Published: 2021-09-19 21:23:00
Updated: 2021-09-20 14:16:29
Posted September 19, 2021 9:23 p.m. EDT
Updated September 20, 2021 2:16 p.m. EDT
By Tony Rice, NASA Ambassador
Each of the 12-13 full Moons throughout the year has a name, often several. Many have their roots in Native American names and reflect what tribes were doing at the time – hunting, fishing or with their crops. According to folklore, the Harvest Moon gets its name from the light it provided farmers to bring in their crops into the early evening as days grow shorter.
But that is true of any full Moon, which rises around sunset and shines brightly until sunrise. What sets the Harvest Moon (Monday 7:30 p.m.) apart is that it is the full Moon occurring closest to the September Equinox (Wednesday, 2:41 p.m. ET).
The Moon, like the Sun, takes its lowest path through the sky around this time of year, and the brightness of the full, or nearly full, Moon rising so soon after sunset, never lets the sky to get really dark. So, work can continue, at least that's how the story goes.
2021's Harvest Moon is the closest to the equinox since 1980. It won't be closer until 2029 when the full Moon and equinox will be separated by just over an hour.
You may also notice the Moon seems a bit bigger this week for an hour or so after moonrise. The reason for this is a combination of the season and how our brains work.
The Moon appears bigger to us near the horizon because of the Moon illusion. Scientists are divided on exactly what causes this, but there's general consensus that it is because of how our brains process the information our eyes are sending. One explanation is that in comparison to other things visible near the horizon – trees and buildings – the Moon looks huge.
NASA astronauts disagree with this explanation saying that they've also experienced the Moon illusion from space, with nothing in the foreground to compare the Moon to. The Moon still looks bigger to them.
The Moon will also appear to be a yellow-orange color as it rises. This is normal as the moonlight passes through more atmosphere near the horizon, scattering more of the blue light and letting more red light through. Smoke from wildfires on the west coast also enhance this effect making the Moon look even more red.