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Angles of Sun and Moon

by Steve Beyer on


Harvest Moon rising.
Harvest Moon Rising
Credit: Wikipedia User: Roadcrusher

Special alignments of sun and moon catch our attention this month. The harvest full moon of Friday, September 16, and the start of the Northern Hemisphere’s autumn Thursday, September 22, each show us interesting angular juxtapositions specific to this time of year.

Friday, September 16, is the night of the harvest moon. It’s defined as the full moon occurring nearest the autumn equinox, the seasonal marker when most farmers in this part of the world are in harvest mode. Aside from obvious advantages of having bright moonlight sustaining harvesting operations through the night, there’s an added advantage with moonlit harvesting around the September equinox.

During final weeks of summer and early autumn, the ecliptic makes a relatively low angle with our eastern horizon compared with other times of the year. That provides shorter delays and less darkness between sunset and moonrise over several evenings just after the night of the Harvest Moon. The effect was a bonus to farmers in former times, providing some extra moonlit minutes for harvesting.

On September 16, the harvest moon rises at 7:10 p.m. EDT in New York City, eight minutes after sunset. The next evening's sunset is at 7 p.m., followed 47 minutes later by the rise of a bright gibbous moon (virtually indistinguishable from a full moon by eye). For comparison, this past March 24, one night after full moon with the ecliptic in our eastern sky close to its maximum angle from the eastern horizon, moonrise was one hour and sixteen minutes after sunset, a 62% longer span between sunset and moonrise than we experience this month the night just after harvest moon.

On the night of the 16th, that full harvest moon is in the constellation Pisces, just south of the asterism known as the Great Square of Pegasus, representing the body of that mythological flying horse.

Autumn begins in the northern hemisphere at 10:21 a.m. EDT, Thursday, September 22. At that time, the sun is above the equator and over the Atlantic Ocean, about 300 miles northeast of the Brazilian coast. At that moment, the angle between earth’s rotational axis and a line between centers of our planet and the sun is exactly equal to 90° of arc. Afterward, the northern part of earth’s axis increasingly leans away from the sun’s direction until start of our northern winter on December 21.  After that, the angle between earth’s  axis and the planet’s equatorial plane again will decrease, eventually bringing us to the vernal equinox of 2017, and start of another cycle of seasons.

September is also a month of significant transitions prompted by the accelerating drop in the sun’s daily position in our sky.

Lowering of the sun’s sky angle diminishes duration and intensity of its energy reaching us. By September surplus warmth stored in ground and sea during springtime has been sapped. Beach attendance is down, the new academic year is underway, and crops will soon need harvesting.

Increasing speed of the Sun’s apparent descent climaxes with the autumn equinox September 22. That day everyone on the planet experiences essentially 12 hours with the sun above, and 12 hours of the sun beneath the horizon. The sun continues to appear lower in our sky each day after the equinox, but the daily rate of that angular decline slows until the beginning of winter when the changes reverse and daily duration of sunlit hours for us start to increase.