If you look at the full moon tonight, you may not notice anything unusual, but you’re seeing something special: a Harvest Moon in October. As National Geographic's Victoria Jaggard reports that this is the first time the Harvest Moon has occurred in October since 2009.
At least one full moon fills the sky each month of the calendar year, though sometimes the heavens stiff the short month of February. The Harvest Moon, Jaggard writes, is the full moon that appears closest to the autumnal equinox, marking the arrival of fall. Since the equinox usually takes place near September 22, the Harvest Moon is most often the full moon in September, while the full moon in October is known as the Hunter’s Moon.
But by a quirk of astronomy, the last full moon occurred on September 6—16 days before the equinox. Tonight’s full moon is taking place just 13 days after the equinox, earning the title of Harvest Moon.
Pre-industrial cultures around the world kept time by tracking the moons, giving each one distinct names. In North America, the names are derived from Native American cultures and early farming practices. For instance, May’s moon is known as the Flower Moon or Planting Moon. July has the Thunder Moon or Full Hay Moon. And November has the Beaver Moon or Frost Moon.
The names are primarily a way of tracking natural changes through the year and don't usually imply any changes in size or color of the glowing orb. But there is something very special about the Harvest Moon. As Deborah Byrd at EarthSky reports, near the autumnal equinox, the moon makes a very narrow angle with the horizon near sunset, which means the moon will rise closer to sunset.
Normally, the moon rises 50 minutes later each day. But because of this narrow angle, the full moon rises just 35 minutes later each day for several nights in a row. As Byrd explains, in the days before electricity, the early-rising full moon meant farmers could continue harvesting their fall crops late into the evening.
The Harvest Moon sometimes also appears orange, like a fat pumpkin hovering in the sky. That’s because extra dust particles in the atmosphere, some of which are created by farmers harvesting crops, scatters the moon’s light. The lower the moon is on the horizon, the more atmosphere its light passes through, meaning more scattering. Only the longest wavelengths of light, the reds and oranges, make it through, causing the fall colors.
For most skywatchers, however, looking at a Harvest Moon will likely appear little different than full moons any other time of year. “The harvest moon is a full moon, but not one that really provides any specific unique-viewing opportunity—other than that you might have great observing from a pumpkin patch,” Andrea Jones, the Lunar Reconnaissance Orbiter public engagement lead at NASA tells Jaggard.
Even if you’re not bringing in the wheat, the Harvest Moon—which we respectfully suggest should be renamed The Pumpkin Spice Moon—is still a great time for a nighttime walk or a nice celebratory bonfire.
Editor's note October 5, 2017: The headline of this article has been updated to reflect that while the event hasn't happend in a decade, October Harvest Moons aren't necessarily rare.