2023, September 21: Mercury Near Greatest Elongation, Evening Ophiuchan Moon

September 21, 2023: Mercury is putting on its best morning display of the year in the eastern sky.  The nearly half-full moon is in front of Ophiuchus, above Scorpius.

Venus, Mercury, Moon, Spica, November 16, 2020
2020, November 16: Brilliant Venus shines in the east-southeast during morning twilight. It is 3.8° to the upper left of Spica and 13.0° to the upper right of Mercury.

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by Jeffrey L. Hunt

Chicago, Illinois:  Sunrise, 6:37 a.m. CDT; Sunset, 6:50 p.m. CDT.  Check local sources for sunrise and sunset times. Times are calculated by the US Naval Observatory’s MICA computer program.

Summaries of Current Sky Events
Summary for Venus as a Morning Star, 2023-24

Here is today’s planet forecast:

Morning Sky

2023, September 20: Venus and Sirius are about the same altitude – height above the horizon – at during morning twilight.
Chart Caption – 2023, September 20: Venus and Sirius are about the same altitude – height above the horizon – at during morning twilight.

Mercury nears its largest separation from the sun, known as greatest elongation, tomorrow.  This is the best morning display of the year.  The planet is only 17° from the sun.  This favorable view is from the ecliptic’s large angle with the eastern horizon.  The elongation is relatively small as Mercury nears its closest point to the sun, known as perihelion.

At one hour before sunrise, the speedy planet is about 5° above the eastern horizon and 10.3° to the lower left of Regulus, Leo’s brightest star.  Mercury brightens each morning and is noticeably brighter than the star.

Brilliant Venus, nearly 25° up in the east, is stepping eastward in front of Cancer, 13.6° to the upper right of Regulus.  Their conjunction occurs October 9th.  Watch the gap close each morning.

Venus and Sirius, night’s brightest star and about 40° to the right of the planet in the southeastern sky, are at about the same altitude – height above the horizon – this morning, continuing during the balance of September.

Look for Procyon, above an imaginary line from Venus to Sirius.

2023, September 21: Before daybreak, Jupiter is high in the southwest, retrograding in front of Aries.
Chart Caption – 2023, September 21: Before daybreak, Jupiter is high in the southwest, retrograding in front of Aries.

Farther westward at this hour, Jupiter is high in the southwest, retrograding in front of Aries, 13.4° to the lower left of Hamal, the constellation’s brightest star, 11.2° to the upper right of Menkar, Cetus’ nostril, and 16.1° to the lower right of the Pleiades star cluster, part of Taurus.

As Jupiter seems to move westward against the starfield, it crosses an imaginary line from Menkar to Hamal.  Watch this alignment slowly occur during the next several days.

2023, September 21: Forty-five minutes before sunrise, Venus, Mercury, and Regulus are in the east.
Chart Caption – 2023, September 21: Forty-five minutes before sunrise, Venus, Mercury, and Regulus are in the east.

Look again for Mercury at forty-five minutes before sunrise, when it is higher in the sky.

Evening Sky

Mars from Hubble during 2018
Photo Caption – NASA’s Hubble Space Telescope photographed Mars on July 18, 2018, during a dust storm and near its closest approach to Earth since 2003. (NASA photo)

Dim Mars continues to slowly slide into brighter evening twilight, setting about forty minutes after the sun sets.

2023, September 21: The moon is to the upper left of Antares, Scorpius’ bright stars after sundown.
Chart Caption – 2023, September 21: The moon is to the upper left of Antares, Scorpius’ bright stars after sundown.

An hour after sundown, the waxing crescent moon, 42% illuminated, is only 15° above the south-southwest horizon, in front of Ophiuchus.  The phase is half-full (First Quarter) tomorrow at 2:32 p.m. CDT.

This evening the lunar orb is 8.7° to the upper left of Antares, the Scorpion’s heart and 10.5° to the upper right of Shaula and Lesath, also known as the Cat’s Eyes, at the tail.

Ophiuchus, the Snake Handler, extends southward, from high in the sky. It seems to be standing on the plane of the solar system, also known as the ecliptic, where the sun, moon, and planets appear.  In Ophiuchus the ecliptic is nearly 19° long, compared to nearly 6° in Scorpius.  This year the sun appears in front of Scorpius from November 24th to the 29th and in Ophiuchus from November 30th through December 17th.

2023, September 21: An hour after sunset, Saturn is in the southeast in front of Aquarius.
Chart Caption – 2023, September 21: An hour after sunset, Saturn is in the southeast in front of Aquarius.

Farther eastward from the moon, Saturn is nearly 20° up in the southeast, 9.6° to the upper right of Skat, meaning “the leg,” and 10.0° to the right of Lambda Aquarii (λ Aqr on the chart).  The Ringed Wonder is retrograding – appearing to move westward compared to the stars – in front of Aquarius.  This is caused by Earth overtaking and passing between the distant planet and the sun.

Bright Jupiter rises less than two hours after nightfall.  An hour later it is over 10° above the eastern horizon.  By tomorrow morning, it is high in the southwestern sky.

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