2021, September 1: Morning Moon, Venus Targets Spica

Astronomy, Sky Watching
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September 1, 2021:  Before sunrise, the crescent moon is near a star cluster that is visible.  In the western evening sky, Venus targets Spica before their conjunction in four evenings.

2021, September 1: Through a binocular, the crescent moon is near the star cluster Messier 35.
Chart Caption – 2021, September 1: Through a binocular, the crescent moon is near the star cluster Messier 35.

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by Jeffrey L. Hunt

Chicago, Illinois:  Sunrise, 6:17 a.m. CDT; Sunset, 7:24 p.m. CDT.  Check local sources for sunrise and sunset times for your location.

Morning Sky

This morning, before sunrise, the moon’s tour of star clusters continues. Find the lunar crescent near the feet of the Gemini Twins.  In a binocular locate the star cluster Messier 35 (also known as M 35), 1.2° to the right of the crescent.  The stars Propus and Tejat Posterior are in the same binocular field.

Like the Pleiades and Hyades clusters in Taurus, M 35 is a cluster in the Milky Way galaxy. It is about 3,000 light years away.

Stars are formed in bunches. Like the Pleiades, M 35 has some bright blue stars, in addition to giant yellow and orange stars that indicate this cluster is older than the famous Taurus star cluster.  According to astronomical theory, blue stars burn their nuclear fuels quicker than the sun.  When the hydrogen at their cores is exhausted, they begin to burn helium, swell in size, and shine as bright yellow or orange stars, so-called red giants.  The more red giants that are present, the older the cluster.

Evening Sky

Chart Caption – 2021, September 1: Forty-five minutes after sunset, Venus is 4.7° to the right of Spica.

In the evening sky, Venus begins its approach to Spica before their conjunction in four evenings.  About forty-five minutes after sunset, find the planet about 8° above the west-southwest horizon.  It is 4.7° to the right of Spica.  Each evening, notice that Venus is closer to the star.

Chart Caption – 2021, September 1: Two hours after sunset, use a binocular to find the starry background with Jupiter and Saturn.

Farther eastward, Jupiter and Saturn are in the southeastern sky. After Venus, Jupiter is the next brightest “star” in the sky this evening. Venus, Saturn, and Jupiter are visible simultaneously, until Venus sets nearly 90 minutes after sundown.

Jupiter and Saturn are in front of the background stars of Capricornus in the southeast. Saturn is 17.5° to the upper right of Jupiter.  To gauge their separation in the sky, extend your arms fully.  Make fists and place your thumbs together.  Hold your left hand so that Jupiter is near the left pinky finger.  Tilt your hands up slightly, Saturn is near the pinky finger on your right hand.

Jupiter and Saturn are retrograding, appearing to move westward compared to the distant sidereal background.  This movement is an illusion as our planet passes between the planets and the sun.  The backwards direction continues for the next month. Jupiter and Saturn move slower against the background stars than Venus. The changing place of Venus against the starry backdrop is easily spotted.  The giant planets move slowly and a few evenings pass before their movement is noticeable with a binocular.

By two hours after sunset, Jupiter and Saturn are higher in the sky in the southeast.  Use a binocular to spot the distant stars.  Look again later in about a week to note the planets’ changing places compared to the starfield.

Detailed Daily Note:One hour before sunrise, the crescent moon (23.8 days after the New phase, 30% illuminated) is nearly 50° up in the east, among the stars of Gemini, 3.8° above Tejat Posterior (μ Gem, m = 2.8).  None of the bright, naked-eye planets are visible at this hour, although Uranus and Neptune are in the sky.  Uranus (m = 5.7) is over two-thirds of the way up in the south.  It is to the right of a line from Pi Arietis (π Ari, m = 5.2) to Sigma Arietis (σ Ari, m = 5.5), 1.0° to the upper right of σ Ari.  Neptune (m = 7.8) is over 23° up in the west-southwest, mixed in a starfield of 6th magnitude stars, 4.7° to the upper left of Phi Aquarii (φ Aqr, m = 4.2). The sun is in the sky for 13 hours, 7 minutes.    Darkness, the time between the end of evening twilight and the beginning of morning twilight, is 7 hours, 36 minutes long. Four of the naked eye planets are in the evening sky.  Mars is not visible.  It is only 12° east of the sun, setting a few minutes after Civil Twilight.  Mercury (m = −0.1), 24° east of the sun in heading toward its greatest elongation, is about 4° up in the west-southwest at 25 minutes after sunset.  The ecliptic is tilted at a low angle compared to the western horizon and the planet is nearly 1° below the ecliptic and moving farther south during this apparition.  This evening the planet sets nearly 50 minutes after sunset.  Forty-five minutes after sunset, Venus – 40° east of the sun – is about 8° up in the west-southwest, 4.7° to the right of Spica (α Vir, m = 1.0) and 2.3° to the lower right of Theta Virginis (θ Vir, m = 4.4). Through a telescope Venus displays an evening gibbous, 15.2” across and 73% illuminated. Venus races eastward at over 1.1°, from evening to evening.  Saturn and Jupiter are in the southeastern sky at this hour.  Saturn is about 19° up in the southeast.  Jupiter – 17.5° of ecliptic longitude east of Saturn and 125.9° east of Venus – is 13.0° up in the east-southeast.  Two hours after sunset, Saturn is over 26° above the south-southeast horizon, 1.0° below Upsilon Capricorni (υ Cap, m = 5.1). Jupiter – nearly 24° above the southeast horizon, 3.5° to the upper right of Iota Aquarii (ι Aqr, m = 4.2), 0.7° to the lower right of Mu Capricorni (μ Cap, m =5.1), and 2.2° to the upper left of Deneb Algedi (δ Cap, m = 2.8).  This evening Jupiter is on a line between the two stars. Both planets are retrograding in Capricornus.  Through a telescope, Jupiter is 48.6” across, while Saturn’s globe covers 18.3”.

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