NASA ScienceCasts: The Harvest Moon

The full Moon closest to the northern autumnal equinox is coming this week.
According to folklore, every full Moon has a special name. There’s the Wolf Moon, the Snow Moon, the Worm Moon, the Sprouting Grass Moon, the Flower Moon, the Strawberry Moon, the Thunder Moon, the Sturgeon Moon, the Harvest Moon, the Hunter’s Moon, the Beaver Moon, and the Long Night’s Moon. Each name tells us something about the season or month in which the full Moon appears.

The Harvest Moon is the full Moon that falls closest to the autumnal equinox, the beginning of northern autumn. In 2013 the Moon is full on Sept. 19th (the night of Sept. 18-19 in North America) while the equinox follows close behind on Sept. 22nd. The coincidence sets the stage for a nice display of Harvest moonlight.

In the days before light bulbs, farmers relied on moonlight to help them harvest their crops. Many crops ripen all at once in late summer and early autumn, so farmers found themselves extremely busy at this time of year. They had to work after sundown. Moonlight became an essential part of farming and, so, the Harvest Moon was born.

There’s more to the Harvest Moon, however, than just an old-fashioned name. It really is special.

Throughout the year the Moon rises, on average, about 50 minutes later each day. But near the autumnal equinox, this difference shrinks to only 30 minutes. The reason is, at the beginning of autumn the moon’s orbital path makes a narrow angle with the evening horizon.

To a non-astronomer, that might sound like celestial trivia. But to sky watchers it makes a huge difference. For several nights in a row around the time of the Harvest Moon, the moon rises at about the same time–sunset. And you know what happens when the moon rises at sunset….

Low-hanging moons are reddened by clouds and dust. Not only that, they are swollen to outlandish size by the Moon illusion, a well-known but still mysterious trick of the eye that makes low-hanging Moons seem much larger than they really are.

When you add these effects together the Harvest Moon often looks like a great pumpkin. The experience is repeated for several nights in a row around the equinox.

A great pumpkin-colored Moon rising in the east is a nice way to kick off northern autumn. And it’s a nice way to end the day. At sundown on Sept. 18th, go outside, face east, and enjoy the Harvest Moon.

Author: Dr. Tony Phillips