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Mercury’s greatest evening elongation January 7

Greatest evening elongation: Chart showing moon and planets, January 4 to 7, 2022.
View larger. | Moon and planets – west after sunset – January 4 to 7, 2022. Mercury will reach its greatest evening elongation – its greatest distance from the sun on our sky’s dome – on January 7, 2022. But watch for Mercury all this week and through the weekend, in the fading twilight glow. It’s near 2 other bright planets, Jupiter and Saturn, and the waxing crescent moon. Notice that the lighted portion of the crescent will point to Mercury on Thursday and Friday evenings (January 6 and 7). That’ll be true no matter where you are on the globe. Chart via Guy Ottewell’s Astronomical Calendar 2022.

Mercury’s greatest evening elongation

Mercury – the most elusive planet – is actually quite bright. When Mercury is visible, it’ll often look brighter than most stars, shining in morning or evening twilight. Mercury is named for the fleet-footed messenger god of the Romans. Today, we know it’s our sun’s innermost planet. We know it moves fast around the sun, completing an orbit in only 88 days. And that’s why we on Earth see Mercury zipping in and out of our sky, always near the sunrise or sunset, morning, evening, morning, evening … back and forth half a dozen times over the course of an earthly year. We never see Mercury high in a dark sky. We never see it far from sunrise or sunset. Our best chances to bag Mercury come when the innermost planet stretches its farthest from the sun from our point of view, either before sunrise or after sunset.

One of those times happens this week. On January 7, 2022, Mercury will reach the end of its tether with respect to the sun in our sky, an event called greatest eastern elongation by astronomers. That position lifts Mercury a bit higher into our western twilight sky at sunset. Watch for it shortly after the sun goes down!

See the moon phase for every day in 2022. EarthSky lunar calendars now available! Going fast!

Look for Mercury in the evening

The exact time of Mercury’s greatest elongation is 11 UTC or 5 a.m. CST on January 7, 2022. So either the evening of January 6, or the evening of January 7, would be a great time to look for Mercury.

At this greatest elongation, Mercury will be 19.2 degrees from the sun and shining at magnitude -0.6. It’ll be brighter than Saturn, which floats above it, but not brighter than Jupiter, which is higher still.

Just remember … Mercury is bright, not faint. It isn’t more noticeable because we typically see it in a twilight sky, where it competes with the glow of sunset or sunrise. It’s also often behind trees, buildings, hills and other obstructions near the western horizon.

Chart comparing Mercury elongatons in 2022.
View larger. | Mercury elongations compared. Gray areas represent evening apparitions (eastward elongation). Blue areas represent morning apparitions (westward elongation). The top figures are the maximum elongations, reached at the top dates shown beneath. Curves show the altitude of the planet above the horizon at sunrise or sunset, for latitude 40 degrees north (thick line) and 35 degrees south (thin). Maxima are reached at the parenthesized dates below (40 degrees north bold). Chart via Guy Ottewell’s 2022 Astronomical Calendar.

How to spot Mercury after sunset

Mercury won’t venture very far above the horizon after sunset. As soon as the sun disappears below your horizon, the clock starts ticking. Will you see the glowing point of light that is Mercury before it follows the setting sun? To find out when the sun sets at your location and what time Mercury sets, use these links to the Old Farmer’s Almanac (U.S. and Canada) or timeanddate.com (worldwide). Or check out Mercury by looking at the online planetarium program Stellarium.

To help you spot Mercury, use the chart above. You’ll see Mercury and Saturn in the sunset glare along with Jupiter and the moon above and the bright stars Fomalhaut and Altair on either side. Can you spot Mercury before it disappears below the horizon? How about Saturn?

If you spot Mercury on any of the dates shown on the chart above (January 4 to 7), follow it for some nights after that. As both Mercury and Saturn edge toward the sunset horizon, they’ll also be edging closer to each other. They won’t quite meet, though, before Mercury sinks back below the horizon, ending this evening apparition. When will that happen? It depends on your location on the globe, your sky conditions, and other factors. But, for most of us, Mercury will probably be gone again by mid-month.

Lifeguard tower in the foreground, crescent moon and Mercury in a twilight sky.
View at EarthSky Community Photos. | Chix RC captured this image on January 3, 2022, from Hermosa Beach, California. See Mercury to the upper right of the crescent? Chix wrote: “A faint young moon at 1% illumination and Mercury.” Thank you, Chix!

A comparison of elongations

Not all of Mercury’s greatest elongations are created equal. Some are greater than others. The farthest from the sun that Mercury can ever appear on the sky’s dome is about 28 degrees. The least distance is around 18 degrees.

Elongations are also better or worse depending on the time of year they occur.

In the autumn for either hemisphere, the ecliptic – or path of the sun, moon and planets – makes a narrow angle to the horizon in the evening. But it makes a steep slant, nearly perpendicular, in the morning. So – in autumn from either hemisphere – morning elongations of Mercury are best. Then Mercury appears higher above the horizon and farther from the glow of the sun. Evening elongations in autumn are harder to see.

In the spring for either hemisphere, the situation reverses. The ecliptic and horizon meet at a sharper angle on spring evenings and a narrower angle on spring mornings. So – in springtime for either hemisphere – evening elongations of Mercury are best. Morning elongations are harder to see.

Diagram showing solar system from above, and Mercury at eastern and western elongation.
We’re looking down at the north side of the solar system plane. In this view, Mercury and Earth circle the sun counterclockwise. We see Mercury at greatest elongation when it’s farther to the side of the sun from our earthly vantage point. At greatest eastern elongation, Mercury is west after sunset. At its greatest western elongation, Mercury is east before sunrise. Image not to scale. Mercury’s mean distance from the sun is about 0.39 times Earth’s distance from the sun. Image via Donna the Astronomer.

Bottom line: Mercury will be at its greatest eastern elongation on January 7, 2022. So this is a great time to spot Mercury in the sky after sunset!

Posted 
January 5, 2022
 in 
Tonight

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