Have you been watching the moon’s nightly progress toward Jupiter this week? Well, tonight’s the big night. We’ll have a full Harvest Moon and it will sit right atop Jupiter. The night’s two brightest characters join together for a wonderful naked eye sight all night long.
Although the actual moment of full moon occurs Thursday morning, it will be fullest during evening hours tonight. The full moon nearest the autumnal equinox is called the Harvest Moon. 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 minutes instead of the usual hour. That means the moon comes up not long after sunset several nights in a row. No doubt this helped farmers harvest their crops into the night back in the days before electricity. Early moonrises at full moon guaranteed nearly continuous light from sunset to sunrise.
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 is tipped at a narrow angle to the eastern horizon. As the moon scoots along the ecliptic at the rate of about one outstretched fist to the east each day, it rises later each night … but not by much. If you’re a lover of moonlight like the old-time farmers, or just enjoy more light for an evening walk, you’ll appreciate the difference an angle can make.
The easiest way to understand the rising time difference is to compare the full moon’s path in fall versus that of spring. The moon is only a little lower below the horizon each night around the fall equinox, so only a short time has to pass between successive moonrises. In spring, the ecliptic cuts the horizon at such a sharp angle, even though the full moon still moves just a fist eastward night to night, it has to “climb” a long way up to the horizon between successive moonrises.
The difference in rise times is amazing. For Duluth, Minn., it’s 19 minutes on the first day of fall and one hour and 20 minutes around the first day of spring. You’ll see for yourself what the farmer looked forward to in times past if good weather prevails in the coming nights. Happy moonbeam harvesting!