September's full moon will light up the sky tonight but for many people in the U.S. it won't become a Harvest Moon until after sunrise.

During the year the moon rises approximately 50 minutes later each day. During the fall equinox however the day-to-day difference in the "local time of moonrise" is just 30 minutes according to NASA.

This is important for many reasons.

"In the days before electric lights, farmers depended on bright moonlight to extend the workday beyond sunset," said NASA's Dr. Tony Phillips according to Weather.com. "It was the only way they could gather their ripening crops in time for market. The full moon closest to the autumnal equinox became the Harvest Moon, and it was always a welcome sight."

Though the moon will look full all night, astronomers define a full moon as the instant when the moon "lies most directly opposite the sun" for the month according to EarthSky.

That moment will take place at 7:13 a.m. on Sept. 19 meaning the moon will be full after sunrise.

Click here to live stream the Harvest Moon.

"In traditional skylore, the Harvest Moon is the full moon closest to the autumnal equinox," EarthSky said according to Weather.com. "Depending on the year, (it) can come anywhere from two weeks before to two weeks after the autumnal equinox."

In 2010 there was Super Harvest Moon, the first one in two decades and the last until 2029, according to Phillips.

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