2021, September 22: Autumnal Equinox

Astronomy, Sky Watching
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September 22, 2021:  The sun’s light is most direct at the equator at 2:21 p.m. CDT, signaling the beginning of the autumn season in the northern hemisphere.

2021, September 22: Arcturus and the Big Dipper are in the western sky after sunset during early autumn.
Chart Caption – 2021, September 22: Arcturus and the Big Dipper are in the western sky after sunset during early autumn.

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by Jeffrey L. Hunt

Chicago, Illinois:  Sunrise, 6:38 a.m. CDT; Sunset, 6:47 p.m. CDT.  Check local sources for sunrise and sunset times for your location.

In the northern hemisphere, the sun’s rays are most direct at the equator at 2:21 p.m. CDT.  This begins the autumn season for northern lands.  The sun’s light shines south of the equator until March 20, 2022.  On that date, the sun’s rays shine directly on the equator and then on regions north of the equator.

Notice from the sunrise and sunset times above that daylight and nighttime are not equal today.  While theoretically correct that the length of daylight and nighttime match, the actual rising and setting are affected by our definitions of sunrise and sunset as well as refraction that either makes the sun look higher or lower in the sky when it is near the horizon.

Daylight continues to shrink until December 21, 2022, when winter begins in the northern hemisphere and our readers south of the equator are at the beach celebrating the northern hemisphere’s winter holidays.

At the equinox the sun rises at the east direction point.  Each day thereafter, it rises south of east, rising farther southward along the horizon each day.  Its southerly course ends at the winter solstice, and the rising point starts northward.

There are celestial signs of the changing season.  The stars of the Orion region of the sky shine brightly in the southeastern sky before sunrise.

In the evening sky, Arcturus, the brightest star in the northern half of the sky, is low in the western sky while the Big Dipper is low in the northwest.  The star Fomalhaut appears in the southeastern sky during the early evening hours.  Bright Jupiter is to the upper right of the star.

Along with Jupiter, Saturn and Venus are in the evening sky this year as the new season begins.  Saturn, dimmer than Jupiter, is to the Jovian Giant’s upper right in the south-southeastern sky. 

Venus sparkles from low in the west-southwest during evening twilight.  It is now setting after the end of evening twilight.

The bright Harvest Moon continues to rise in the eastern sky after sunset.

Happy Equinox!

Detailed Daily Note: One hour before sunrise, the bright moon (15.4d, 98%), among the dim stars of Cetus, is 26.0° above the west-southwest horizon.  The sun is at the autumnal equinox at 2:21 p.m. CDT, signaling the beginning of the new astronomical season in the northern hemisphere.  At this time, the sun’s rays are most direct at the equator.  The sun’s declination is below the celestial equator until March 20, 2022, 10:33 a.m. CDT.  Three bright planets are in the evening sky.  Brilliant Venus is over 8° up in the west-southwest, 2.4° to the lower right of Zubenelgenubi.  Tomorrow evening, Venus passes the star.  This evening, Venus is 26.2° to the lower right of Antares. Venus sets 96 minutes after sunset.  On average its sets about 1 minute later each evening through October 18 when it sets two hours after sunset.  This evening, Jupiter and Saturn are farther eastward. Bright Jupiter is over 20° in altitude above the southeast horizon.  Saturn, 16.2° to the right of Jupiter, is slightly higher than the Jovian Giant, but above the south-southeast horizon. Both planets are retrograding in Capricornus.  By 90 minutes after sunset, the moon (16.0d, 95%) is about 5° up in the east.  At this hour, Jupiter, over 27° above the south-southeast horizon, is 2.8° to the lower right of μ Cap, 1.4° to the upper right of Deneb Algedi, and 1.8° to the upper left of Nashira.  Saturn is at about the same altitude farther south along the horizon and 1.3° to the lower right of υ Cap.

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