Dr. Jean Meeus of Belgium, recognized as a world authority in spherical and mathematical astronomy, has defined a planetary trio as when three planets fit within a circle with a minimum diameter smaller than 5 degrees.

How small a region of sky is this? Your clinched fist held at arm's length is equal to roughly 10 degrees; the pointer stars at the end of the bowl of the Big Dipper are separated by just more than five degrees.

"That limit of 5 degrees has been chosen more or less arbitrarily," Meeus says, "but we have to make a choice."

Here is what to look for, and when:

From now through May 10, an official trio will be readily visible low in the west-northwest sky for about two hours after sunset. The three planets are Venus (magnitude -3.9), Saturn (+0.1) and Mars (+1.6). The circle will be at its smallest (2.7 degrees) at 9 p.m. EDT on May 6.

Be sure to check out the view after sundown on May 5 to find Venus, Saturn and Mars forming a nearly perfect, and visually striking equilateral triangle, with each side of the triangle measuring roughly 2.5 degrees in length.

Since Venus and Mars appear to move much faster than Saturn does in our sky from one night to the next, they will seem to quickly leave it behind as the week progresses.

On the evening of May 7 and especially May 8, the planet triangle will have become a narrow isosceles.

On the evening of May 10, Venus will move to within only one-third of a degree of Mars; both will be visible together in binoculars or a low-power telescope. Venus will appear 175 times brighter than Mars, because while Venus will be 132 million miles from the Earth on that night, Mars will be 223 million miles away.

Through this seven-day interval there will also be separate conjunctions between Mars and Saturn (2.2 degrees apart on May 4) and Venus and Saturn (2.4 degrees apart on May 7). In addition, this entire sequence of events takes place in central Taurus, just to the north of the orange first-magnitude star Aldebaran and the beautiful V-shaped Hyades star cluster.

Amazingly, a fourth planet, Mercury, will not be too far away. It passed just south of the Pleiades star cluster on April 29 and arrives at a very favorable elongation, 21 degrees east of the Sun, on May 4. Mercury will sit roughly 8 degrees below and to the right of the planetary trio on May 6, while shining at magnitude +0.8.

Jupiter is the only one of the five bright naked-eye planets that is not directly involved in the extraordinary clustering of celestial bodies. Yet, it's really not so far away. Shining brilliantly at magnitude -1.9 in the middle of the constellation Gemini, it will stand well up in the western sky at dusk. And during the final week of May, brilliant Venus will be drawing noticeably closer to Jupiter.

The overall span of the five planets across the western evening sky has been steadily shrinking in recent weeks. It was 54 degrees in length on April 20, then 40 degrees long on April 30, and it will be 36 degrees on May 5. Mercury, Venus, Mars and Saturn will squeeze into a narrow sector of the sky reaching a minimum span of just 9 degrees on May 7.

Mercury will then gradually fade to invisibility during the second week of May.

In his recently published book, "More Mathematical Astronomy Morsels," Meeus points out that "such a gathering of three or more planets in a restricted area of the sky should really be called a 'grouping,' not 'an alignment.'"

He goes on to point out that an "alignment" is when three or more planets and the Sun are on a straight line in space. However, strictly speaking, such an alignment never occurs, because three or more planets can never be exactly aligned, Meeus says.

I have been referring to this gathering (or grouping) as "The Great Celestial Summit Meeting" to members of my family and close friends. This especially will be true on the evenings of May 13 and 14 when a skinny crescent Moon will again be passing through the same area of the sky as the planets.

Truly, that will be the icing on this celestial cake!

According to astronomer Robert C. Victor at Michigan State University's Abrams Planetarium, after the spectacular planetary array of 2002 passes into history, future generations will witness similar compact gatherings of the five naked-eye planets in September 2040, July 2060 and November 2100.

But Victor adds an important disclaimer regarding the visibility of these future planet bunchings:

"The groupings of 2040 and 2100 will take place with some of the planets barely above the horizon at mid-twilight for northern latitudes and hence they may only be visible with binoculars. Only with the compact gathering due in July 2060 will we again have a chance to clearly see all five naked-eye planets in a single glance."