Celestial News: Catch the twilight planet this week
April 18, 2016
The one naked-eye planet most folks probably have never seen is the planet Mercury. Mercury is not only the closest planet to the sun, but it also became the solar system's smallest planet after Pluto was demoted to dwarf planet status in 2006.
Mercury is challenging to spot for a number of reasons. First, it is small, so it doesn't reflect very much sunlight our way and doesn't appear as bright as the other planets in our night sky. Also, as it swings around the sun and approaches Earth, Mercury turns its dark nighttime hemisphere towards us, causing it to fade as it draws nearer.
Second, because it orbits so close to the sun, it moves very fast in its orbit and zips through our sky in only a couple of weeks before vanishing. It's no wonder the Romans named it after their swift messenger god.
Furthermore, because it hugs the sun so closely, Mercury always rises or sets within an hour and a half of the sun and is seldom seen in a completely darkened sky. Mercury is the "twilight planet."
Fortunately, Mercury swings out far enough from the sun that we can get a good peek at it several times each year. Sometimes, this happens in the morning sky and sometimes, in the evening sky. These times of greatest elongation are the times you want to try and spot the innermost planet.
One of this year's best evening elongations of Mercury is in progress this week. You can glimpse Mercury on any clear evening in the next week or so, approximately one hour after sunset.
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Start scanning just above the west-northwest horizon around 8:45 to 9 p.m. Mercury will look like a yellowish "star," about as bright as the nearby star, Aldebaran, that marks the eye of Taurus the Bull. Having reached its greatest elongation 20 degrees east of the sun on April 18, Mercury will descend slowly toward the horizon, a little earlier each night, until it is swallowed up by the sun's glare in early May.
And, then, something truly amazing will happen; planet Mercury will make a rare transit across the face of the sun as it zips between our star and Earth on the morning of May 9. Student members of the SKY Club at Colorado Mountain College will have safe solar telescopes set up so visitors can drop by the campus and view the tiny eclipse for themselves. Stay tuned for more details about this unusual transit of the planet Mercury on May 9.
Consider yourself very lucky if you catch sight of Mercury, the elusive "twilight planet." Some of history's greatest astronomers, including Nicolaus Copernicus, are said to have never been so fortunate.
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College’s Alpine Campus. His “Celestial News” column appears weekly in Steamboat Today. Check out Westlake’s astrophotography website at jwestlake.com.