Happy September. September means two good things (at least): the beginning of nights with no mosquitoes and it’s getting dark earlier. I know that the shorter days are not generally a popular thing to call “good”, but it means that there are more awake-hours to spend out under the night sky. Two more things of interest are the Harvest Moon and the Autumnal Equinox.
The Harvest Moon, besides just being a great excuse to dance with someone special out under the stars, is the full moon that occurs closest to the Autumnal Equinox (more on just what the equinoxes and solstices are, we’ll talk about later). This year, with the Equinox occurring on the 23rd, the dates of the full moon that fall on either side of it are September 12th (11 days before the Equinox) and October 11th (18 days after the Equinox). So, the Harvest Moon is on September 12th (enjoy your dance!). What makes the Harvest Moon special? The short answer is that for a few days in a row, that big, bright, nearly full moon is in the sky and ready to take over lighting fields by the time the Sun has completely done its job for the day, giving farmers more time to bring in their crops. The (slightly) longer answer is that the path the Moon (and the Sun, the planets and most asteroids) follows across the sky (called the Ecliptic) makes a shallow angle with the Eastern horizon at this time of year. That may be a bit hard to picture so, here’s a picture (the circles represent the Moon on successive days and the tilted lines are the Ecliptic):
As the Moon orbits the Earth, it moves along the Ecliptic from West to East (right to left as we see it in the sky). Since it takes the Moon about 30 days to move from Full Moon to Full Moon, it moves 1/30 of the way along in its orbit each day. This means that the Moon rises later and later each day. Our days are 24 hours long. On the equator the Ecliptic is exactly vertical to the eastern horizon at the equinoxes and, if the Moon was near full then, it would rise 24/30 of an hour later (48 minutes) than it did the day before. But, as the Earth moves in its orbit around the Sun, the angle between the Ecliptic and the eastern horizon at moonrise changes throughout the year. When the Ecliptic is at its steepest angle at moonrise (at the Vernal Equinox) the daily rising of the Moon differs the most from day to day (about 84 minutes). When the Ecliptic is at its shallowest angle to the horizon at moonrise (at the Autumnal Equinox), the time between moonrise each day changes the least (about 24 minutes).