On Sept. 24 (and for a few days after) make sure to look up after sunset: If the skies are clear, you’ll see the harvest moon — a beautiful way to mark the start of autumn.What is the harvest moon?The harvest moon is the full moon that happens closest to the autumnal equinox, or the official start of fall, according to Sky & Telescope. The specific date of the harvest moon varies from year to year and can occur either in September or October. When can we see it this year?The 2018 autumnal equinox is on Sept. 22 and the next full moon occurs two days later on Sept. 24. Of course, this is only true for the northern hemisphere; in the southern hemisphere, the harvest moon comes in March or April, EarthSky explains.Where does the name come from?It’s called a harvest moon because the moonrise occurs shortly after sunset, giving farmers lots of bright moonlight early in the evening while they harvest their crops, according to The Old Farmer’s Almanac. The harvest moon rises near the time of sunset for several nights in a row, making it unique among other full moons during the year — it might even seem like there’s a full moon for around a week.EarthSky also notes the term “harvest moon” became popular because of the 1903 song “Shine on Harvest Moon,” which was more recently covered by Leon Redbone. What does it look like?If you catch a glimpse, you'll notice that this annual spectacle looks particularly beautiful. Compared to your average moon, it can appear bigger, brighter or more orange, if you look right after sunset when it's close to the horizon. It’s a visual trick called “the moon illusion,” but it’s a gorgeous one. The harvest moon also occurs right around when birds are migrating south for the winter — so depending on where you live, you might be able to see birds fly by the full moon for a truly stunning sight.