Harvest Moon

harvest_moon-150x150September or early October holds one of the most striking Moons of the year, the so-called “Harvest Moon” which traditionally marked the harvest and lent light to farmers working late in their fields.  The Harvest Moon is the full Moon closest in time to the autumnal equinox which marks the first day of autumn in the northern hemisphere.

The Harvest Moon is always the last full Moon of summer or the first full Moon of autumn in the northern hemisphere.  It comes as early as two weeks before the first day of autumn and as late as two weeks after.  The Moon is not especially large or bright at this time compared to other full Moons unless it occurs by chance when the Moon is near perigee, its closest approach to Earth during its monthly revolution.

The full Moon along the ecliptic (red line) in mid-September (left) and mid-March (right) in the northern hemisphere. Click to enlarge.

The full Moon along the ecliptic (red line) in mid-September (left) and mid-March (right) in the northern hemisphere. Click to enlarge.

But the Harvest Moon does have one unusual feature.  It occurs when the ecliptic, the circle around the sky that marks the path of the Sun, Moon, and planets, makes a very shallow angle with the eastern horizon in September.  This means the difference in time between moonrise in the days before and after the Harvest Moon is just 30-35 minutes compared to the average 50-minute period between moonrises each night during the year.  So during the week of the Harvest Moon, a bright Moon seems to linger over the eastern horizon for an unusually long time just before or after sunset.   This was no doubt a big help to farmers bringing in their crops.

In the southern hemisphere, the same effect occurs in mid-to-late March for exactly the same reason.

The Harvest Moon is also sometimes called the Full Corn Moon, the Fruit Moon, the Barley Moon, and the Wild Rice Moon.