Shine On Harvest Moon For Me And My Gal

The Harvest Moon from a few years back rises over Lake Superior at Brighton Beach in Duluth, Minn. Photo: Bob King

Autumn blew in last weekend with a chill and flourish of color. This weekend we get fall v2.0 with an appearance by the Harvest Moon, the full moon closest to the autumnal equinox.

The Harvest Moon harks back to our agrarian past when farmers could continue harvesting by its light for several evenings in a row. Because the angle of the full moon’s path to the horizon is very shallow in September and October, the time difference between successive moonrises is only about 20-30 minutes instead of the usual 50-60. With moonrise happening on the heels of sunset, farmers could harvest their crops into the night in the days before electric lighting.

The angle of the moon’s path to the horizon makes all the difference in moonrise times. At full phase in spring, the path tilts steeply southward, delaying successive moonrises by over an hour. In September, the moon’s path is nearly parallel to the horizon with successive moonrises just 20+ minutes apart. Times are shown for the Duluth, Minn. region. Illustration: Bob King

The moon, planets and sun all travel along the ecliptic, an invisible circle in the sky that defines the plane of the solar system. During early fall, the ecliptic runs nearly parallel to the eastern horizon for the northern hemisphere. As the moon scoots eastward along this path at the rate of one fist held at arm’s length each day, its rising times vary by a half hour or less. For several nights in a row a big moon seems to rise at nearly the same time – a constant companion of sorts.

Exactly the opposite happens 6 months later in spring, when the moon’s path is tipped at a steep angle to the horizon. While it moves the same amount each night – one outstretched fist – the moon is much farther below the horizon on successive nights, delaying moonrise by an hour or more. You’ll see the full moon one night and wonder why it’s taking so long to rise the next.

We all love to see a big orange moon on the horizon. If you want to know when it’s coming up in your neighborhood this weekend, go to the U.S. Naval Observatory website , select the link (data for the sun and moon for one day or the whole year) and specify your location. When the table pops up, be sure to add an hour for Daylight Saving Time to the times shown. For Duluth, Minn. the full moon rises at 6:21 p.m. tomorrow night (Sept. 29) or 30 minutes before sunset, 6:46 p.m. on Sunday and 7:14 p.m. Monday. Happy gazing!

The title of this blog refers to Shine On, Harvest Moon, a Tin Pan Alley song from the early 1900s. Click the video to see and hear it sung by two of my favorite comedians – Laurel and Hardy.

7 Responses

  1. lynn

    What a beautiful picture that is Bob I would like that on my wall :-). Laurel and Hardy, you just have to laugh everytime you watch them they are classic lol

    1. astrobob

      Thanks! That’s cool you also enjoy Laurel and Hardy. I have current favorite comedians but I’ll always love these guys.

      1. caralex

        Bob, you say: “the full moon rises at 6:21 p.m. tomorrow night (Sept. 29) or 30 minutes before sunset”

        Is the moon truly full then, if it rises half an hour before sunset, or is that just another effect of the shallow angle of the ecliptic?

        1. astrobob

          For sure it’s because the moment of full moon happens late that evening. If the moon was exactly full near sunset, it would rise closer to sunset. I suspect that the angle also plays a part.

  2. MBZ

    Hey, but you got lucky in February and March. At our age, that ain’t too bad 😉
    You guys enjoy yours up there while we’re still too warm down here at 31.29’N

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