Harvest Moon is just a name. It’s the name for the full moon nearest the autumnal equinox. In the Northern Hemisphere, the Harvest Moon sometimes falls in September and sometimes falls in October. But the Harvest Moon is more. Nature is particularly cooperative around the time of the autumn equinox to make the full moonrises unique around this time.
On average, the moon rises about 50 minutes later each day. But when a full moon happens close to the autumnal equinox, the moon (at mid-temperate latitudes) rises only about 30 to 35 minutes later daily for several days before and after the full Harvest moon. Why? The reason is that the ecliptic – or the moon’s orbital path – makes a narrow angle with the evening horizon around the time of the autumn equinox. The narrow angle of the ecliptic results in a shorter-than-usual rising time between successive moonrises around the full Harvest Moon.
These early evening moonrises are what make every Harvest Moon special. Every full moon rises around sunset. After the full Harvest Moon, you’ll see the moon ascending in the east relatively soon after sunset for a few days in a row at northerly latitudes. The lag time between successive moonrises shrinks to a yearly minimum, as described in the paragraph above. Because of this, it seems as if there are several full moons – for a few nights in a row – around the time of the Harvest Moon.
* the above excerpt is taken from here, where you can also read more about the harvest moon
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