A Harvest Moon is the full moon that occurs closest to the autumnal equinox (around September 22 or 23). What makes it special is that its rise times on successive evenings are much closer together than usual, often only about 10 to 12 minutes apart rather than the typical 50-minute delay that happens during other times of the year. This phenomenon happens because the angle of the Moon’s orbit relative to the horizon is at a minimum near the autumn equinox, causing it to rise quickly one night after another. The effect is that the bright Moon rises around the time of sunset for several nights in a row, providing extended bright moonlight in the early evening that historically helped farmers with harvesting their crops.
The Harvest Moon often appears orange or reddish when it is near the horizon, due to the lunar light passing through more of Earth's atmosphere which scatters blue light and allows the red hues to filter through. The Moon itself is not actually bigger or brighter than usual, though it can sometimes coincide with a supermoon when it is closest to Earth in its orbit, appearing slightly larger.
In summary, the key characteristics making a Harvest Moon unique are:
- It is the full moon closest to the autumnal equinox.
- It rises about the same time for several nights consecutively.
- This creates a longer period of bright moonlight after sunset historically useful for harvesting.
- The moonlight has a characteristic orange or reddish hue near the horizon due to atmospheric effects.