Harvest Moon 2012: Why The Moon Will Be Big And Colorful This Weekend

Usually the moon rises about 50 minutes later each day throughout the year. But at this time of year in the Northern Hemisphere, the moon rises only about half an hour later each day for a couple days, which in less electrified times meant more light for farmers to bring in their crops, hence the tradition that the full moon that falls closest to the autumnal equinox is dubbed the Harvest Moon. The earlier moonrise times are due to the fact that the moon’s orbit around Earth has a slightly narrower angle with the horizon in the Northern Hemisphere at the time of the fall equinox.

The Harvest Moon often looks orange or yellow for the same reason that a setting sun has a spectacular color display -- we’re seeing them close to the horizon. When we look a celestial body close to the horizon, we’re seeing the light given off by the sun or moon through the thickest possible amount of atmosphere, which allows red light to pass through but traps blue light.

Nearness to the horizon also means that the moon looks much larger than it normally does, but this is an optical illusion. Scientists are still debating exactly why the moon looks inflated near to the horizon. Some think this “Moon Illusion” results because at the horizon, we see the moon closer to foreground objects like trees and buildings as opposed to a moon hanging out alone in the sky. It’s an example of the classic Ponzo Illusion, where converging lines can make makes two bars of equal length look different sizes.

However, as NASA notes, airline pilots have witnessed the moon illusion at high altitudes, where there are very few buildings about. This leads some to think that the apparent hugeness of the moon results from how humans perceive the sky. We see it as a flattened dome, which means our brains miscalculate the size of the moon when we see it on the horizon.

One University of Wisconsin-Whitewater researcher proposes a different explanation, based on another kind of “angular size” illusion.

In 2010, the Harvest Moon fell directly on the autumn equinox, making it appear especially big and bright -- a Super Harvest Moon. Such a coincidence will not occur again until 2029, according to NASA astronomer Tony Phillips.

Every full moon has a traditional name, according to the Farmer’s Almanac -- the one following the Harvest Moon, which in 2012 falls on Oct. 29, is known as the Hunter’s Moon.