It gets its name from farmers who viewed the light from the full moon as an extra dose of light (and thus, some extra work time) to harvest their crops before the days grow even shorter and the harsh winter weather set in.
This year's version was a bit early -- on Sept. 10, although it can occur as early as Sept 8 or as late as Oct. 7.
Then, there's a geometrical quirk in the orbits of the Earth and Moon around the equinox Usually, the moon rises about 50 minutes later each day. But during the Harvest Moon, the moon only rises about 20-25 minutes later each day, thus the moonrise is right about equal to sunset for three days instead of one, allowing farmers constant light.
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