2021, September 3: AM Crescent, Twins, PM Planets

Astronomy, Sky Watching
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September 3, 2021:  Before sunrise, the crescent moon is in the eastern sky near the Gemini Twins.  Three bright planets shine during evening hours.

Chart Caption – 2021, September 3: The morning crescent moon is 5.2° below Pollux.

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by Jeffrey L. Hunt

Chicago, Illinois:  Sunrise, 6:19 a.m. CDT; Sunset, 7:20 p.m. CDT.  Check local sources for sunrise and sunset times for your location.

For the next two days, the length of daylight is about a minute short of 13 hours.  Daylight is rapidly leaving the mid-northern latitudes as the autumnal equinox approaches. 

Typically, the public press notes that the day and night are the same lengths at the equinox.  At Chicago’s latitude, this occurs on September 25, 12 hours of daylight and 12 hours of night.

Morning Sky

An hour before sunrise, a thin crescent is about one-third of the way up in the east.  It is below the Gemini Twins, Castor and Pollux.  The lunar sliver is 5.2° below Pollux.

Like yesterday morning, look for earthshine on the night portion of the moon.  This is sunlight reflected from Earth’s oceans, clouds, and land.

Evening Sky

Evening Star Venus, Jupiter, and Saturn shine during the early evening hours.  About 45 minutes after sundown, brilliant Venus shines from the west-southwest.  It is only about 8° above the horizon, low enough to be behind the neighbor’s trees or the house next door.

Chart Caption – 2021, September 3: After sunset, Venus nears Spica, approaching their widely-spaced conjunction in two evenings.

Venus is moving toward a widely-space conjunction with the star Spica in two evenings.  This evening, the gap is 3.1°.  Spica is 3.6° to the lower left of the brilliant planet.

Watch Venus move closer to and pass by Spica during the next few evenings. Unlike the pokey big planets, Jupiter and Saturn, Venus moves rapidly compared to the background stars. 

Use a binocular to note that the dimmer star Theta Virginis (θ Vir on the chart) is 3.1° to the upper right of Venus. Venus sets nearly 90 minutes after sunset.

Chart Caption – 2021, September 3: Bright Jupiter and Saturn are in the southeastern sky after sunset.

Farther eastward, bright Jupiter is less than 15° above the southeastern horizon.  It is in front of the stars of eastern Capricornus.  Dimmer Saturn is over 17° to the upper right of Jupiter.  It is in western Capricornus.

Both planets are moving westward compared to the background stars.  Normally, the planets move eastward compared to the sidereal backdrop.  When our planet moves between them and the sun, the distant worlds seem to backup.

While the stars and planets rise in the east and set in the western sky from our planet’s rotation, the solar system’s worlds move eastward against the distant suns.  The illusion of when they appear to move westward against the stars is known as retrograde motion.

By two hours after sunset, Jupiter is the brightest “star” in the sky.  It is higher in the southeast.  Note that it is 2.0° to the upper left of the star Deneb Algedi – “the kid’s tail.” 

Along with Jupiter, only two other stars – Arcturus and Vega – are brighter than Saturn.  The Jovian Giant is nearly 20 times brighter than Saturn.  Because of its proximity to Jupiter, Saturn does not appear very bright.

Detailed Daily Note: One hour before sunrise, the crescent moon (25.8d, 14%) is about 30° above the east horizon.  It is 5.2° below Pollux (β Gem, m = 1.2).  Twenty-five minutes after sunset, Mercury is nearly 5° up in the west-southwest.  Twenty minutes later, Venus – about 8° up in the west-southwest – is 3.6° to the upper right of Spica and 3.1° to the lower left of θ Vir.  Farther eastward, Saturn is over 19° above the southeastern horizon, 17.3° to the upper right of Jupiter.  Both planets are retrograding in Capricornus.  At this hour Jupiter is nearly 14° up in the east-southeast.  Two hours after sunset, Saturn is over 26° above the south-southeastern horizon, 1.0° below υ Cap.  Lower in the sky, Jupiter – over 24° above the southeast horizon – is 3.8° to the upper right of ι Aqr, 0.9° to the lower right of μ Cap, and 2.0° to the upper left of Deneb Algedi. A binocular is helpful to find dimmer stars with the planets.

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