2021, August 20: Five Planets, Harvest Moon Effect

Astronomy, Sky Watching
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August 20, 2021: Five bright planets are visible after sunset this evening.  During the next few evenings, a month before the Harvest Moon, get a preview of the harvest moon effect.

2021, August 20: Through a binocular, Mercury is 1.9° to the upper left of Mars.
Chart Caption – 2021, August 20: Through a binocular, Mercury is 1.9° to the upper left of Mars.

by Jeffrey L. Hunt

Chicago, Illinois:  Sunrise, 6:04 a.m. CDT; Sunset, 7:43 p.m. CDT.  Check local sources for sunrise and sunset times for your location.

This evening, the five brightest planets are visible again, although Mars and Mercury are a challenge to see.  They are low in the sky, twenty-five minutes after sunset.  A binocular is needed to first locate Mercury.  Dimmer Mars is 1.9° to the lower right of the speedy.

Chart Caption – 2021, August 20: Saturn is above the moon and in the same binocular field as the lunar orb. Jupiter is to the lower left of the Ringed Wonder.

If you’ve been looking for the five planets and had difficulty finding Saturn.  This evening the moon is 4.7° below the Ringed Wonder.  Both fit into the same field of a binocular.  Place the lunar orb at the bottom of the field of view.  Saturn is near the top of the field.

Bright Jupiter, is 18.3° to the lower right of Saturn.  One night after its opposition, Jupiter is in the sky nearly all night.  Find it in the southwest as morning twilight begins tomorrow.

Chart Caption – 2021, August 20: Venus and Spica are in the western sky.

Venus is the brilliant “star” shining in the western sky after sunset.  About 45 minutes after sundown, it is about 8° up in the sky.  It is moving toward Spica that is 18.5° to the upper left of the planet.

Watch the bright moon for the next few evenings.  It is in a different place in the sky each evening, but it is nearly the same height (altitude) above the eastern horizon.  This is known as the “Harvest Moon Effect.”

The Harvest Moon is the full moon nearest the first day of autumn in the northern hemisphere.  This year it occurs on September 20. Traditionally, the bright moonlight was used to bring in the harvest.  For a few evenings the moon seemed to hang in the same place in the sky after sunset.

Astro-geometry is the reason for this.  Each evening the moon moves eastward compared to the stars.  Tomorrow evening, the moon is near Jupiter.  Compared to the horizon, the moon is nearly the same altitude for a few evenings.

The moon is approaching the vernal equinox.  The term is used to name the first day of spring and it is the origin of the solar system’s coordinate system in the sky.  The sun is at the vernal equinox point in the sky when spring begins.

Now as the sun approaches the autumnal equinox, that point in the sky is near the horizon in the west and the vernal equinox is approaching the eastern horizon.  When this geometry occurs and the moon is approaching the vernal equinox, the lunar orb’s altitude in the eastern sky does not change much.  The harvest moon effect can be observed whenever the astro-geometry syncs with the moon’s location.  Get a preview of the lingering bright moon in the easter sky during the next few evenings.

Detailed Daily Note:One hour before sunrise, bright Jupiter is nearly 11° up in the west-southwest.  Twenty-five minutes after sunset, Mercury (m = −0.4) is less than 4° up in the west, 1.9° to the upper left of Mars.  Again, this evening all five planets are visible at this time interval after sunset with an optical assist from a telescope or binocular. Saturn is easier to find with the help of the moon.  The Ringed Wonder is nearly 5° above the lunar orb.  Twenty minutes later, Venus is nearly 8° up in the west, 1.1° to the lower left of Zaniah, and 18.5° to the lower right of Spica that is over 12° above the west-southwest horizon.  Venus continues its eastward trek along the ecliptic stepping nearly 1.2° from evening to evening.  The Venus – Jupiter gap is 141.4° of ecliptic longitude.  Through a telescope Venus shows an evening gibbous that is 76% illuminated and 14.1” across.  At this hour the moon (12.5d, 97%), over 10° up in the southeast, is 4.7° below Saturn.  Jupiter, 18.3° of ecliptic longitude east of Saturn, is about 8° up in the east-southeast. At this time during the next few evenings, notice the “Harvest Moon Effect.”  Normally, we think of the Harvest Moon that occurs nearest the autumnal equinox. Traditionally, in the northern hemisphere, the harvest moon signaled the end of the agricultural season, the harvest time.  The full moon’s light aided in cutting and gathering the summer’s bounty after sunset.  There is an astronomical reason for this and it can be observed during most months, although the moon is not full at these times.  When the moon approaches the vernal equinox in the sky (coordinates 0° ecliptic longitude, and 0° ecliptic latitude) and that point is near the eastern horizon, the moon’s altitude is about the same for a few evenings.  The moon moves eastward along its path, but that path is nearly parallel to the eastern horizon, from mid-northern latitudes.  Each night starting this evening and through August 24, watch the moon move eastward compared to the bright planets and note its location compared to the horizon.  Its azimuth is lower – farther north – each evening, but its altitude does not change much from night to night. Tonight, as midnight approaches, the moon is over 25° up in the south, to the lower left of Saturn.  Use a binocular to see the Ringed Wonder in the starfield, 1.3° to the lower left of υ Cap.  Jupiter, over one-third of the way up in the south-southeast, is 2.1° to the upper right of ι Aqr, 1.0° to the lower left of μ Cap, and 3.5° to the upper left of Deneb Algedi.

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