Mars' closest approach to Earth in 17 years - and the best Northern Hemisphere view of the red planet in 113 years - has backyard astronomers peering excitedly through telescopes.

A network of 500 mostly amateur astronomers in 33 countries has been organized for Mars Watch '88 by the International Mars Patrol and the Planetary Society, which has dubbed the occasion "a close encounter of the red kind.""Unless you're on the North Pole or South Pole, you'll get good views of Mars" through telescopes until mid-December, said Dr. Donald Parker, a Coral Gables, Fla., anesthesiologist and "Mars recorder" for the patrol, which is part of the Association of Lunar and Planetary Observers.

"You can see the surface, the clouds and the polar ice caps and really realize it is a whole world," Parker said. "Occasionally we've seen shadows cast by big dust storms. It's really neat."

"For observers in the Northern Hemisphere, Mars is more favorably positioned than at any time from 1875 through 2025" because it's not only closer, but farther north in the sky than usual, said Jeff Beish, an Eastern Airlines flight simulator technician in Miami who also collects reports for the Mars Patrol.

Mars watchers already have provided 1,200 reports, photographs and sketches to Parker and Beish, who send key findings to Jet Propulsion Laboratory astronomer Stephen Edberg, who coordinates Mars Watch '88 during his spare time.

The observations mean "when we send cosmonauts or astronauts to Mars, we're going to have a better understanding of the planet," said Edberg. "Mountaineers use telescopes to look at routes before they try them. The same goes for preparing for manned exploration of a planet."

The Pasadena-based Planetary Society is a nonprofit group lobbying for manned exploration of Mars.

At 8:18 p.m. PDT on Sept. 21, Earth and Mars were 36.54 million miles apart - the closest since they were 34.92 million miles apart in August 1971, said telescope demonstrator Anthony Cook, of Los Angeles' Griffith Observatory.

Mars and Earth are close now because on Aug. 12 Mars was at perihelion - its closest approach to the sun - while at 8:25 p.m. PDT Sept. 27 it will reach opposition, meaning it is on the opposite side of Earth from the sun. They won't be so close again until Aug. 27, 2003, when they get within 34.65 million miles. They sometimes are as far as 248 million miles apart, and as far as 63 million miles apart during non-perihelion oppositions.

Oppositions occur about every 26 months. But favorable, or close, oppositions, occur every 15 to 17 years. Even then, excellent views usually are available only to Southern Hemisphere residents.

To the naked eye or through binoculars, Mars now is easily visible all night as a reddish-orange spark, the evening sky's second brightest object after the moon.

For Northern Hemisphere viewers, Mars appears low in the east-southeast after dark. At midnight, it is halfway between the southern horizon and a point directly overhead. Before dawn, it is low in the west-southwest. For Southern Hemisphere observers, Mars is in the east-northeast in the evening, appears halfway between the northern horizon and a point directly overhead at midnight, then is visible before dawn low in the west-northwest.

People who want detailed views can attend telescope sessions sponsored by the Planetary Society, planetariums, colleges, science centers and astronomy clubs.

The smallest telescopes capable of providing detailed views of Mars are 3-inch refractors or 4-inch reflectors, each with 150-power magnification, Cook said. Because time on big telescopes is dominated by observations of distant stars and galaxies, "most of the round-the-clock watch of the planet is kept by amateur astronomers."

Parker, 49, has spent nights outdoors to watch Mars for 40 years. "I have very bad arthritis and get mosquito bites, but I look at this planet and it makes my whole day," he said. "I hope in my lifetime men will get to Mars. It's always been a dream of mine."