Sky-watchers this week get a rare chance to catch sight of a tricky cosmic target: Mercury. This most elusive of naked-eye planets puts on its best appearance of 2014 in the evening sky starting on Friday.
Locating this baked, barren world is usually a serious challenge, even for seasoned stargazers, because the planet never strays far from the glow of the sun. Mercury is the closest planet to our star—never traveling more than 43.5 million miles (70 million kilometers) in its orbit—so it always sets soon after sundown and rises just before sunrise.
But from now until February 4, Mercury is the farthest it will be from the sun all year—what astronomers call its great eastern elongation. This means folks in the Northern Hemisphere will be able to spot the planet in the twilight sky with the naked eye. It will be relatively high in the southwestern sky, ten degrees (about the width of your fist at arm’s length) above the setting sun.
Mercury will remain visible to the naked eye for up to one hour after sunset until early February, giving sky-watchers a small window of opportunity to catch sight of the planet. Thereafter, the planet resumes its place in the sun’s glare.
Knowing where and when to look will make it easier to spot this speedy little planet. First, find a viewing location that has a totally clear line of sight toward the west. Go out with binoculars about 30 minutes after sunset and scan the southwest horizon for Mercury to appear as a yellow-orange star.
Look for the razor-thin crescent moon 20 degrees above the southwestern horizon at dusk on February 1. Mercury will appear about ten degrees below it. By Sunday evening, the crescent moon will rise to double its distance above and to the left of Mercury.
If you miss this viewing opportunity, you will have to wait until May for another glimpse of Mercury—but your chances won’t be as good then because the planet will be buried deeper in the glow of sunset.