Shine on Harvest Moon

As this week begins, the moon is in waxing gibbous phase, on its way to being full on the morning of September 23. This full moon occurs just six hours after the fall equinox, which is at 10:13 pm the previous night.  Therefore, this is this year’s Harvest Moon.  Every year, the full moon nearest to the fall equinox is the Harvest Moon, even if the two don’t coincide as well as they do this year.  If the full moon occurs very early in September, the Harvest Moon is the full moon of early October.  To understand why the full moon nearest the fall equinox would be special to early farmers, we need to understand some celestial geometry. 

Consider two geometric planes. One is your horizon, a flat plane tangent to the Earth at your location.  The other is the plane of the Earth’s orbit around the sun.  This plane, projected against the sky, is called the ecliptic; we see the Sun shift position along this plane throughout the year as we orbit it.  The solar system as a whole is so flat that all planets orbit in nearly the same plane.  Because the moon formed from a collision between Earth and a Mars-sized object, it orbits Earth with five degrees of Earth’s orbital plane.  (Had the moon formed with the Earth, it would orbit in the plane of Earth’s equator.)  Thus the sun, all the planets, and even our moon always appear near the ecliptic in the sky.   

On winter and spring evenings, the ecliptic forms a steep angle to the horizon.  In the northern hemisphere, this is particularly true in March, as winter turns to spring.  With that steep angle, the moon’s daily displacement along the ecliptic causes it to rise just over an hour later each day.  For example, moonrise on March 29, 2010, in Houston was at 7:33pm CDT; while on the next night the Moon rose at 8:40.

On summer and autumn evenings, however, the ecliptic intersects the horizon at a shallow angle.  For observers in the northern hemisphere, this is especially true in September, when summer turns to autumn.  With that shallow angle, the time of moonrise does not change as much due to the moon’s daily displacement along the ecliptic.  If the moon is rising in the east at dusk, it will rise only about a half hour later for several days in a row.  For example, moonrise on Wednesday night, September 22, is at 6:44pm CDT.  The Moon rises on Thursday, September 23, at 7:14pm, and on Friday, September 24, at 7:45pm. 

Harvest Moon
Creative Commons License photo credit: Jay Scott Photography

This was a great help to early farmers bringing in the harvest.  On a typical evening, work would have to cease at nightfall. A full moon, though, meant that a new source of light rose right as the sun set.  Thus, harvesters could continue to work into the night by moonlight, without having to stop.  And at the Harvest Moon, the moon would rise near sunset for a few days in a row.  Harvesters had several days  of round the clock labor to bring in everything their fields had produced, leaving as little as possible to wither on the vine. 

Time and the advance of technology have diminished our connection to the cycles of nature; for many of us the coming full moon is just one of the twelve full moons this year.  However, in this month when many of us have returned to work from vacation, and have even taken time to celebrate laborers, we can reflect on how the light of our nearest neighbor helped laborers of old harvest their fields.

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