The full moon that occurs nearest to the autumnal equinox, which is coming up on Sept. 22 — is nearly upon us. Which means, for all you infinity scarf and pumpkin spice disciples — squee! — fall is almost officially here. Let’s delve into why this moon is so unique, and when to see it for yourself.
What is the ‘Harvest Moon’?
The “Harvest Moon” behaves differently than other full moons. For several evenings before and after its peak, due to the angle of the moon’s orbit relative to the Earth’s horizon being at a minimum, the moon rises above the horizon much faster than usual — soon after sunset.
Usually, when a moon is full it rises around sunset and then rises about 50 minutes later each day after. But in the case of the Harvest Moon in mid-temperate latitudes, that successively increasing rise time is cut in half, to 20-25 minutes. (It’s even shorter in northern latitudes like Alaska, where the moon will rise at nearly the same time for a whole week.) So for several days before and after the full moon on Sept. 20, there will be no substantial period of darkness between sunset and moonrise. The Harvest Moon will provide a rare abundance of light from dusk-till-dawn.
How did the Harvest Moon get its name?
While we don’t know exactly where the name came from, according to EarthSky, it “probably sprang to the lips of farmers throughout the Northern Hemisphere, on autumn evenings, as the Harvest Moon aided in bringing in the crops.” It was further popularised by a 1903 song “Shine on Harvest Moon,” written by married vaudeville duo Nora Bayes and Jack Norworth (who — side note — also wrote the lyrics to “Take Me Out to the Ball Game”).
Hollywood films of the same name followed in the 1930s and ‘40s, as well as more than two dozen song recordings by other artists throughout the century. (Neil Young’s “Harvest Moon” is his original version, in case you’re wondering.)
Why does it look orange?
The Harvest Moon has a reputation for being larger and more reddish-orange in colour than other full moons, but this is largely a misconception. The Harvest Moon is no bigger than any other full moon — and its size differs every year. In 2019, in fact, it was the smallest full moon of the year; in 2020, it graduated to second-smallest. This year it will be of average size. While it’s not necessarily any closer or larger than other full moons, it may appear bigger because of its location near the horizon.
While it does take on a burnt sienna hue, this is a result of the moon being low in the sky, not the colour of the moon itself. Because of its proximity to the horizon, we are looking at it through a greater thickness of the Earth’s atmosphere than when it is high overhead. The density of air molecules scatter more blue light, thus allowing more red light to pass through. The colour illusion is amplified on muggy or hazy nights.
When can you see it?
The peak of the 2021 Harvest Moon is on Sept. 20. Be sure to gaze at its quiet majesty, perhaps while regaling your friends or family with new facts about its history, appearance, and singular attributes.