Tonight’s Harvest Moon Takes Us Back In Time

A painting titled “The Harvest Moon” (c. 1872) by George Mason depicts farmers harvesting by the light of the Moon in the era before electric lighting. Public Domain

Tonight’s moon is one we’ve all heard of — the Harvest Moon. It’s the full moon that occurs closest to the fall equinox, and since that was only two days ago, this Harvest Moon is a good fit. What makes it special is that for several nights in a row, it rises only about 20-25 minutes later instead of its usual 50 minutes. Successive risings at nearly the same time provided extra light to bring in the harvest in the days before electric lighting.

The moon travels east (to the left for observers in the northern hemisphere) across the sky as it orbits the Earth. The evening crescent sets shortly after sunset, but by the time the moon is half, it’s due south at sunset and sets around midnight. A full moon lies directly opposite the sun — it rises at sunset and stays up all night till the next morning’s sunrise.

The waxing gibbous moon a few days before full rises over the Aerial Lift Bridge in Duluth, Minn. Saturday night. Watch for the big, golden Harvest full moon to rise in the eastern sky around sunset tonight. Bob King

On average, the moon moves its own diameter (½°) each hour for a total of 12° per day. That’s a little more than one fist held at arm’s length against the sky. That eastward motion also causes the moon to rise about 50 minutes later each night. In the evenings following full moon, the moon continues to do its thing, rising later and later. Normally, if it rose tonight at say 7 o’clock tonight, we’d see it rise on Wednesday night shortly before 8 and Thursday around 8:45 p.m.

The angle of the Moon’s path to the horizon makes all the difference in moonrise times. At full phase in September, the Moon’s path is more nearly parallel to the horizon, so successive moonrises are just 20+ minutes apart. At full phase in spring, the path tilts steeply southward, delaying successive moonrises by over an hour. Times are shown for the Duluth, Minnesota, region. Stellarium with additions by the author

Successive moonrise times vary from about 25 to 75 minutes (50 minutes is an average) depending upon the angle the ecliptic makes to the eastern horizon at rise time. The ecliptic is the invisible freeway followed by the sun, moon and planets across the sky created by extending Earth’s orbital plane across the solar system. In September, the ecliptic meets the horizon at a shallow angle around the time of full Moon. As the Moon moves to the east, the path it takes is more nearly parallel to the eastern horizon. It doesn’t dip very far below the horizon for a few nights so moonrise times are closely spaced.

These diagrams show the Harvest Moon’s path on three evenings, September 24–26, from Minneapolis, a mid-northern latitude city, and Nome, in the Arctic. The Harvest Moon “effect” reaches its extreme at high latitudes with moonrise times (shown next to each Moon) that vary only a few minutes from night to night. Stellarium with additions by the author

The farther north you live, the more the moon’s path parallels the horizon. From Nome, Alaska, successive moonrises are delayed by only a few minutes! North of there and moon actually rises earlier each night for a while. Crazy stuff.

I hope you like lots of moonlight because you’re going to be getting a more than usual in the coming nights. Click here to find out when the moon rises from your location. If you really want to get into the old-time spirit, pick the remaining tomatoes in your garden by moonlight this week. I bet there will be plenty enough light to see your way around.