Tuesday, April 29, 2008
Planets will wander over next few months
John Goss
John Goss is chairman of the Mid-East Region of the Astronomical League and a former president of the Roanoke Valley Astronomical Society.
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Blog
Ancient sky watchers considered the bright planets to be wandering stars. And wander they do, especially over the next four weeks when planet gazers spy the varied movements of Mercury, Mars and Saturn.
While most planets can be seen every clear night for several months straight, Mercury pops into our twilight sky for a couple of weeks then drops from view only to return a few weeks later on the opposite side of night. Because of this and because it is never far from the sun, Mercury is tough to spot.
Until May 15, this little planet lies in a favorable position in its close orbit around the sun. It can be found at 9 p.m. -- if the western sky is clear and if there are no mountain ridges blocking the view. Even so, it will be a challenge.
Our thin crescent moon, descending in the bright western twilight, acts as a helpful guide for locating Mercury on the night of May 6. Try using a pair of binoculars. Just to the moon's lower left, perhaps four of its diameters away, shines another object: Mercury. Much farther to their left is another object, though not quite as bright as Mercury. This is the star Aldebaran in the constellation Taurus.
Four nights earlier at about 8:50 p.m., careful observers find Mercury closer to the horizon and to the left of the Pleiades star cluster. This definitely will be a challenge, but the view through binoculars makes the effort worthwhile.
Mars' motion is different than Mercury's fast rise and fall. The Red Planet continues growing dimmer throughout the next few months as our Earth pulls away from it. In December, Mars shone brighter than any star in the night sky. Now, it has tripled its distance from us and, consequently, shines only as bright as Gemini's two famous stars, Castor and Pollux.
As April ends, Mars forms an interesting isosceles triangle with those twin stars. As it's been doing for the past three months, Mars moves to the east relative to the background stars, and on Saturday it falls into alignment with Castor and Pollux. Gemini transforms from twins into triplets.
Nearly three weeks pass after Mars moves out of Gemini when it encounters the dim star cluster known as both M44 and the Beehive. On May 22, Mars will be in the west at 10 p.m. Bring out the binoculars again for a good look at the Red Planet scooting above the faint, twinkling lights of the Beehive. Consider this: The light you see from Mars reflected off that planet about 15 minutes ago, while the light reaching us from the cluster's stars left them when Columbus explored the New World.
Our third planetary wanderer, Saturn, exhibits yet another type of orbital motion. Because of Earth's motion around the sun, for the past four months Saturn appeared to move opposite its normal eastward direction across the sky. We see Saturn slowly inching westward toward Regulus, Leo's brightest star. They reach their closest approach to each other on Saturday.
Measure the change in their separation by using your index finger. Simply extend your arm and place your forefinger between Regulus and Saturn. At the end of April and the beginning of May, your finger just fills the gap between them. By the end of May, there will be plenty of play between Saturn and Regulus, and by July 1 two fingers easily fit between them. You have direct evidence that plodding Saturn moves.
The next few weeks give you an opportunity to see for yourself changes in the sky. This is a time for the wanderers: Mercury moves, Mars marches and Saturn saunters.
