Monday, September 8, 2014

The harvest moon and lunar elevation

Tonight is the full moon, and since it's the moon closest to the autumnal equinox, it's the "Harvest Moon". I found a number of explanations of why it's called the harvest moon online (e.g. this one by Bob King), and everyone mentions the fact that for several days, the moon rises near the sunset time. The moon therefore allows people to bring in crops by moonlight, hence "harvest" moon. I'm not a historian, but I have a very strong suspicion that this is only a part of the story.
autumnal equinoxa
autumnal equinox
autumnal equinox
autumnal equinox
autumnal equinox

Moonrise over Nationalpark M├╝ritz by Alejandro Sanchez de Miguel is licensed
under a Creative Commons Attribution 3.0 Unported License.

If you think back to a memory of walking through the country on an extremely bright moonlit night, the odds are very good that you'll be thinking of a snow-covered winter landscape. Snow of course makes the landscape much brighter, but full moons are also brighter in winter because the full moon rises much higher in winter than it does in summer. (A useful way to remember this is that the full moon always does the opposite of the sun: in summer it's low in the sky, and in the winter it's high).

But it's a bit more complicated in the spring and fall. At those times, the highest elevation moons are during either the first quarter (spring), or the third quarter (fall). Now here's where I think the "harvest" moon comes in. In the days shortly before the full moon in autumn, the moon sets shortly after midnight, and the landscape isn't particularly brightly illuminated because the moon doesn't rise very high in the sky (both of which are not so useful if you want to work all night). In contrast, the moon soars high in the sky in the days immediately after the full moon in autumn.

This year in Berlin, the moon on September 6 (3 days before the full moon) reaches only 24° above the horizon. On September 12 it's 47° above, and that makes the landscape about 78% brighter (if I accounted properly for the light absorbed and scattered by the atmosphere, it would increase that number even further).

I'd love to hear from a historian whether I'm right about the nighttime harvest taking place on the days after the full moon rather than before. If you anyone knows, or can find information about this from a reliable source on the Internet, please make a post in the comments.

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