March 24, 2024: Mercury is at its best evening appearance of the year for northern hemisphere sky watchers. The Full (Worm) moon occurs tonight.
by Jeffrey L. Hunt
Chicago, Illinois: Sunrise, 6:46 a.m. CDT; Sunset, 7:08 p.m. CDT. Check local sources for sunrise and sunset times. Times are calculated by the US Naval Observatory’s MICA computer program.
Mercury at Greatest Elongation
Mercury is at greatest elongation east this evening, tongue-twisting and brain-teasing terminology. Here’s the breakdown: While Mercury is in the western sky this evening, it is east of the sun. When facing the central star in the northern hemisphere, east is left and right is west. When a celestial body sets during the evening, it is east of the sun. Tonight, Sirius and Orion are east of the sun in the southern sky after night falls.
The term “elongation” is a synonym for angle. A planet’s elongation, usually from the sun, is the angular measure the planet makes with the central star. As seen from above the solar system, the sun-Earth-Mercury angle is at its largest and so the name greatest elongation.
Notice on the accompanying chart that Mercury’s orbit is not a perfect circle, but an ellipse. It is the most non-circular orbit of the eight planets in the modern solar system model. The speedy planet was at perihelion, its closest point to the sun a week ago.
Mercury’s Orbital Inclination
Mercury’s best visibility depends on the season, not the elongation. With Earth’s tilt, the plane of the solar system – known as the ecliptic – has a changing angle with the horizon before sunrise or after sunset. Think of riding a bicycle with a bent rim. The tire wobbles from side to side as it spins. In a sense the ecliptic does the same along the horizon. The ecliptic’s largest angle occurs during spring evenings and autumn mornings. At other seasons, angles are shallow and even with a large elongation, the planet sets during brighter twilight.
Tonight, the ecliptic and planet orbits are steeply inclined with the horizon. With a small elongation, Mercury is about 10° up in the west at 45 minutes after sundown. The accompanying chart shows Mercury’s orbit at sunset. The apex of the orbit is nearly above the sunset point, reflecting the ecliptic’s high angle.
This greatest elongation is the smallest evening value (18.7°) of the year. The maximum evening elongation (26.9°) occurs July 21st, but that one is not easily visible. The best morning appearance occurs September 4th (18.0°), occurring four days before perihelion.
Mercury’s May Elongation
The largest morning elongation (26.4°) occurs May 9th, but the ecliptic has a shallow angle with the horizon. Mercury rises 51 minutes before the sun. In bright morning twilight, Mercury is less than 5° up in the east, making the visibility very unfavorable. At sunrise, it is less than 10° above the horizon.
For northern hemisphere sky watchers, spring evenings and autumn mornings are the best times to easily find Mercury.
Summaries of Current Sky Events
Summary for Venus as a Morning Star, 2023-24
Here is today’s planet forecast:
Morning Sky
Morning Moon
An hour before sunrise, the bright moon, 99% illuminated, is about 10° up in the west and over 11° to the lower left of Denebola, Leo’s tail. This Worm moon is at the Full phase in less than 24 hours.
Mars, Saturn, and Venus
Three bright planets are west of the sun, rising before sunup. Mars rises 69 minutes before the sun. About 30 minutes later it is nearly 5° above the horizon. Find it with a binocular.
Saturn, rising 30 minutes after Mars, is lost in the sun’s glare.
Venus, moving toward superior conjunction during early June, follows Saturn across the horizon five minutes after the Ringed Wonder. Fifteen minutes before sunrise, the Morning Star is less than 3° above the horizon. It is difficult to see in the glare of approaching dawn.
Evening Sky
Jupiter and Mercury after Sunset
At 45 minutes after sundown, look westward. Jupiter is “the bright star” less than one-third of the way from the horizon to overhead. Mercury, bright but dimmer than Jupiter, is to the lower right and about 10° up in the sky.
Your extended fist, from pinky knuckle to thumb knuckle, is about 10°. Orient your fist so that the pinky finger is at the west horizon. Mercury is near the thumb. Jupiter is two fists or about 20° to Mercury’s upper left. The planet is bright, but a binocular is helpful with the initial identification, as the planet is shining through the colorful hues of evening twilight.
After tonight, Mercury is about the same height for a few evenings, slightly dimmer than the previous night. Then it dims and appears lower as it races back into brighter twilight. It is visible for the remainder of a month, but a binocular is needed to see it near month’s end.
Worm Moon
On this night of the Worm moon, the lunar orb is over 10° up in the east at this hour, over 15° to the lower right of Denebola. The moon is officially at the Full phase at 2 a.m. CDT tomorrow.
Look for the star Arcturus, nearly 5° up in the east-northeast. Shining as the northern hemisphere’s second brightest star, it rises at sunset in nearly a week.
Jupiter and Uranus
Try to see Jupiter and Uranus in the same binocular field. This is challenging as the moon’s light washes across the sky and seems to cover dimmer stars. Place Jupiter toward the lower right portion of the field. Uranus is toward the upper left.
Use the accompanying chart to identify the bright stars, such as Sigma (σ Ari on the chart), Pi (π Ari) and Rho (ρ Ari) in Aries. Uranus is to the upper left of dim 53 (53 Ari), although the star might be washed out.
Jupiter sets less than four hours after sunset and before midnight.
The moon is in the sky all night, appearing in the western sky before sunrise.
Penumbral Lunar Eclipse
A penumbral lunar eclipse begins at 11:51 p.m. CDT and peaks at 2:12 a.m. tomorrow. While not as beautiful as a total lunar eclipse, the moon passes through Earth’s outer shadow region. Unless observing carefully, the moon’s light is only mildly dimmed. This is a prelude to the April 8 total solar eclipse that is visible across the Americas.
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