Full moon (copy)

The harvest moon isn't defined by any change in its color, but rather by the time of year it arrives -- closest to the autumnal equinox.

Q: What is different about the harvest moon, and why is it significant?

A: The harvest moon isn’t different in terms of its perceived size or color, Jim Lattis of UW Space Place said. Instead, it is significant because of the time it appears in the sky.

The harvest moon is defined as the full moon closest to the autumnal equinox, which was Sept. 22, when days and nights are the same length in the fall. Because the moon’s orbital orientation is slightly tilted from the Earth’s rotation, the harvest moon rises just shortly after sunset, Lattis said.

“This means that we see a nearly full moon illuminating the sky just after dark for the better part of the week,” Lattis said. “That is, there is a chain of brightly moonlighted evenings, which doesn’t occur quite like that for any other full moon of the year.”

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While the harvest moon isn’t notable for appearing a different color, the Smithsonian Magazine notes that viewers may occasionally see an orange harvest moon “like a fat pumpkin hovering in the sky.”

This is because of dust particles scattered in the air, according to the Smithsonian. When viewing the moon at the horizon, you are looking at it through more atmosphere than looking straight up, and since the atmosphere scatters blue light, more red and orange light makes it to your eyes. This can happen anytime of year, but it could also be affected by dust kicked up through farmers harvesting their crops.

— Shelley K. Mesch

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