September 15-22, 2021: The moon passes evening planets Saturn and Jupiter as a lead in to the Harvest Moon and the geometrical effects of the sky, known as the Harvest Moon Effect.
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by Jeffrey L. Hunt
During the early evening hours three bright planets are visible in the sky. Brilliant Venus is about 8° up in the west-southwest at 45 minutes after sunset. Bright Jupiter and Saturn are in the southeast.
On September 15 and for the next three evenings, the bright moon is near Jupiter and Saturn.
On September 15, the moon, 74% illuminated, is 16° to the lower right of Saturn. On the next evening, the lunar orb, 83% illuminated, is 4.8° to the lower right of the Ringed Wonder. Jupiter is nearly 19° to the upper left of the moon.
On September 17, the moon, 90% illuminated, is 6.4° to the lower right of Jupiter. On the next evening, the bright gibbous moon, is 10.1° to the lower left of the Jovian Giant.
On the chart above, note the location of the moon in the sky, compared to the horizon. For the most part, the moon’s nightly change is roughly parallel to the horizon.
The moon completes one revolution every 27.3 days. On average, the moon moves 13.2° to the east compared to the starry background from night to night. Before the full moon, the moon phase is growing each evening. On average, the moon rises about 50 minutes later each day.
On the chart above, note the moon’s nightly change compared to the horizon at two hours after sunset.
The full moon that occurs nearest the autumnal equinox is known as the Harvest Moon. This year that is September 20. Traditionally, at mid-northern latitudes, the full moon’s light, at this season, aided with the fall harvest.
Near the Harvest Moon, the moon’s orbital plane is lowly inclined with the horizon so that when it moves eastward compared to the starry background, the lunar altitude does not change much each evening. Near the time of the full moon the difference in the moon’s rising time interval from night to night is less than 30 minutes. To demonstrate the short difference in the nightly rising time difference, the table below shows the rising times for Chicago Illinois.
| Moon Rising Times (Chicago, Illinois) | ||
| Date | Rising Time | Rising time difference from previous date (minutes) |
| September 18, 2021 | 6:19 p.m. CDT | |
| September 19, 2021 | 6:46 p.m. | 27 |
| September 20, 2021 (Full) | 7:10 p.m. | 24 |
| September 21, 2021 | 7:32 p.m. | 22 |
| September 22, 2021 (Equinox) | 7:54 p.m. | 22 |
On September 22, evening twilight does not end until 8:21 p.m. CDT. The bright moon is up before the sky is completely dark from natural lighting.
Before the invention of lighted-farm machinery, farmers could continue to work into the early evening with the aid of a bright moon providing enough light to drive a team of horses, gather harvested crops, and generally move about.
Compare this to the rising moon, six months later at the full moon near the Vernal Equinox. On the chart above, compare the rising moons with the autumnal moon rising with the chart at the beginning of the article.
During the springtime full moon, the moon’s orbital plane is highly inclined to the eastern horizon. The difference in nightly rising time is reaching a maximum. The interval between moonrises averages nearly 71 minutes. At mid-northern latitudes, there is little activity in the fields. The last frost date is still about two months away and there is no help from a bright moon for successive evenings of outdoor work. On March 19, 2022, evening twilight ends at 8:35 p.m. CDT. On the night following the spring full moon, the sky is completely dark before the moon rises. The table below shows the moon rising times for the full moon during March 2022.
| Moon Rising Times (Chicago Illinois) | ||
| Date | Rising Time | Rising time difference from previous date (minutes) |
| March 16, 2022 | 5:19 p.m. CDT | |
| March 17, 2022 | 6:27 p.m. CDT | 68 |
| March 18, 2022 (Full) | 7:37 p.m. CDT | 70 |
| March 19, 2022 | 8:48 p.m. CDT | 71 |
| March 20, 2022 (Equinox) | 10:02 p.m. CDT | 74 |
The Harvest Moon Effect occurs at any moon phase occurs when the lunar orbital plane like it is at early evening hours during late September and early October each year. It is most prominent with the autumnal full moon when it is in the public eye. This effect occurred last month and will occur again next month when gibbous moons reach the minimum moonrise time. Later, it occurs with a last quarter moon and then with crescent moons.
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