Editor’s Note: This story has been updated to correct the length of days and nights after the autumnal equinox. It also clarifies the annual timing of the Harvest Moon.
SAN FRANCISCO (KRON) — The Harvest Moon will be glowing at its peak Monday night.
The Harvest Moon is the full moon that rises closest to the autumnal equinox, which is Sept. 22, officially marking the start of the fall season for the Northern Hemisphere.
According to the Farmer’s Almanac, the moonlight of the Harvest Moon brightens the early evening when crews are harvesting their summer crops.
The Harvest Moon can occur in either September or October, the Farmer’s Almanac says.
“The section of the zodiac band in which the full Moon travels around the start of autumn is the section that forms the most shallow angle with the eastern horizon,” according to The Farmer’s Almanac.
“Because the Moon’s orbit on successive nights is more nearly parallel to the horizon at that time, its relationship to the eastern horizon does not change appreciably, and the Earth does not have to turn as far to bring up the Moon,” it says.
According to NASA, the Harvest Moon will appear full for about three days this year, with peak illumination around 7:55 p.m. ET Monday night. It will last through Wednesday morning.
The equinox occurs when the sun is directly over the equator. At this time, the sun’s rays move from north to south. During the vernal equinox, they move from south to north. Daylight hours are roughly the same in the Northern and Southern hemispheres during the fall equinox.
After the autumnal equinox, days become shorter and nights longer. This comes to an end with the December solstice, which brings with it longer days and shorter nights.