Here’s the science behind this full moon.
Why is the Harvest Moon special?
Harvest Moon is just a name. It’s the name for the full moon nearest the autumnal equinox. In the northern hemisphere, you’ll always see the Harvest Moon in either September or October. In the southern hemisphere, a moon with these same characteristics always comes in March or April.
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.
Here’s what happens. 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 minutes later daily for several days before and after the full Harvest moon. Why? The reason is that the ecliptic – or path of the sun, moon and planets – 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 several days in a row. 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 several nights in a row – around the time of the Harvest Moon.
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