We now have a fine opportunity to view the planet that many astronomy guide books refer to as the most difficult of the naked-eye planets to see. The planet in question is Mercury.
Beginning now and running through the end of the second week of March, this somewhat overgrown version of the moon will have an evening appearance about as favorable as we northerners ever get to see.
In general, the most favorable elongations of Mercury — morning or evening — are those when the planet rises or sets in a dark sky, and that situation will occur for a five-day interval beginning later this week. From March 5 through March 10, Mercury will set at, or shortly after the end of evening twilight, more than 1 hour 30 minutes after sunset for mid-northern latitudes.
A bad rap
Mercury is popularly known as the “elusive” planet. Read what the New Handbook of the Heavens — considered one of the best guides to the stars for the amateur astronomer — said about Mercury:
“It stays close to the sun like a child clinging to its mother’s apron strings, because you must look for it so soon after sunset or before sunrise, there was a famous astronomer, Nicolaus Copernicus, who never saw the planet in all of his life.”
And yet, despite its reputation, Mercury is not really that difficult to spot; just find a reasonably unobstructed horizon. A clear, haze-free sky also helps. The remark about Copernicus is rather ironic, for he was the man who pushed hard for placing the sun and not the Earth at the center of our solar system.
Although the climate of Copernicus’ homeland (Poland) tended to be rather cloudy and misty, one would have to believe that such a noteworthy figure in the field of astronomical calculation must have surely tried on those occasions when the weather was more favorable. Indeed, Mercury was far from impossible to glimpse during elongations as favorable as the upcoming one.
Venus and the moon direct you to Mercury
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During the first two weeks of March, Mercury will have an advantage in that it will be positioned not very far from the most dazzling of all the planets, Venus. In fact, we strongly encourage you to use Venus as your landmark to locate Mercury. And on March 1, another celestial object will help point the way to this rocky little planet: the moon.
Currently, Mercury sets about 90 minutes after sunset; about a half hour after sundown look for it about 10 degrees above the west-southwest horizon. Your clenched fist held at arm’s length measures roughly 10 degrees, so “one fist up” from the horizon should bring you to Mercury. You’ll have no trouble in seeing it as a very bright “star” glowing with just a trace of a yellowish-orange tinge.
On March 1, Mercury is shining at a very bright magnitude of –1.0. In fact, among the stars and planets, Mercury will rank only behind Venus, Jupiter and Sirius (the brightest star) in terms of brightness.
But to make absolutely sure that you’re looking at Mercury, brilliant Venus will be hovering about 15 degrees (“1.5 fists”) above and slightly to the right of it, while floating about 10 degrees (“one fist”) above and slightly to Mercury’s left will be a slender, 5% illuminated, waxing crescent moon, almost mimicking the smile of a Cheshire cat against the twilight sky; a great occasion to make a positive identification of the so-called elusive planet using our nearest neighbor in space as a pointer.
After March 1, the moon will quickly move away to the east, but Mercury will continue to interact with Venus, drawing closer to it, while moving to its lower left. In the evenings that follow, Mercury will diminish — slowly at first — in brightness, but it will also be reaching its greatest elongation, 18-degrees to the east of the sun, on March 8. Shining then at a magnitude of -0.2 (just a trifle brighter than the similarly hued star Arcturus), Mercury should be readily visible, appearing low in the western sky and setting more than 1.5 hours after the sun.
Rapid fade out
Finally, on the evening of March 12, Mercury and Venus will be closest to each other, separated by 5.5 degrees, and appearing almost side-by-side — Mercury positioned to the left of Venus — as they descend down the western sky. By that time, Mercury will have faded to magnitude +0.7.
Mercury, like Venus, appears to go through phases like the moon. Right now, it resembles a gibbous phase, about 73% illuminated, which is why it currently appears very bright. By the time it arrives at its greatest elongation, it will appear less than half-illuminated (44%) and the amount of its surface illuminated by the sun will continue to decrease in the days thereafter. So, when it begins to turn back toward the sun’s vicinity after March 8th, it will fade at a rather rapid pace.
In fact, on the evening of March 15, Mercury’s brightness will have dropped to magnitude +1.6; as bright as the star Castor in Gemini and only 1/11 as bright as it is now. In telescopes it will appear as a narrowing crescent phase only 16% illuminated. 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 invisible after mid-March. With binoculars, you might try scanning to the left of Venus for a couple of more evenings after the 15th to try and extend your view of Mercury, but for all intents and purposes it will be gone by St. Patrick’s Day.
A double identity
In old Roman legends, Mercury was the swift-footed messenger of the gods. The planet is well named for because it’s the closest planet to the sun it’s also the swiftest of the sun’s family of eight planets, averaging about 30 miles per second and completing one circuit of the sun in only 88 Earth days.
Interestingly, the time it takes Mercury to rotate once on its axis is 59 days, so that all parts of its surface experiences periods of intense heat and extreme cold. Although its mean distance from the Sun is only 36 million miles (58 million km), Mercury experiences by far the greatest range of temperatures: nearly 900°F (482°C) on its day side; -300°F (-184°C) on its night side.
In the pre-Christian era, this planet actually had two names, as it was not realized it could alternately appear on one side of the sun and then the other. Mercury was called Mercury when in the evening sky, but was known as Apollo when it appeared in the morning. It is said that Pythagoras, about the fifth century B.C., pointed out that they were both one and the same.
Want to see planets like Mercury in the night sky? Our guides to the best telescopes and best binoculars are helpful tools for anyone hoping to get a closer look at the cosmos.
And if you’re looking to take photos of the night sky or the planets, check out our guides on how to photograph the planets, how to shoot the night sky, and our guides to the best cameras for astrophotography and best lenses for astrophotography.
Joe Rao serves as an instructor and guest lecturer at New York’s Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers’ Almanac and other publications.
Editor’s Note: If you get a good photo of Mercury, and would like to share it with Space.com’s readers, send your photo(s), comments, and your name and location to spacephotos@space.com.
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