This month yields the year's best opportunity to spy the relatively elusive planet Mercury. What's more, May presents two great conjunctions of Mercury with fellow celestial objects - one with Jupiter and another with the crescent moon and Jupiter. Finally, now is your last chance to catch Jupiter and Saturn in the evening sky, before they venture into the morning realm.

As the nearest planet to the sun, Mercury spends much of its time hidden in the twilight glow of sunrise and sunset, making it difficult for skygazers to spot. But the good news is that the swift planet is nearing the position in its orbit around the sun that astronomers call Greatest Eastern Elongation.

"This is the best shot at observing Mercury for the year," says astronomy popularist and Astronomy Magazine columnist Bob Berman. "By the time Mercury comes around again in another four months, and the time after that, it will be buried in the glare of the sun."

Throughout the course of its 88-day orbit, Mercury steadily shifts position in the sky with respect to the sun, sometimes wandering as far as 28 degrees east or west of the sun at elongation. (A fist held at arm's length spans about 10 degrees.) In principle, the farther Mercury appears to be from the sun, the easier it is to see. But good earthly views of Mercury also depend on the time of year.

"You only get one or two favorable evening elongations of Mercury per year," adds Berman. "Late winter or springtime are the only times when you have the angle of Mercury's orbit about the sun being most nearly vertical to the horizon."

Mercury is currently making a swift progression toward Greatest Elongation, giving would-be observers only a handful of days to catch a good glimpse. On May 1st, Mercury set a mere 50 minutes after the sun. But when Mercury officially reaches Greatest Elongation on May 22nd - when it will be relatively high in the western sky, 22 degrees above the setting sun - it won't set until about two hours after the sun.

Some of the best views of the swift planet can be found during the evenings leading up to, and following, Greatest Elongation. On May 16th Mercury floats just 3 degrees (about the distance from fingernail to first knuckle on an index finger held at arm's length) to the north (or slightly right) of Jupiter. Be careful not to confuse yellow-orange Mercury with the similarly colored bright star Aldebaran, in Taurus, located just below the Jupiter-Mercury duo. And on May 24th, the crescent moon hovers just 4 degrees to the south (or left) of Mercury, while Jupiter lies to the lower right of the pair.

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