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Fast Facts About Thursday's Harvest Moon

Why does it look so big and orange?

Harvest Moon October

Boston Globe/Getty Images

The harvest moon was once important to farmers who had to work by the light of the moon.

Thursday night will see the rise of this year's harvest moon, and although you’ve probably heard the term, you might not know what it really means. So why is this full moon different from all the others? Here are some reasons.

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  • It’s the closest full moon to the beginning of fall. That means it usually rises in September, but this year it’s Oct. 5 (in a given year, the harvest moon may rise from two weeks before the autumn equinox to two weeks after it).
  • Moonrise comes close to sunset, a fact relating to the origin of the name harvest moon. The harvest moon was once important to farmers and workers who were harvesting end-of-season crops, as they needed the moonlight to see what they were doing. For several nights there will be bright moonlight early in the evening. "It may almost seem as if there are full Moons multiple nights in a row!” the Old Farmer’s Almanac writes.
  • The moonrises over several days are closer together than they are the rest of the year. Typically, the moon rises about 50 minutes later than it did on the previous night. But that difference shrinks to 30 minutes near the autumn equinox, the almanac notes. It's the shortest gap of the year.
  • The harvest moon often looks bigger and more orange than other full moons. The size is an illusion, science website notes (there’s even an approximate way to demonstrate this with an aspirin). The orange color results from the moon's position on the horizon. The atmosphere is thicker near the horizon than it is at higher altitudes, and that atmosphere scatters blue light but lets red light pass. The result is that the moon appears to be orange in color (or at least yellow or reddish). 

Of course, the harvest moon is also the subject of songs and poetry. Henry Wadsworth Longfellow and Carl Sandburg, among others, waxed rhapsodic about the event. On a mildly darker note, English poet Ted Hughes wrote of petrified cows and sheep. 

On the music front, "Shine on, Harvest Moon" was a popular song of the vaudeville era (you may remember jazz traditionalist Leon Redbone performing it in the early days of Saturday Night Live), and Neil Young named a critically acclaimed album and song after the celestial event.

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