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The next few weeks will afford skywatchers an unusual opportunity.

In any year, several comets will typically come within range of good binoculars or small telescopes. But a comet bright enough to be readily visible to the unaided eye comes along usually only once or twice per decade.

If Ikeya-Zhang attains third magnitude, it would be comparable to the star Megrez, the star in the Big Dipper that joins the handle with the bowl.

Of course, a comets appearance is much different and less well defined than the sharp image provided by a star. In 1974, Kenneth Weaver, an assistant editor of National Geographic magazine, provided an excellent description of a comet possibly comparable to Ikeya-Zhang. Weaver had traveled to the dark skies of Kitt Peak National Observatory near Tucson, Arizona, for a glimpse of comet Kohoutek. He wrote:

When Weaver wrote these words nearly three decades ago, he was describing a moderately bright comet from a location completely free of light pollution. Today, with the glare of bright lights seemingly metastasizing everywhere, it is far more difficult to find such dark, starry and pristine conditions. Urban skyglow has robbed many of us of our night skies, and the vast majority of the population of the United States now live in regions where the stars are mostly blotted out by overly excessive lighting.

If you live in an area where light pollution is severe, the first prerequisite in obtaining a good view of comet Ikeya-Zhang will be to travel to a darker location where you can see many of the fainter stars. Anything less will likely mean that, at best, the comet may only be barely visible through the bright light haze, and more likely not at all.

Comet Ikeya-Zhangs overall appearance could change radically between the time it makes its closest approach to the Sun in late March and when it passes closest to the Earth near the end of April.

If it proves to be a particularly active comet, noticeable changes may occur even from night-to-night. It probably won't lose much in overall brightness during most of April, as it approaches Earth and swells in apparent size. Although its tail will then be shrinking in physical size, it also will be swinging toward the Earth, causing our viewing angle of the whole tail structure to change.

It should be stressed, however, that the shape and size of a comets tail is very hard to predict.

The most important factor is the amount of dust released from the comet nucleus as its surface ice vaporizes in the heat of the Sun.

John Bortle, a long-time comet consultant for Sky & Telescope magazine, has previously noted that the tail of Ikeya-Zhang is likely to be composed chiefly of gas and not dust, meaning it will likely appear rather faint with a bluish hue. The gas tail is a fairly thin, straight line pointing away from the Sun.

On the other hand, comets that produce prodigious amounts of dust become eye-catching because the dust tails appear much brighter. The dust tail is a flat sheet of material spread out in the plane of the comets orbit with one edge immediately adjacent to the gas tail.

As a good example of how these factors can affect appearances, most people were far more impressed with comet Hale-Bopp in 1997, as opposed to comet Hyakutake in 1996. Although both comets were similar in brightness, Hale-Bopp shed significant tails of both dust and gas, as opposed to Hyakutake whose tail was mainly gas. Even people in brightly-lit cities could trace some of Hale-Bopps tail.

Yet in comparison, although Hyakutakes narrow gas tail stretched across a much greater expanse of the sky, only those blessed with very dark skies, far from any light pollution, actually saw it. A similar fate may await those who attempt to view Ikeya-Zhang.

The only way youll know for sure is to actually go out some evening soon after sunset or, better yet, on a morning during early- or mid-April before dawn breaks, and attempt to see the comet for yourself.

The comet's production of gas or dust could halt without notice, making the tail vanish and causing Ikeya-Zhang to appear much like comet Halleys disappointing apparition back in 1986. Or you might witness a sudden increase in dust or gas production, and the tail might lengthen before your eyes.

Ikeya-Zhang was discovered by Kaoru Ikeya of Japan and Daqing Zhang of China on Feb. 1, 2002. While this was Zhangs first comet discovery, it was Ikeyas sixth.

At one time, Ikeya was considered one of the worlds most prolific comet hunters. He was the discoverer or codiscoverer of five comets, bagging them between January 1963 and December 1967. One of these, comet Ikeya-Seki, went on to become the brightest comet of the 20^th Century during late-October 1965.

But after 1967, nothing had been heard from Mr. Ikeya for more than three decades. Exclaimed one Japanese astrophotographer after hearing of Ikeyas latest find: "He is the phoenix!"