"The full moon closest to the September equinox [is] the traditional definition of the harvest moon," said Alan MacRobert, an amateur astronomer and senior editor at Sky & Telescope magazine in Cambridge, Massachusetts.

"And you can't get any closer to the September equinox than this."

The moon is set to rise on the autumnal equinox at 6:27 p.m. ET, though—sticklers, take note—it won't officially be full until 5:17 am ET on September 23, a handful of hours past the start of fall. (Take National Geographic's moon quiz.)

MacRobert said the full moon falling on the autumnal equinox is simply one of many astronomical coincidences that happen to be occurring this month.

The planet