Venus and Jupiter, August 18, 2012

The Harvest Moon


Full moon
Full Moon (NASA Photo)

The Harvest Moon is the full moon that occurs nearest to the autumnal equinox in the northern hemisphere.

by Jeffrey L. Hunt

The Harvest Moon is the moon that occurs nearest the first day of autumn in the northern hemisphere.

In the Central Time Zone, the equinox occurs on September 22 from 2020 – 2030, except for 2027 when it occurs September 23.  Here are the Harvest Moon dates for the same time interval:

Harvest Moons 2020-2030 (CDT) – Full Moons nearest the autumnal equinox

  • October 1, 2020
  • September 20, 2021
  • September 10, 2022
  • September 29, 2023
  • September 17, 2024
  • October 6, 2025
  • September 26, 2026
  • September 15, 2027
  • October 3, 2028
  • September 22, 2029 (11:29 a.m. CDT); Autumnal Equinox (12:38 p.m. CDT)
  • September 11, 2030

Notice that a nearly “pure” Harvest Moon occurs when the full moon and the equinox occurs on the same date in 2029.

Traditionally, the full moon’s light at this season aided the fall harvest that occurred at this time of year at mid-northern latitudes. In times before Daylight Saving Time, the sun set around 6 p.m. (standard time), when clocks became important.

During the harvest time work could run longer than the 12 hours of daylight. So any extra illumination would help during the long work days of the season.

The full moon is bright enough to illuminate the ground so that the human eyes can easily maneuver outside and continue to work without artificial light.

As daylight wanes after late September, the moon’s reflected sunlight was important to help harvesters reap the summer’s bounty.

There’s an astronomical reason for added moonlight at this time of year. Other astronomical concepts are important:

  1. The moon’s is full when it is precisely 180° from the sun.  On nights before the “official” full moon, it may look like its that phase; it is missing 1-2% of its full moon light.
  2. The vernal equinox is the location of the sun in the sky on the first day of spring.  The equinox is the origin (0,0) for at least two coordinate systems.
  3. The autumnal equinox is the location of the sun on the first day of autumn.  Its coordinates are 180° from the vernal equinox (180,0).
  4. The celestial equator is the extension of the earth’s equator into the sky.
  5. The ecliptic is the plane of the solar system.  The sun seems to move along this great circle in the sky. The planets and moon appear to move near the ecliptic.  The moon’s orbit is inclined 5° to the ecliptic.
  6. Seeming to travel along the ecliptic, the sun crosses the celestial equator at the equinoxes.  When the sun is farthest from the equator (23.5° north or south), the solstices occur.
  7. The moon crosses the ecliptic twice a month.  If the sun is located at the spot where the moon crosses, a solar eclipse occurs.  If it crosses 180° from the sun’s location (full moon), a lunar eclipse occurs.

As the autumnal equinox approaches, the sun moves toward its namesake point in the sky. As the sun sets, the vernal equinox point rises.  If the moon is approaching its full phase simultaneously – a rare occurrence – it is approaching the vernal equinox coordinates.

As the list above shows, the nearest full moon to the beginning of the fall season can occur several days before or after the equinox date.

The vernal equinox lacks bright stars nearby to indicate its location.  (The north pole in the sky has Polaris nearby. Several bright stars – Regulus, Spica, Antares, Aldebaran, and Pollux – are near the ecliptic.)  The equinox point is among the faint stars of Pisces, southeast of the “circlet” of dim stars that outlines the Western Fish of the constellation. The bright planets are sometimes nearby to provide cues for the invisible point’s location. During the evenings of harvest time in 2020, Mars is over 25° east of the equinox. During the 2022 Harvest Moon, Jupiter is about 5° east of the point. During 2024, Saturn is about 15° west of the equinox at harvest time; 2° west in 2025; and 12° east in 2026.

When the vernal equinox is near the eastern horizon, the plane of the solar system (ecliptic) makes a minimum angle with the eastern horizon.  As the moon moves eastward, it covers the 13° segment of its orbit compared to the starry background.  Compared to the horizon, the moon does not lose much altitude.

The moon revolves about 13° to the east each day.  It rises later – about 50 minutes each day – and sets later. During the full moon at harvest time, the moon rises only about 30 minutes later on successive afternoons and evenings. This effect occurs for a few evenings after the full phase as well. Consequently, the Harvest Moon illuminates the landscape after sunset around this traditional harvest time.

As the gibbous phase occurs around the time of the autumnal equinox, begin to spot the nightly change of the moon compared to the horizon.  The moon is slightly lower, not as much as the average nightly change, and seemingly a little farther north along the horizon.

This Harvest Moon effect can be observed anytime the vernal equinox is near the horizon and the moon is moving toward that important coordinate, even if the moon is at a waning crescent phase, such as near the time of the vernal equinox in March.

The Harvest Moon gets more notice because of its traditional agrarian role and the publicity the moon receives in the popular press, such as blue moon, supermoon, and such.

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