Sky Show Tuesday: Four Moons to Sweep Across Saturn

Victoria Jaggard
National Geographic News
February 23, 2009

Four of Saturn's moons will cross the planet's face tomorrow in the first such event since 1996.

Sky-watchers using medium to large telescopes should be able to see Titan, Mimas, Dione, and Enceladus gliding together across Saturn's disk around the same time.

Astronomers will also be pointing the Hubble Space Telescope in Saturn's direction to capture the event.

"It was brought to our attention by an amateur astronomer … in the Philippines, and it sounded like a good opportunity for us," said Keith Noll, of the Space Telescope Science Institute in Baltimore, Maryland.

"It's unusual to get that many [moons] on the disk at the same time."

Even with small telescopes, sky-watchers will be able to see the largest moon, Titan, begin transiting, or crossing in front of, Saturn's northern hemisphere around 5:55 a.m. ET. People with at least mid-size telescopes will be able to see the smaller moons as well.

All four moons and their shadows will be visible at 9:25 a.m. ET. This means viewers on the Pacific Coast of North America—along with those in Alaska, Hawaii, Australia, and East Asia—will have the best views.

And a Green Comet, Too?

Naked-eye observers in northern latitudes will see Saturn as a bright golden star in the constellation Leo in the predawn skies of February 24.

As luck would have it, the green comet Lulin will be making its closest approach to Earth at the same time, and the comet will be visible in very dark skies just below Saturn. (See a sky map of Saturn and comet Lulin as they will appear on Tuesday morning from the Northern Hemisphere.)

Hubble likely won't capture Lulin in the same frame as the transits, Noll said, because the space telescope "will be zooming in to get great detail" of Saturn and its moons.

But observers on Earth might be able to see the comet and Saturn dotted with moons in the same telescope view, he said.

In addition to capturing what promises to be a spectacular image, Hubble scientists think the data could reveal new scientific information.

"People are particularly interested in using Saturn as a 'backlight' to study Titan," Noll said. As the large moon passes in front of the planet, astronomers will be able to examine Titan's atmosphere from a unique perspective.

Likewise, the viewing angles that make the quadruple transit visible from Earth create interesting conditions for studying Saturn's rings, Noll said.

Similar transit events have been witnessed before, Noll added. But because these transits happen only about every 15 years, "this will be the last one Hubble can observe."

Hubble is currently awaiting its last servicing mission, after which the space telescope is expected to keep working only until about 2014.

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