People who have no advance notice will almost certainly wonder, as they cast a casual glance toward the waxing Moon on these evenings, what those "stars" are.

Similar occasions have brought a sudden rash of phone calls to local planetariums, weather offices and police precincts. Not a few of the callers excitedly inquire about the UFO thats closely hovering in the vicinity of Earth's natural satellite.

It's a rare opportunity. In more than 35 years of skywatching, I have seen just five planet occultations. Most Americans won't see another Saturn occultation this good for two decades. In the majority of North America, the next similar chance will come in the year 2024, followed by 2036 and 2037.

Unfortunately, for observers over much of the central and southwestern United States, both the disappearance and reappearance of Saturn will occur against daylight. By the time darkness falls, the occultation will be over with the Moon having left Saturn behind to its right (west) -- still a worthwhile grouping to witness.

Residents of the Pacific Northwest are completely shut out of this event, but they will have a chance to witness a dark sky occultation on April 25, 2007, when the Moon is just one day past First Quarter. Alas, this event will be ended abruptly by moonset for most of those in its viewing range.

Right now, Saturn is about as spectacular as it can be, with its great rings in all of their glimmering, icy elegance, tilted toward the Earth at a 25.9 angle -- very near their maximum possible inclination.

And Saturn is a rather bright object, shining at magnitude 0.0. The brightest star-like object in the sky, when it is visible, is Venus, which can reach magnitude -4.7; the dimmest stars that can be seen with the naked eye are magnitude +6.0.

The Moon is just past its First Quarter phase and is 57 percent illuminated. Saturn will disappear behind the Moons dark portion.

Owing to the relatively large size of Saturn and its ring system as seen on the sky -- something astronomers call angular size -- the occultation will occur at a rather leisurely pace. The Moons slow eastward drift will require up to 50 seconds to cover the planets disk and up to 120 seconds to traverse the entire ring system and completely hide the planet. Then, about 30 to 90 minutes will pass, depending on your location, before the planet re-emerges.

Saturns vanishing act can be watched with just your unaided eye or binoculars, although a telescope will afford the very best views. Using a magnification of 30-power or more, Saturn and its rings will appear to be gradually swallowed up by the Moons nearly invisible dark rim -- an electrifying sight.

Later, Saturn will slowly emerge from behind the Moons bright limb. Observers who train their telescopes at the right spot will see Saturn slowly "rise" into view above the bright lunar landscape.

The northern limit of the occultation track runs roughly from near Grover City, California, northeast through the northern Great Plains, exiting the United States just east of International Falls, Minnesota. The track continues on an east-northeast trajectory into Canada, passing just to the south of Lake Nipigon and the southern tip of James Bay, then runs east through central Quebec before heading out over the Atlantic at the coast of southern Labrador.

Anyone south of this line will see a regular occultation; the planet and rings being entirely hidden as the Moon passes in front of them. Those positioned north of this line, however, will see the Moon pass below the planet and rings.

For an observer fortuitously positioned on the line -- actually a narrow band about 22 miles wide -- the upper limb of the Moon will appear to literally graze Saturn as it passes by. Here, an unusual partial occultation should last about 10 to 15 minutes, during the midpoint of which Saturns disk and rings may ever-so-briefly appear to be completely covered by the Moons bright northern limb. Otherwise, the mountains and crater rims close to the lunar north pole will be seen superimposed on Saturn as the Moon glides on by.

The best region to appreciate this in the United States will be where the Sun has set while both the Moon and Saturn are visible against a deepening twilight sky. This happens between approximately 6:15 and 6:30 p.m. central time, and runs from near Mobridge, South Dakota to Fargo, North Dakota continuing on across northwest Minnesota.

Once you've found Saturn and the Moon, you can use them as guideposts to find some stars. Look for the bright orange star, Aldebaran, marking the angry eye of the constellation Taurus, the Bull. Aldebaran will be to the left of the Moon. About twice this distance to the Moons right will the famous Pleiades star cluster, looking at first glance like a diffuse patch of light.

Large telescopes will also reveal several of Saturns satellites. Appearing as tiny "stars" of magnitude 10 to 13, Tethys, Mimas, Enceladus, Dione and Rhea will be closely clustered around Saturn.

If your telescope fails to show these, you still have a chance of getting a glimpse of Saturns largest moon, eighth magnitude Titan, which lies farther to the east and should disappear about 6 to 8 minutes after Saturn depending on your exact location.

Editor's Note: This is the debut column for Spacewatch Friday. Return each week for another feature article highlighting things anyone can see in the night sky.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.