2023, September 18: Mercury Closest to Venus, Evening Lunar Crescent

Photo Caption – 2022, June 27: Venus, Mercury, and the crescent moon.


by Jeffrey L. Hunt

Chicago, Illinois:  Sunrise, 6:34 a.m. CDT; Sunset, 6:55 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

Chart Caption – 2023, September 18: Saturn is low in the west-southwest during the early morning hours.

Three bright planets are visible before morning twilight begins.  They are Venus, Jupiter, and Saturn that are along an arc that begins at the east horizon, curves up high in the south, and ends at the west-southwest horizon.  Saturn, the dimmest of the three planets, is low in the west-southwest.  This is becoming a challenging view, not only from obstructions along the horizon, but from the blurring effects of the air near the horizon that reddens and dims the sun and moon.

The planet appears near Skat and Lambda Aquarii (λ Aqr on the chart).

Chart Caption – 2023, September 18: Bright Jupiter is high in the south between Hamal, Menkar, and the Pleiades.

Bright Jupiter is high in the south, retrograding in front of Aries, 13.4° to the lower left of Hamal, Aries’ brightest star, 11.2° to the upper right of Menkar, in Cetus, and 16.0° to the lower right of the Pleiades star cluster.  Jupiter is retrograding, from Earth lapping the planet from an orbital path closer to the sun and at a faster pace than the Jovian Giant.  During the next several mornings, watch the planet pass between Hamal and Menkar.

Venus is low in the east completing the lineup.  It is considerably brighter than Saturn and easily seen near the horizon even from the detrimental sighting effects described earlier.

As twilight picks up, Saturn sets and Jupiter is farther westward.  Venus rises higher in the sky.

Chart Caption – 2023, September 18: An hour before sunrise, Venus and Sirius are the same altitudes in the eastern sky. Mercury is below Regulus in the east.

By one hour before daybreak, the brilliant Morning star, in its interval of greatest brightness, is over 20° up in the east, the same altitude – height above the horizon – as Sirius, over 40° to the right of the bright planet. 

Venus and Sirius, the Dog Star, do not have conjunctions.  The star is too far from the plane of the solar system, where the planets and moon move, for the solar system bodies to appear near it.  For our observing pleasure, we see a brilliant planet and the night’s brightest star at the same altitude in the eastern sky each morning before sunrise throughout the month.

Procyon, known as the Little Dog Star, is above an imaginary line from Venus to Sirius.  It rises about 30 minutes before Sirius at the mid-northern latitudes.  Its name means “before the dog,” because it rises before Sirius.  When Procyon is above the eastern horizon, Sirius rises soon thereafter.

Venus is stepping eastward toward Regulus, the brightest star that is closest to the plane of the solar system.  Frequently the moon and bright planets pass the star. This morning Venus is 15.2° to the upper right of Regulus.  Their conjunction occurs October 9th.

Chart Caption – 2023, September 18: Forty-five minutes before sunrise, Venus, Regulus, and Mercury are in the east.

Mercury is in its best morning appearance of the year. At this hour, the speedy planet is about 4° above the horizon and 8.4° to the lower left of Regulus, too far away to fit into the same binocular field with the star.

In another fifteen minutes, Mercury is nearly 7° above the horizon.  It is noticeably brighter each morning.  Today, the planet is the closest it gets to Venus for this Mercurian appearance.  The separation is 23.1°.  When the speedy planet returns to the morning sky during the new year, the minimum separation is 11.1° during mid-January.

Evening Sky

Chart Caption – 2023, September 18: After sundown, the crescent moon is in the west-southwest below the Scorpion’s claws.

The crescent moon, 14% illuminated, is less than 10° above the west-southwest horizon at 45 minutes after sundown.  Look carefully for Zubenelgenubi, the Scorpion’s southern claw, 6.1° to the upper left of the lunar crescent.  The second claw, Zubeneschamali, is 9.2° above the southern appendage.

Photo Caption, 2022, July 30: The crescent moon with earthshine.

Look carefully at the moon, the night portion, surrounded by the cusps or horns, is softly illuminated by sunlight reflected from Earth’s oceans, clouds, and land, known as earthshine.

Saturn rises before sunset and at this hour, it is over 15° above the east-southeast horizon.  Wait until the sky is darker to see the starry background with the planet.

Chart Caption – 2023, September 18: Two hours after sunset, Saturn is in the southeast near Skat and Lambda Aquarii (λ Aqr).

At two hours after nightfall, the Ringed Wonder is over 25° up in the southeast, 9.5° to the upper right of Skat, the leg, and 9.8° to the right of Lambda Aquarii.  The planet continues to retrograde, moving westward compared to these two stars.  A binocular may be needed to see them.  They do not fit into the same binocular field of view with Saturn.  Start at the planet and move the appropriate directions to find the referenced stars.

By three hours after sundown, bright Jupiter is about 10° up in the east.  During the night the wheel of the sky seems to turn westward carrying the stars and planets with it.  Before twilight begins tomorrow, the three planets are again along the arc of the solar system.


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