Earth, glowering Mars align tomorrow

November 26, 1990|By Luther Young

If you haven't noticed the pale-orange "star" high in the northeast sky on recent clear nights, shame on you. The planet Mars is bigger and brighter than it will be again this century, and even city dwellers can enjoy the show.

"When you look at the stars, people have this vague feeling that things are so far away," said Jim O'Leary, director of the Maryland Science Center's planetarium. "But Mars is a neighbor, and its beauty can be appreciated with the naked eye when we're this close."

The Red Planet is positioned about midway between two familiar celestial guideposts: the ethereal Pleiades star cluster and Aldebaran, a bright star with an orange tinge of its own.

"Aldebaran twinkles," Mr. O'Leary said. "Mars, like all the planets, has a steadier light."

But observers won't have any trouble finding Mars, which is blazing away at minus 2 on the magnitude scale. Brighter celestial objects are assigned negative magnitudes (minus 12 for the full moon, minus 1.5 for the brightest star), and the fainter ones have positive numbers.

Except for creamy white Jupiter -- shining in the east after midnight at minus 2.3 -- Mars is the brightest object in the night sky, with the moon just a crescent in late November. The planet has increased in brightness 20-fold since last December. And that dramatic brightening makes Mars appear larger. It now looks about the same size as the brightest stars to observers with the naked eye or binoculars, Mr. O'Leary said, although those with powerful telescopes can measure precise increases in "apparent size" of the planet. The prime viewing of Mars from Earth this week is the result of the complex dance of the two planets around the sun and the size and shapes of their individual orbits.

Mars,the fourth planet in the solar system,takes 687 days to orbit the sun,while Earth -- the third,at an average distance of 93 million miles -- completes its year in 365 days. So Earth periodically overtakes its outward planetary neighbor.

That occurs about every 780 days,with the Earth briefly positioned in a straight line between the sun and Mars in a configuration known as "opposition." The current opposition will

take place at 3:27 p.m. tomorrow,when Mars is 48.4 million miles away.

But because the two planets' orbits are slightly lopsided, not perfect circles,the actual closest approach doesn't coincide with the opposition. Mars passed nearest to Earth a week earlier,on the night of Nov. 19-20,at a mere 47.9 million miles.

Although it won't come that close again until 2001,the most recent encounter,on Sept. 28, 1988,was even closer -- 36 million miles -- and provided some of the best viewing in decades,particularly of several huge dust storms that may have encompassed the entire planet. Viewing for the 1990 opposition could be almost as good for denizens of the northern hemisphere,with Mars high and bright,and scientists will use the orbiting Hubble Space Telescope on Dec. 13 and 14 to grab a global mosaic of Mars.

"Our top priority is to see if there was a global dust storm this year," said Philip James,chairman of physics and astronomy at the University of Toledo in Ohio and leader of the Hubble observing project. "If we're extremely lucky,we may see a storm in progress." Mr. O'Leary cautioned that ground observers who want to study the many intriguing surface features of the 4,200-mile-wide planet will need good telescopes,patience and excellent "seeing" conditions. Those features include ice caps that advance and retreat with the Earth-like seasons. It's now late summer in the southern hemisphere,with the South Pole tilted toward Earth.

And amateur astronomers will be searching diligently for Mars' two tiny, elusive moons, Phobos and Deimos, which are 200,000 and 600,000 times fainter,respectively,than their mother planet.

Mars will quickly dim as it speeds away from opposition,but not quickly enough for the Maryland Science Center to avoid curious,sometimes urgent,inquiries about the strange orange-red object in Baltimore's sky.

"People call up and wonder what it is," said Mr. O'Leary. "They always seem relieved to hear it's just Mars."

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