Stargazing for September 2021

Full Harvest Moon and the Autumnal Equinox

August 31, 2021
Moon and Girl

Ready for fall? Here are a couple fun astronomical tidbits to enjoy this month.

Catch the Harvest Moon

September’s full Moon, which is traditionally called the Harvest Moon, will appear on the night of Monday, September 20! Look southeast after sunset to watch it rise above the horizon. Read more about the Harvest Moon here.

The Autumnal Equinox

This year, the autumnal equinox—also called the September equinox or fall equinox—arrives on Wednesday, September 22!

The autumnal equinox marks the astronomical start of the fall season in the Northern Hemisphere and of the spring season in the Southern Hemisphere of Earth.

What Is an Equinox?

The word “equinox” comes from Latin aequus, meaning “equal,” and nox, “night.” On the equinox, day and night are roughly equal in length.

During the equinox, the Sun crosses what we call the “celestial equator”—an imaginary extension of Earth’s equator line into space. The equinox occurs precisely when the Sun’s center passes through this line. When the Sun crosses the equator from north to south, this marks the autumnal equinox; when it crosses from south to north, this marks the vernal equinox (the spring equinox).

After the autumnal equinox, the Sun begins to rise later and nightfall comes sooner. This ends with the December solstice, when days start to grow longer and nights shorter.