For those in the Northern Hemisphere, the autumnal equinox marks the start of fall. This year, the equinox falls on Wednesday, September 22 at 3:21 P.M. EDT. At that time, the Sun will cross the celestial equator — the projection of Earth’s equator out into space in all directions — heading from north to south.
The Full Moon that occurs closest to the autumnal equinox has a special name: the Harvest Moon. This year, the Harvest Moon falls just a few days before the equinox, on Monday, September 20, at 7:55 P.M. EDT. And because of some nifty celestial geometry, it brings with it a special effect called, appropriately, the Harvest Moon effect.
What is the Harvest Moon effect? Put simply, it’s a span of several days during which the Full (or nearly Full) Moon rises at almost the same time each night. This means several nights in a row receive lots of bright moonlight — a boon to farmers working hard to bring in the harvest before the advent of artificial lights.
What causes the Harvest Moon effect?
Okay, so what’s really going on? As the Moon circles Earth, we see its changing phases as the amount of illumination it receives from the Sun also changes. When it is Full, it sits directly opposite the Sun, rising around sunset and setting around sunrise. On any two successive nights, moonrise generally varies by about 50 minutes as our satellite goes from New to Full and New again. Then the cycle repeats.
But that 50-minute change from day to day is only an average. And it’s related to the angle of the Moon’s orbit with respect to Earth’s equator — because unlike many satellites in the solar system, our Moon doesn’t orbit Earth’s equator. Around the vernal equinox, which kicks off spring in the Northern Hemisphere, that interval can stretch to a 70-minute change in moonrise times from night to night. And around the autumnal equinox, it shrinks to just 20 minutes or so. (Note that these times and dates are flipped for the Southern Hemisphere, which gets the most moonlight around the vernal equinox and the least around the autumnal equinox.)