On This Harvest Moon

On September 11th through the 14th be sure to look to the east at sunset and behold the most famous of all full moons, the Harvest Moon. Its nostalgia goes back to the days before electricity, when it was used as a night light by farmers to work longer in the fields to get the fall crops in, hence the name. But the Harvest Moon stands out for other reasons as well.

The Harvest Moon is identified as the full moon closest to the autumn equinox. All full moons rise at sunset, and on average they rise 50 minutes later each succeeding night. But at mid-northern latitudes like where we live, the Harvest Moon rises only 25 to 30 minutes later for several evenings in a row. So there is no long period of darkness between sunset and moonrise, and farmers back in the day could continue working their fields uninterrupted by the shorter days of autumn. This shorter rise time also occurs on the following full moon, called the Hunters Moon.

The reason for the shorter time period between sunset and moon-rise is hard for me to grasp, but the simplest explanation I’ve seen is that the ecliptic (the path that the moon moves through the sky as seen from Earth) makes a narrow angle with the evening horizon around the time of the autumn equinox. This narrow angle results in a shorter-than-usual time between moon rises, and the full moon phase appears to extend for several days.

The Harvest Moon has a reputation of being bigger and brighter than other full moons, but this is probably that nostalgia thing kicking in. Actually, all full moons appear larger when they first clear the horizon. The larger appearance is your brain being tricked by an optical illusion called the Moon Illusion. It may satisfy you to know that the astronomy guys are still fighting over exactly why this occurs. There are at least 8 hypotheses out there, and the debate is lively.

The Harvest Moon (and other full moons) often appears yellow or orange when it first rises. This is due to the fact that you are looking through more atmosphere when you look towards the horizon than when you look overhead. The thicker atmosphere lets more red light pass through to your eyes, and other factors like humidity and dust in the air makes for varied color at moon-rise.

Try to make time to see the Harvest Moon rise this year and enjoy its beauty and tradition. Maybe even make it a family tradition with the kids. There is an old Chinese saying that “when the moon is full, mankind is one.”

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