Ring shadowplay on a Saturn moon

Ring shadowplay on a Saturn moon

Ring shadowplay on a Saturn moon

Bad Astronomy
The entire universe in blog form
Dec. 23 2009 1:30 PM

Ring shadowplay on a Saturn moon

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

cassini_janusshadows

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This animation is made up of images taken by Cassini (of course) in August. At that time, it was the equinox on Saturn, so the Sun was shining straight down on Saturn's equator... which happens to be the plane of the orbit for both the rings and the moons. In other words, the Sun was shining straight along the rings. During this brief time, twice per Saturn orbit of 29 years, the moons can cast long shadows across the rings, and the rings can cast shadows on the moons.

Janus really is dinky, just 179 km (111 miles) across, which is why it's not really all that round. Its gravity isn't strong enough to crush itself into a sphere. Other moons are bigger, of course, and the Cassini folks just released several other astonishing animations of them as well, showing the moons dancing and eclipsing each other, with Saturn's rings as the backdrop. This one showing Rhea and Janus is particularly beautiful.

What more can I add? Cassini continues to deliver, over and again. Amazing.