What is the Harvest Moon effect?
For those in the northern hemisphere, the fall 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 the Earth’s equator into space in all directions – heading north to south.
The Full Moon that occurs closest to the Fall Equinox has a special name: the Harvest Moon. This year, the Harvest Moon falls a few days before the equinox on Monday, September 20 at 7:55 p.m. EDT. And because of a clever celestial geometry, it brings a special effect called, appropriately, the Harvest Moon effect.
What is the Harvest Moon effect? Simply put, this is a period of several days in which the Full (or near) Full Moon rises at almost the same time each night. This means that several nights in a row receive plenty of brilliant moonlight – a boon to farmers who worked hard to harvest before the advent of artificial lights.
What causes the Harvest Moon effect?
So what’s really going on? As the Moon revolves around the Earth, we see its phase change because the amount of light it receives from the Sun also changes. When full, it sits directly in front of the sun, rising at sunset and setting at sunrise. On two consecutive nights, the moonrise usually varies by about 50 minutes as our satellite goes back to full and back to again. Then the cycle repeats.
But that 50 minute change from day to day is just an average. And this is related to the angle of the Moon’s orbit relative to the Earth’s equator – because unlike many satellites in the solar system, our Moon does not orbit around the Earth’s equator. . Around the spring equinox, which marks the onset of spring in the northern hemisphere, this interval can extend up to a 70-minute change in moonrise times from night to night. And around the fall equinox, it shrinks to just about 20 minutes. (Note that these times and dates are reversed for the southern hemisphere, which receives the most moonlight around the vernal equinox and the least around the fall equinox.)