Solar System (near enough to the ecliptic) by about 5.2°. The crossing point where the Moon's orbit intersects the ecliptic precesses, cycling slowly around over 18 years. So sometimes the Moon's orbital tilt partially cancels out the the tilt of the ecliptic and sometimes it partially augments it. When the Moon reaches its maximum declination, as it is doing once a month this fall, the Moon's orbital tilt has been added on to the tilt of the ecliptic. This means that the Moon rises and sets much farther north and spends a far longer period of time in the sky (for mid-northern observers). In two weeks the pattern will be reversed, and the Moon will only be in the sky for a rather short period of time. For my latitude, the Sun tonight (through tomorrow morning) will spend about 16h15m in the sky. By contrast on December 13, the Moon will be in the sky for 8h 48m. This latter passage, short in time and low in altitude across the southern sky will not be visible because the Moon will be only a day from New.

I encourage any of you with clear skies to get outside tonight shortly after the time of moonrise. I was out picking up a printing order around that time last night, and I assure you, the Moon was wrong! It just seemed to be in the wrong place by azimuth. Of course, the Moon was right, and my casual impressions of how things should be was wrong. I drove up to a stop sign facing Narragansett Bay, and in my head the road there points roughly north. And there was the Moon, dead ahead --due north by the (incorrect!) compass in my head. Astronomical objects provide us with the ultimate natural compass. And it's almost free of charge -- available to us on any clear night for the small price of learning the stars. But the Moon can surprise unless we remember that it's motion is more exotic and more interesting than the other celestial bodies.

A final note: Anyone in southern Florida? Tonight (actually tomorrow by standard civil time in the early a.m. hours) the Moon will pass just within a couple of degrees of the zenith. And if you happen to be more than about 20 miles south of the latitude of Orlando, the Moon will pass north of you. You're inside the zone of the Moon's "tropical" latitudes.

Frank Reed
Clockwork Mapping / ReedNavigation.com
Conanicut Island USA