A harvest moon—or the full moon that happens nearest to the fall equinox—is due to appear in the sky on the night of Friday the 13th. It may be an inauspicious date, but fans of lunar phenomena will find themselves feeling lucky, because something rare is set to happen this year. The harvest moon often appears large and orange, since many people observe it as it surfaces above the horizon. But in 2019, the harvest moon will seem unusually small.
As Jenna Amatulli reports for the Huffington Post, this phenomenon is known as a “micromoon,” which occurs when a full moon happens close to the lunar apogee, or the point at which the moon is farthest from Earth. (A supermoon, which appears large in the sky, happens when the full moon coincides with the perigee, or the moon’s closest approach to our planet). To people watching from the ground, a micromoon looks around 14 percent smaller than a typical full moon, according to the Time and Date.
The Harvest micromoon is a rare occurrence, according to Amatulli. Typically, the moon rises at an average of 50 minutes later each day, but around the time of the autumnal equinox, that difference shrinks to just 30 minutes each day. “The reason for this seasonal circumstance is that at this time of the year, the path of the moon through the sky is as close to being along the horizon as it can get,” the Farmer’s Almanac explains. “Thus, from night to night the moon moves more horizontally than vertically and thus rises sooner from one night to the next.”
This early moonrise allows farmers to continue working after sunset by the light of the full moon during the height of the harvest season—hence the moon’s name.