Saturn emerges from behind the Moon in a Sept. 19, 1997

A map from the International Occultation Timing Association

Artist's concept of Cassini braking into orbit at Saturn on July 1, 2004


For sky watchers across the United States, Earth's Moon will briefly slide in front of the planet Saturn on Wednesday, Feb. 20.

"This will be a good opportunity for people to watch the moon gobble up Saturn. Seeing that kind of celestial motion is fun," said JPL astronomer Stephen Edberg, a member of the science planning team for the Cassini-Huygens mission to Saturn.

The event is called an occultation. You can get a sense of the Moon's motion even if you just look up a couple of times during the evening. Viewers with favorable locations using binoculars or telescopes will watch the dark edge of the Moon dramatically progressing across Saturn's width.

The occultation will begin after sunset in eastern states but, unfortunately, during daylight on the West Coast. Even in western states, though, good binoculars may reveal Saturn beside the Moon in the blue sky just before the occultation, and the Moon will still be near enough to Saturn after dark that changes in their relative positions will be easily noticeable.

The Moon hid Saturn for viewers in some parts of the world in December and January, but those occultations occurred with the Moon nearly full, making Saturn harder to see against the glare. During the Feb. 20 event, the last for several years in North America's night sky, the Moon will be slightly more than half-illuminated and high in the sky.

Saturn will be next to the night side of the Moon before the occultation and next to the day side afterwards. One advantage of the event is that such a prominent place marker makes Saturn easy to find even for novice sky watchers. The sixth planet from the Sun is best known for its glamorous rings, visible with virtually any telescope.

Cassini-Huygens, a joint mission of NASA and the European Space Agency, will reach Saturn in 2004 after a seven-year journey from Earth. The spacecraft swung near the fifth planet from the Sun, Jupiter, at the end of 2000. You can spot Jupiter as the brightest point of light in the eastern evening sky this month, about two fist-widths toward the horizon from Saturn. Look about halfway between Jupiter and Saturn and you'll be gazing toward Cassini-Huygens, though the spacecraft is far too small to be visible from any telescope on Earth at its current distance. Cassini will become the first spacecraft to enter orbit around Saturn on July 1, 2004. It will release the Huygens probe to descend through the thick atmosphere of the moon Titan about six months later.

A simple way to appreciate the Feb. 20 event is just to look toward the Moon high in the sky at dusk, then again once or twice later in the evening about an hour apart. Depending on where you are, you should see a bright dot nearby that noticeably changes position relative to the Moon and may disappear or appear. The visible change in positions results from the Moon's progress in orbiting the Earth, not from Saturn's motion around the Sun. For a more memorable sight, especially from eastern states, watch Saturn with binoculars or a telescope as the edge of the Moon begins to cover it, Edberg recommended.

Local times for when Saturn will disappear behind the Moon on Feb. 20 vary from 3:15 p.m. PST in San Diego, Calif., to 7:40 p.m. EST in Bangor, Maine. A chart from the International Occultation Timing Association gives Saturn's disappearance times for more than 300 North American cities. Saturn may reappear momentarily or stay hidden for up to about 90 minutes, depending on your location.

Through telescopes, the occultation will offer a fleeting view of part of the Moon's dark skyline silhouetted against sunlit Saturn. "The Moon isn't a perfect sphere," Edberg said. "You can see mountains along the edge as it passes in front of Saturn."

Saturn actually has a diameter more than 30 times the diameter of the Moon, but it will appear small beside the Moon because it is more than 3,000 times farther away.

For more information about the Cassini-Huygens mission, see http://saturn.jpl.nasa.gov. JPL, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Office of Space Science, Washington, D.C.

Other Web pages display photographs from previous occultations of Saturn. One is SpaceWeather.com's site, at http://spaceweather.com/planets/gallery_nov01.html.