Every year it’s the same story. Harvest moon tonight, says the television weatherperson, and all across the country people think one of two things. Some believe the harvest moon has no observational significance, that it’s just another archaic name like April’s grass moon or March’s sap moon, a throwback to simpler times when moonlight still had relevance for everyday life. Others assume that the harvest moon looks special in some way: bigger or redder or something.
They’re all wrong. First of all, let’s dispense with the notion that the year’s most famous--and misunderstood--full moon appears different from other full moons. It doesn’t. One way the moon changes its naked-eye appearance is by wandering closer to or farther from Earth in its elliptical orbit. For example, August’s full moon appeared about 12 percent smaller, because it’s farther away from Earth, than will the giant moon coming up in March.
Then there’s our atmosphere, with its own bag of tricks. In the haze and humidity of summer the full moon looks more orange, especially since it then travels its lowest path of the year and spends more of its time in the thicker air near the horizon. It’s even been suggested that the term honeymoon derives from the amber-colored full moons of June. Full moons near the winter solstice, on the other hand, are the year’s highest and usually shine through drier, crisper air, making them whiter and a bit brighter than most. But September’s harvest moon? No, nothing special there.
Then you’ve got the famous moon illusion. Perhaps the most powerful mirage of all, it makes the moon seem enormous when it’s down near the horizon, a visual effect that may be caused by the moon’s proximity to foreground terrestrial objects. But again, that natural bit of sorcery acts on all moons equally.
So if it doesn’t look special, what’s the fuss? It all boils down to that old mischief-maker, the tilt of Earth’s axis. Our slanted spin makes the moon’s path seem to wobble like a tire out of balance, which in turn affects its rising and setting. On average the moon comes up about 50 minutes later each night. But in the autumn the moon’s orbital plane makes such a skinny slant to the eastern horizon that it is barely below the skyline. Then the nightly interval can be cut in half, to only about 25 minutes between moonrises, or even much less if you live at a high latitude.
Like all full moons, which are 180 degrees across the sky from the sun, the harvest moon rises at sunset. The result? Picture it: The harried farmer trying to finish harvesting chores is running out of daylight. The sun sets, and bingo--the full moon rises to add welcome light. And this goes on for several nights in a row. Therefore, while the official harvest moon, usually defined as the full moon nearest to the autumn equinox, occurs on the night of September 11-12 this year, the harvest moon effect is seen for a couple of nights before and after that date.
Since the harvest moon always floats very nearly over Earth’s equator, the worldwide equilibrium of the upcoming equinox is nicely represented as well. A study in symmetry, the harvest moon rises upward and to the left for those in the southern hemisphere, while the rest of the world sees it come up and to the right.
It’s a rare equilibrium, as if heaven and Earth are pausing for one ephemeral breath, balanced and motionless, before rushing headlong toward the northern winter and the southern summer.