A comet discovered only two weeks ago will be visible in the northern skies tonight as it approaches the Earth more closely than any comet has in 200 years.

Although smaller than most comets and apparently almost burned out from old age, the comet will move rapidly across the sky and should be visible just after dark tonight 60 degrees above the horizon, just below the bowl of the Big Dipper near the north star Polaris.

Moving at a speed of 27 miles a second, the comet has a 15,000-mile-long tail and it should be visible as a moving ball of light in the Northern Hemisphere if the skies are clear and dark.

"If it is not visible to the naked eye, it will be with binoculars," Dr. Paul Feldman, an astronomer on the staff at Johns Hopkins University, said yesterday.

Although seen only for the first time on April 25 by an Earth-orbiting satellite called the Infrared Astronomical Satellite, the new comet has aroused worldwide astronomical interest because of its approaching close encounter with Earth. The comet will pass as close as 3.5 million miles tonight and pass within 3.1 million miles Wednesday morning.

"The last comet to pass that close to Earth came by in 1770," said the Jet Propulsion Laboratory's Ray Newburn, a comet historian. "That was Comet Lexell, which was a larger and brighter comet than the one headed our way right now."

The new comet has been named Iras-Araki-Alcock, for the satellite and the two astronomers who first identified it. Genichi Araki is a Japanese amateur astronomer, and G.E.D. Alcock is a British amateur astronomer. They reported sighting the comet simultaneously with a science team in the United States, Britain and The Netherlands studying the infrared pictures sent back by the IRAS satellite.

No larger across than six tenths of a mile, the nucleus of the new comet is nevertheless boiling off enough water, ice and gas as it nears the sun to create a tail that is about 15,000 miles long.

Although the tail may not be visible even with binoculars because it is so thin and diffuse, it should be visible to amateur astronomers with telescopes.

Naked-eye viewing of the comet will be difficult because it is small and ancient, meaning it has lost much of the water, ice, dust and gas that light up the sky when a larger and younger comet approaches the sun.

"We would probably never even see it if weren't coming so close," said Robert Harrington of the U.S. Naval Observatory. "It is as bright as the brightest star in the heavens, but its light is so diffuse it will be spread out over an area larger than a full moon. It will also be 20 times farther away than a full moon, which will of course reduce its brightness still more."

Nevertheless, the sighting of a comet by anybody but the most dedicated astronomers is a rare event. The best viewing times will be tonight in the northern skies, Wednesday morning in the southwest and Wednesday night in the western skies as it passes in front of the constellation Cancer.

Telescopes all over the world and astronomical satellites everywhere in space have been reconfigured and repositioned to observe the comet. It is so close to Earth that it will give astronomers a rare look at a comet's nucleus, whose chemical makeup is as old (4.6 billion years) as the solar system because it has been undisturbed since the dawn of time.

Comets are the remnants of the formation of the solar system and are believed to be the most pristine bodies in the solar system, undisturbed by weather, the impact of meteorites or the radiation of the sun.

They are as old as the observable universe, but less has been known about these "heavenly ghosts" than anything else in the skies. Only in the last 20 years have astronomers known that they are made of dust, ice and snow.

More than 600 comets have been observed inside the solar system, most of them one-timers that appear as infrequently as every 100,000 or even 1 million years and then drift far beyond the orbit of Pluto.

All comets are believed to have their origins in the so-called Oort Cloud, an astral swirl of perhaps millions of comets 40 times as far from the sun as Pluto. The cloud is named for Jan Oort, the Dutch astronomer who discovered it.