2023, September 6:  Mercury’s Inferior Conjunction

September 6, 2023: Mercury is at inferior conjunction between Earth and Sun.  Venus, Jupiter, and Saturn are easily visible during nighttime hours.

2023, September 6: Mercury at inferior conjunction, between Earth and Sun.
Chart Caption – 2023, September 6: Mercury at inferior conjunction, between Earth and Sun.

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by Jeffrey L. Hunt

Chicago, Illinois:  Sunrise, 6:21 a.m. CDT; Sunset, 7:16 p.m. CDT.  Check local sources for sunrise and sunset times. Times are calculated by the US Naval Observatory’s MICA computer program.

Mercury passes between Earth and Sun today.  This is known as inferior conjunction.  The planet races around the sun every eighty-eight days and overtakes and laps our planet every one hundred, sixteen days.

This event is not easily visible by conventional means because Mercury is with the sun.

After today’s inferior conjunction, the planet zips into the morning sky for its best predawn display of the year.  The planet reaches its largest separation from the sun, known as greatest elongation, on the 22nd.  The closest separation it has to Venus is 23.1° on the 18th.  This puts four bright planets that are visible after midnight and before sunrise.

Summaries of Current Sky Events

Summary for Venus as a Morning Star, 2023-24

Here is today’s planet forecast:

Morning Sky

2023, September 6: Jupiter and the moon are in the south during morning twilight.
Chart Caption – 2023, September 6: Jupiter and the moon are in the south during morning twilight.

Of those four planets, Saturn sets early during morning twilight that begins over ninety minutes before daybreak. 

At forty-five minutes before sunrise, the bright moon, 55% illuminated, is high in the south-southeast, 8.9° above Aldebaran, Taurus’ brightest star, and over 20° from bright Jupiter.  The moon is at the morning half phase, called Last Quarter, at 5:21 p.m. CDT, when the lunar orb is below the horizon in the Americas.

Jupiter is retrograding in front of Aries, nearly 16° to the lower right of the Pleiades star cluster.  Use a binocular to see the star cluster in this moonlight.

2023, September 6: Venus, Sirius, and Procyon are in the eastern sky before sunrise.
Chart Caption – 2023, September 6: Venus, Sirius, and Procyon are in the eastern sky before sunrise.

Brilliant Venus, rising over two hours before the sun, is over 15° up in the east-southeast.  It outshines everything this morning except the moon.  The planet gains about six minutes of rising time each morning compared to sunrise.

Look for Sirius, over 40° to the right of Venus and slightly higher.  The Dog Star is too far from the ecliptic, the plane of the solar system, for the planets to pass closely.  So, we see the night’s brightest star and the brightest planet in the eastern sky.  On about the 10th, Venus and Sirius are at about the same altitude during morning twilight.  This lasts for the remaining days of the month.

Notice Procyon, above an imaginary line from Venus to Sirius.  Procyon, meaning “before the dog,” is known as the Little Dog Star.  It rises about 30 minutes before Sirius at the mid-northern latitudes.

In about a week, Mercury appears above the eastern horizon.  Scout out a location with a clear eastern horizon to spot the planet during its best morning appearance of the year.

 Evening Sky

2007, December 1: Late winter in the northern hemisphere shows clouds above the northern polar cap and some above the southern cap.
Photo Caption – 2007, December 1: Late winter in the northern hemisphere shows clouds above the northern polar cap and some above the southern cap. (NASA Photo)

With Mercury moving into the morning sky, Mars is fainter than expected, setting less than an hour after nightfall.

2023, September 6: Saturn is in the southeast during the early evening.
Chart Caption – 2023, September 6: Saturn is in the southeast during the early evening.

Saturn rises before sunset.  At two hours after sundown, the planet is about 20° above the southeast horizon.  It is retrograding in front of Aquarius, 8.9° to the upper right of Skat, the Aquarian’s leg, and 9.0° to the right of Lambda Aquarii.  Use a binocular to see the stars.  They do not fit into the same binocular field with Saturn.  Find the planet and then move the binocular in the direction of each star.

2023, September 6: Jupiter and the moon are in the eastern sky as the calendar day ends.
Chart Caption – 2023, September 6: Jupiter and the moon are in the eastern sky as the calendar day ends.

Jupiter rises in the eastern sky less than two hours, 30 minutes after the sun.  As the calendar day ends, the Jovian Giant is nearly 20° up in the east. The thick crescent moon is rising in the east-northeast.  Capella is the bright star to the moon’s upper left.

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