Bob Moler's Ephemeris Blog

This is Ephemeris for Tuesday, September 29th. Today the Sun will be up for 11 hours and 47 minutes, setting at 7:26, and it will rise tomorrow at 7:40. The Moon, 2 days before full, will set at 6:09 tomorrow morning.

This upcoming full moon is the Harvest Moon. It is the most famous of the named full moons, and was very useful in the days before electric lights. The reason is that the Moon, around the time it is full now doesn’t advance its rising time very much from night to night effectively extending the light of twilight to allow more time to gather in crops. This is because the Moon is moving north as well as eastward. The farther north it is the longer it stays up and retards the advance in rise times. On average the Moon rises 50 minutes later each night. This week the interval is down near 20 minutes advance in moonrise times per day extending twilight and the time each day to harvest the crops for a few more days.

The event times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.

Addendum

The motion of the moon from tonight through next Monday night. This is looking east at where the Moon will rise, and we’re able to see below the horizon. The celestial equator, a projection of the Earth’s equator on the sky, crosses the horizon at an angle equal to 90 minus one’s latitude. Around my location that’s 45.5 degrees. The Moon and stars will rise parallel to the celestial equator. Its daily orbital motion is at the shallow angle of 5 degrees to the ecliptic, the apparent path of the Sun. So the advance in rise times starts off at 20 minutes later each night, rather than the average 50 minutes.