More on the Harvest Moon

More on the Harvest Moon

Grim's moon picture got me thinking how I never can quite keep straight how moonrise and moonset work. I know that the full moon rises as the sun sets, and that the new moon rises as the sun rises. I usually can remember that the moon rises a bit later every night (an average of a little over 50 minutes later, or 24 hours in a revolution of the Earth divided by 27-28 days of moon cycle), so if I concentrate I can figure out that the half-moon waning rises at midnight while the half-moon waxing rises at noon. I also know that the sun rises and sets due east and west at the equinox, but rises and sets south of east and west in midwinter and north of east and west in midsummer. What I never realized is that this means the moon rises in a different direction throughout each month:

Season	Position of Moonrise/Moonset
	New	1st	Full	3rd
Winter	SE/SW	East/West	NE/NW	East/West
Spring	East/West	NE/NW	East/West	SE/SW
Summer	NE/NW	East/West	SE/SW	East/West
Autumn	East/West	SE/SW	East/West	NE/NW

Like the sunrise and sunset positions, the amount of variation to the north or south depends on the latitude. (And this chart, of course, would be reversed in the southern hemisphere.)

Here's something else I didn't know, and can't quite visualize without some globes and lights: The 50-minute lagtime is only an average, which varies between 25 and 75 minutes depending on the season. In summer and fall, the lagtime is at a minimum; in winter and spring, at a maximum. (The distortion is maximized with latitude.) So the Harvest Moon rises only 25 minutes or so later every night this week, and we should look for moonrise in the southeast sky even though the sun is rising and setting nearly due east and west this month.