For sky-watchers in the Northern Hemisphere, the October full moon is pulling off a celestial trick that will be a real treat.
On Thursday night, the sky will be aglow with this year’s harvest moon—the first time a full moon bearing this moniker has fallen in October since 2009. Technically, the moon reaches its full phase on October 5 at 2:40 p.m. ET, which means the harvest moon will be in effect when the lunar orb glides above the horizon at 7:21 p.m. ET.
So why do we call this spectacle a harvest moon, and why is it special?
The late Harvest Moon was caused by the moon being new, with its sunlit side turned fully away from Earth, just two days before the equinox, on September 20. That was the first new moon after the one that caused August's total solar eclipse. September's new moon was also the beginning of a new year in the Jewish and Islamic traditions, which both run on lunar cycles.
North America's moon nicknames are adapted from American Indian terms for the moons. Tribes tied each lunar cycle to what was happening in the natural world. Most of the names now in popular use were coined by members of the Algonquin tribes, who once lived throughout New England and the Great Lakes area.
Other tribes across the continent have other nicknames for the moons—October names include "moon when the wind shakes off the leaves" among the Lakota people and "the moon the birds fly south" among the Cree people, according to Phil Konstantin, a retired NASA scientist with Cherokee roots who collects American Indian celestial terms.
In the southern hemisphere, full moons are also often given nicknames tied to seasonal events in the natural world, but because September's equinox there marks the beginning of the spring, October names instead include "Egg Moon," "Seed Moon," and "Pink Moon."
This year's Harvest Moon is the 10th of 12 full moons this calendar year. The next full moon, due on November 4, will be called the Beaver Moon. Later in November, the night sky will stay interesting thanks to Venus and Jupiter being visible unusually close together.
LIGHTING UP THE NIGHT
Still, any given harvest moon can be a beautiful sight, thanks to the moon’s behavior in the autumn months.
The moon’s orbital path is slightly tilted with respect to Earth’s orbit, so the lunar disk changes its position in the sky with each moonrise. Over the course of a year, the moon pops above the horizon about 50 minutes later each day, on average.
But for several days in a row around the fall equinox, the moon rises only about 30 minutes later in the northern U.S. and just 10 to 20 minutes later in much of Canada and Europe. It sounds like a small shift, but it brings noticeably brighter nights: During this time, the full moon rises almost as soon as the sun sets, first appearing as a plump, orange-colored orb right around dusk. (Find out more about the moon illusion and why the moon always looks bigger near the horizon.)
With little darkness between sunset and moonrise, farmers can work later into the night harvesting crops—the likely origin of this full moon’s name. According to a monthly email about the moon sent by NASA’s Gordon Johnston, the name “harvest moon” hails from Europe, where it’s been used since at least the early 1700s. It’s also been called the travel moon, dying grass moon, or blood moon in various Native American traditions.
This year, the October 5 full moon also heralds Sharad Purnima in India, a Hindu harvest festival held around the first full moon in the lunar month of Ashwin, which marks the end of monsoon season.
“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,” says Andrea Jones, the Lunar Reconnaissance Orbiter public engagement lead at NASA.
She adds that NASA will be celebrating this year’s International Observe the Moon Night on October 28, when our cosmic companion will be in its first quarter phase and only half-lit.
“We encourage everyone everywhere around the world to look up at the moon that day and take the opportunity to learn about and celebrate lunar and planetary science, exploration, and the personal and cultural connections we all have to our nearest neighbor in space,” Jones says.
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