The planet Mercury is often cited as the most difficult of the five brightest naked-eye planets to see. I've always thought this was a bad rap because there are some times during the year when Mercury is surprisingly easy to see.
Mercury is called an "inferior planet" because its orbit is nearer to the sun than that of the Earth. Since it always appears from our vantage point to be in the same general direction as the sun, relatively few people have set eyes on it. There is even a rumor that the famed scientist Copernicus, living in northern Poland, never saw it.
And for those living in the Northern Hemisphere, we're about to move into a very fine "window of opportunity" for seeing Mercury in the evening sky. That window, which will open on Feb. 22 and close after March 12, will provide several good opportunities to see this so-called "elusive planet" with your own eyes. The sky map of Mercury with this column shows how it will appear on Feb. 22 alongside the bright planets Venus and Jupiter.
First, a tip for reference: Remember that held at arm's length, your fist measures about 10 degrees in width so you can use it to make a reasonable estimate of degrees either horizontally or vertically.
If your sky is clear and there are no tall obstructions to your view (like trees or buildings), you should have no trouble in seeing it as a very bright "star" shining with just a trace of a yellowish-orange tinge. Another way to find it is to draw an imaginary line from Jupiter to Venus and then extend that line down toward the horizon; that line will point in the general direction of Mercury.
In the evenings that follow, Mercury will diminish in brightness — slowly at first. But the planet will also be arriving at its greatest eastern point, 18 degrees to the east of the sun, on March 5. This is Mercury's best eastern apparition in 2012, although the elongation angle itself is less than the other two.
In mid-twilight, about 45 minutes after sunset from the latitude 40 degrees north, Mercury will be situated far below and to the right of the far-more brilliant Venus and will appear 9 degrees above the horizon, nearly due west. It will set just after twilight ends. [Video: See Jupiter and Venus in HD]
By this time, Mercury will have faded in magnitude to -0.3 — only about half as bright as it appeared on Feb.22, and yet perhaps easier to see as it will appear higher and be set against a somewhat darker sky.
By the time Mercury arrives at its greatest elongation, it will appear about half-illuminated and the amount of its surface illuminated by the sun will continue to decrease in the days to come. So when it begins to turn back toward the sun's vicinity after March 5, it will fade at a rather rapid pace.
In fact, by the evening of March 12, Mercury's brightness will have dropped to magnitude +1.5; about as bright as the star Castor in Gemini and only one-twelfth as bright as it is now. In telescopes it will appear as a narrowing crescent phase. This, in all likelihood, will be your last view of it, for the combination of its rapid fading and its descent into the brighter sunset glow will finally render Mercury nearly impossible to spot as the tiny orb swings through inferior conjunction with the sun by March 21.
In old Roman legends, Mercury was the swift-footed messenger of the gods. The planet is well named for it is the closest planet to the sun and the swiftest of the sun's family, averaging about 30 miles per second; making its yearly journey in only 88 Earth days.
If you snap an amazing photo of Mercury or any other skywatching target that you'd like to share for a possible story or image gallery, please contact SPACE.com managing editor Tariq Malik at tmalik@space.com.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and is also an on-camera meteorologist for News 12 Westchester, N.Y.
