Tonight – December 10, 2016 – watch for the planet Mercury, sometimes called the most elusive planet, in the west after sunset. Mercury is the innermost planet of the solar system and never strays far from the sun in Earth’s sky. For much of the time, this world is lost in the sun’s glare. But right now is a good time to catch Mercury, because Mercury is on the verge of its greatest evening elongation, when it’ll appear at its greatest angular distance from the setting sun. Depending on where you live worldwide, Mercury will reach its greatest elongation on December 10 or 11.
Mercury has been in our evening sky since October 27, 2016. Its evening reign will end on December 28 when the planet crosses (more or less) between us and the sun and thus enters the morning sky (it won’t pass directly between us and the sun this time, by the way; if it did there’d be a Mercury transit, like the one last May).
At its greatest evening elongation (December 11 at about 5 UTC), Mercury swings to a maximum of 21o east of the sun. For reference, your fist at an arm length spans about 10o of sky.
So Mercury is now at its highest for this evening apparition, in the west at sunset. It stays out for its maximum time after sundown. At mid-northern latitudes, Mercury sets about 80 minutes after sunset; and south of the equator, Mercury sets about 90 minutes after the sun. Click here for an almanac giving you Mercury’s setting time in your sky.
If all goes well, you might see Mercury low in the west with the eye alone within an hour after sunset. Look first for dazzling Venus, the sky’s brightest planet, starting about 30 minutes after the sun goes down. Venus will give you some clue of Mercury’s whereabouts, and, once you spot Venus, you can use binoculars to scan for Mercury closer to the western horizon.
Although Mercury is nearly 3 times brighter Mars, you might see Mars before you see Mercury. That’s because Mercury lurks lower in the sky and closer to the glare of sunset. If you see Mars first, draw an imaginary line from Mars through Venus to locate Mercury near the horizon.
If you miss Mercury this evening, try again after sunset for the next several days. At nightfall this evening, note where Mars and Venus reside in your sky. Then draw an imaginary line from Mars through Venus and down to the horizon. Remember this spot on the horizon, and look for Mercury in this area some 45 to 60 minutes after sunset tomorrow or the next few days. If you can’t see Mercury with the unaided eye, by all means try using binoculars, if you have them.
Bottom line: Given clear skies, it’ll be easy to view Venus and Mars at nightfall and early evening around December 10, 2016. It’ll be more of a challenge to catch Mercury as dusk is giving way to darkness, but look as soon as the sky begins to darken after sunset.