Harvest Moon 2011 Brightens September Skies

Why does it look so large as it rises?

Sept. 12, 2011 — -- This is the night of the Harvest Moon -- the full moon that happens in September and is often one of the year's prettiest.

If you're in a good place and if the weather is kind to you, the moon will loom large in the eastern sky just as the sun disappears in the west.

We're coming up on the autumnal equinox on Sept. 23, when summer turns to fall and the temperature turns too. At this time of year, night and day are the same length, which is why the sun and moon, just opposite each other in the sky, make for a nice show.

There's no real magic to all this, just the bodies of the solar system doing what they do as they follow the rules of orbital mechanics. But there is a pleasant effect on us earthlings, if we pause to enjoy the combination.

A brief review of the terms at play here:

Harvest Moon: Native American tribes gave names to each of the dozen full moons of the year, and the list now is maintained by the Farmer's Almanac. The Harvest Moon is the one closest to the beginning of fall. It's time for the crops to come in.

The next full moon -- which happens in October -- is referred to by the almanac as the Beaver Moon, though it's also called the Hunter's Moon or Blood Moon. Time to set your traps if you need furs for the winter.

Equinox: the moment when the sun appears to pass directly over the Earth's equator -- in this case southbound as we move from summer to fall. It happens twice a year; if you like warm weather, you may prefer the vernal equinox next March 20, when winter will give way to spring.

Moon Illusion: This last term is the one that perhaps does have a touch of magic to it. If you go out tonight just after sunset, and are lucky enough to have good weather and a clear view of the horizon, you'll see the moon, pumpkin-colored, slowly rising in the east -- and, boy, does it look large.

It is, in fact, no larger in the sky than when it's overhead, but our minds fool us, perhaps because we have a reference point -- something on the horizon -- that we lack when it is high among the stars.

"For instance," said astronomer Tony Phillips, "when you see the moon in close proximity to a tree, your brain will miscalculate the distance to the moon, mentally bringing it closer (like the tree) and thus making it bigger. It seems so real, but this beautiful illusion is all in our minds."

It's a quiet, pleasant show that the heavens put on tonight. If the weather favors you, you will literally get to see the stars align.