Steamboat Springs The solar system’s two innermost planets, Mercury and Venus, behave differently than the other planets. Instead of being free to wander all the way around the sky, they seem to be tethered to the sun so that they swing out from one side of the sun to the other and then back again.
Jimmy Westlake's Celestial News column appears Tuesdays in the Steamboat Today.
Find more columns by Westlake here.
As a result, each planet spends a brief time in our sky as an “evening star,” followed by a brief engagement as a “morning star.”
The best time to observe each planet is when it is near its greatest angular distance from the sun in our sky, an event called greatest elongation. Even at greatest elongation, Mercury is never more than 28 degrees and Venus never more than 48 degrees from the sun. To spot Mercury or Venus, one must look east in the sunrise glow just before sunrise or look west into the sunset glow just after sunset.
The little planet Mercury is particularly challenging to catch. It requires a good, clear sky and an unobstructed view down to the horizon and, even then, it can be tough.
It is said that the great 16th century astronomer Nicolaus Copernicus was never successful at seeing it. Mercury is small, not much bigger than our own moon, and so it doesn’t shine as brightly as its larger siblings. And, it moves in and out of our sky very quickly, so favorable conditions must all converge at the right time in order to see it.
Those favorable conditions will converge for us in the latter half of May this year. Mercury will reach its greatest elongation 23 degrees east of the setting sun May 24 and should be visible for about a week on either side of that date.
Look low in the western sky for a yellowish “star” in the sunset glow, about an hour after the sun goes down. The much brighter planet Jupiter will be shining a hand-span or two above and to the left of Mercury’s position.
The slender crescent moon also can serve as your guide to Mercury on the evening of May 30. Look for Mercury about a fist-width to the right of the moon.
Mercury joins the planets Jupiter, Mars and Saturn, already in our evening sky, for an amazing parade of planets. Connect the planetary dots to visualize the ecliptic across the sky. That’s the plane of our flat solar system, projected into the heavens.
NASA’s MESSENGER spacecraft (an acronym for MErcury Surface Space Environment Geochemistry and Ranging) currently is orbiting the innermost planet and sending back amazing close-up images almost daily. You can check out the latest images and scientific discoveries at the website http://messenger.jhuapl.edu.
This window of opportunity to spot Mercury in the evening sky is one of the best that we will have this year, so get out there and see something that eluded the great Nicolaus Copernicus!
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. His "Celestial News" column appears weekly in the Steamboat Today newspaper and his "Cosmic Moment" radio spots can be heard on local radio station KFMU. Check out Westlake's astrophotography website at www.jwestlake.com.