Tonight – November 23, 2017 – watch for the planet Mercury, which lurks a short hop beneath the planet Saturn in the western sky after sunset. Mercury, sometimes called the most elusive planet, 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 pretty good time to catch Mercury, because Mercury is swinging to its greatest evening elongation, or its greatest angular distance from the setting sun. Depending on where you live worldwide, Mercury will reach its greatest elongation on November 23 or 24.
Mercury first entered our evening sky on October 8, 2017. Its evening reign will end on December 13, 2017, 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 in May 2016).
At its greatest evening elongation (November 24 at about 0 UTC), Mercury reaches a maximum of 22o east of the sun. For reference, your fist at an arm length spans about 10o of sky.
So Mercury is now farthest east of the sun for this evening apparition, shining rather low in the west at sunset and staying out for its maximum time after sundown. At mid-northern latitudes, Mercury sets somewhat more than an hour after sunset. At the equator, Mercury sets about one and one-half hours after the sun; and at temperate latitudes in the Southern Hemisphere, Mercury sets about one and three-quarter hours 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. But don’t mistake Saturn, which shines higher up in the western sky, for Mercury. Although Mercury shines a solid two times brighter than Saturn, Mercury sits deeper in the glow of evening twilight.
The waxing crescent moon is the first celestial body to pop out after sunset. Remember that the lit side of the moon points in the general direction of Saturn and Mercury’s place near the horizon. If you can’t see Mercury or Saturn with the unaided eye, by all means try using binoculars, if you have them.
If you miss Mercury (and/or Saturn) this evening, try again after sunset for the next week or two. Mercury and Saturn will come to within 3o of one another on November 28. Three degrees on the sky’s dome is about the width of your thumb at an arm’s length from your eye. That’s a small enough gap for these two worlds to take stage in a single binocular field of view.
Watch for Mercury (Saturn) after sunset on November 28, and keep watching as these two worlds come closer together
Bottom line: Given clear skies, try your luck at catching Mercury (and Saturn) in the western sky after sunset.