September Full Moon: Rare Friday 13th Micro Harvest Moon Will Appear Later This Week

Take note skywatchers, because this weekend a full "harvest" moon will appear in the night sky across the United States—and for some, it may even be visible on Friday 13.

In astronomical terms, the moon is considered "full" when the Earth is positioned between the sun and our natural satellite. In this alignment—which occurs roughly every once a month—the face of the moon that we can see is fully illuminated, appearing like a perfect circle.

Technically, a full moon occurs at a specific moment. For those living in the Eastern Time Zone, this will be 12:33 a.m. on Saturday, September 14, according to the Farmers' Almanac.

However, Americans in the Pacific, Central, Mountain and Alaskan time zones will witness the full moon on Friday 13—a day that is considered unlucky in Western superstition.

It is actually quite rare for the whole of the United States to experience a full moon on the date of Friday 13. The last time this happened was October 13, 2000, and it won't occur again until August 13, 2049.

Even though the full moon only occurs at a specific moment in time, it will appear perfectly circular to our eyes for about 24 hours before and after.

The full moon in September is known as the "Harvest Moon"—a name that may have originated from ancient Native American traditions, or possibly even Anglo-Saxon or old Germanic languages.

In the Northern Hemisphere, the full moon closest to the fall equinox—which this year takes place on September 23—traditionally had a special significance for farmers who were in the middle of the harvest season because it enabled them to work later into the evenings.

Normally, the moon rises around 50 minutes later each day on average. But in the days leading up to the Harvest Moon, this interval shortens to around 27 minutes on average, meaning there is more light available to farmers after sunset.

The upcoming "Harvest Moon" has also been referred to as a "micromoon" as it will appear around 14 percent smaller in the sky. This is because the moon is also nearing its apogee—the point in its nearly month-long elliptical orbit at which it s furthest away from Earth. The moon will be at apogee on September 13 at 9:32 a.m. EDT. Nevertheless, the moon will not appear significantly different in size to our eyes.

The opposite of a micromoon is a "supermoon" when the the moon is at perigee, or at the closest point to the Earth in its orbit.

full Harvest Moon
A nearly full Harvest Moon is seen on October 4, 2017 in New York City. Michael Heiman/Getty Images