2024, December 5:  Mercury Double Play: Inferior Conjunction, Perihelion

December 5, 2024:  Mercury reaches inferior conjunction and perihelion within 24 hours – double play.  It heads toward an appearance in the eastern predawn sky.

Venus, Mercury, Saturn
Photo Caption – 2012, December 13: Venus, Mercury and Saturn before sunrise.

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by Jeffrey L. Hunt

Chicago, Illinois: Sunrise, 7:03 a.m. CDT; Sunset, 4:20 p.m. CST.  Check local sources for sunrise and sunset times. Times are calculated by the US Naval Observatory’s MICA computer program.

The sun is already setting at its earliest time.  This continues through the 14th.  The latest sunrise begins on the 28th and continues through January 9th.

Mercury at Inferior conjunction

Mercury at inferior conjunction
Chart Caption – 2024, December 5: Mercury is at inferior conjunction between Earth and Sun.

Today, Mercury is at inferior conjunction and perihelion within 24 hours.  At inferior conjunction at 8:18 p.m. Central Time, Mercury is between Earth and Sun.  The planet is lost in the sun’s light.  At this configuration, Mercury rarely passes exactly between the two celestial bodies, known as a transit.  It normally passes above or below the line of sight.

Transit of Venus, 2012
2012, June 5: Venus passes between Earth and the sun. It was observed to make a rare solar transit. The dark circle in front of the sun is Venus.

For a transit of Mercury to occur, it must be crossing the plane and line up in inferior conjunction. Today, Mercury passes 1.3° above the ecliptic, the plane between Earth and Sun.

Should Mercury pass precisely between Earth and Sun, the planet is visible against the bright sun as a dark circle through a telescope with a solar filter like that in eclipse glasses.  This last occurred in 2019 and the next transit is predicted for 2032 for sky watchers across Europe, Africa, southern Asia, and the Indian Ocean basin.

Mercury crossed the ecliptic on December 1st, just a few days earlier than the inferior conjunction.

Mercury at Perihelion

In this exaggerated diagram, the relative positions of perihelion, perigee, perigee, and apogee.
Chart Caption: In this exaggerated diagram, the relative positions of perihelion, perigee, perigee, and apogee.(NOAA diagram)

Mercury is at perihelion, the closest point to the sun.  Its orbit is the most elongated or eccentric of the eight major planets.

Mercury in Predawn Sky

Mercury at inferior conjunction
Chart Caption – 2024, December 25: At 45 minutes before sunrise, Mercury is in the southeast to Antares’ upper left. The crescent moon is in the region near Scorpius’ claws, Zubenelgenubi and Zubeneschamali.

After inferior conjunction, Mercury speeds into the predawn eastern sky.  In a week, it rises 73 minutes before the sun and stands 11° above the southeast horizon at sunrise. On the 19th, it is nearly 10° up in the sky at 45 minutes before daybreak. 

On Christmas morning, Mercury stands nearly 10° above the southeast horizon, nearly 7° to Antares’ upper left.  The star, the brightest in Scorpius, is making its first morning appearance after its solar conjunction.

The Scorpion is crawling across the horizon, reaching westward.  Its classic claws, Zubeneschamali and Zubenelgenubi, are nearly 25° to Antares’ upper right. 

The crescent moon, 26% illuminated, is over 13° to the southern claw’s upper right and about one-third of the way from the south-southeast horizon to overhead.

This late-December appearance is the seventh of the year, three during the evening hours.

Five-Planet Display Near Year’s End

A comparison of Venus, Earth, Moon and Mars. (NASA)
Chart Caption – A comparison of Venus, Earth, Moon and Mars. (NASA)

During this apparition, Mercury is again the fifth planet in a planet display that begins with Venus in the southwestern sky after nightfall. With a Venus-Mars opposition occurring at mid-month, four of the planets are shifting toward a view after sunset, although Mars is about 30° up in the west when the inner-most planet is in the southeastern sky.

While Mercury’s inferior conjunction is not easily visible, it signals that the planet is moving toward a favorable appearance before daybreak near year’s end.

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