Friday, April 30, 1999
Watching the skies: Mars in May and June
The planet Mars will be ideally positioned for viewing in the evening sky for the next two months. Because it is more distant from the Sun than Earth, its year is longer, which means roughly every 26 months Earth catches up with and passes Mars. When the two planets are in line with the Sun and on the same side, Mars is said to be in opposition. The "closest approach" between Earth and Mars occurs on nearly the same date as opposition, but because of the elliptical orbits the dates are a little separated. This year opposition occurred on April 23-24 and closest approach will be on May 1-2.
On April 30, the full moon rises in the southeast about one hour after sunset. At that time Mars will be higher and slightly south of the moon, about 18 degrees above the horizon. It is the brightest object, after the moon, in that part of the sky and definitely reddish. Slightly higher again and further south than Mars is the hot blue star, Spica, in spectral contrast to the red planet. It is the brightest star in that part of the sky. Another interesting contrast is Arcturus, a cool red star, brighter than Spica but dimmer than Mars, found by looking further east and higher than Spica. If you can find the Big Dipper, continue the curve of the stars in the handle and this will bring you right to Arcturus.
During the evening of April 30, Mars is due south (transits the meridian) and the highest it gets in the sky for that night is at 12:15 a.m., so it is ideally situated for viewing all evening. It is about 56 million miles or 4.8 light-minutes from Earth.
Mars will dim slightly as the month of May progresses but perhaps of more interest to watch is its westerly motion which will bring it closer to Spica. It will come within about 2 degrees (4 full moon diameters) of Spica at the end of the month, and then reverse direction and make its way eastward. This reversal of motion of the planets among the stars was indeed baffling to the early observers before they understood it was due to the planets having different orbital velocities about the Sun.
If you are interested in looking at Mars with a telescope, early in May is the best time since its angular size is the largest at the time, of closest approach on May 1-2. Even then it is only about 16 arc seconds in diameter and a minimum of 6- to 8-inch diameter telescope is required to make out any surface features. Check Sky and Telescope magazine for April 1999, page 106 for more information on observing this small planet.
© 1999 The Carlisle Mosquito