Ring shadowplay on a Saturn moon

By Phil Plait | December 23, 2009 1:30 pm

This is simply too cool: the shadow of Saturn’s rings moving across the face of its tiny moon Janus.

cassini_janusshadows

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.

Related posts:
Behold, Saturn
Ringless
Titan’s Shadow
Saturn’s rings do the wave

CATEGORIZED UNDER: Astronomy, Pretty pictures
MORE ABOUT: Cassini, Janus, Rhea, Saturn

Comments (14)

Links to this Post

  1. Jay Lake: [links] Link salad waits for the fat man to fly | December 24, 2009
  1. Is Cassini another program that was supposed to have a limited life, and has now wildly exceeded that lifespan (i.e. like Spirit and Opportunity)? All i can say is keep the funding going! This is great stuff!

    P.S. Did you get the FB Wall to Wall that had the “clever” vaccine advert they use in Ontario?

  2. CJSF

    Phil, I think Emily had this on her blog back in August or September!

    CJSF

  3. Alareth

    Inspired by the success of the Army’s video game, NASA is sponsoring an official MMO http://www.astronautmmo.com

  4. Nick G

    Why is it that objects seem to orbit on the same plane? Looking at saturn, the solar system, and the universe it seems that most orbits are restricted to a few degrees difference instead.

  5. Steve Paluch
  6. @Nick G, because the way the system forms. Unless a planet is captured or disturbed somehow, all of them form along an ecliptic plane. I think the Wiki page has a good explanation if you google solar system formation.

  7. Plutonium being from Pluto

    Superluminous! :-D

    So like an asteroid that small Saturnean moon.

    It looks here like we’ve already mapped it though and I must admit I don’t recall seeing any images of it before now.

    Thankyou BA & thanks too to the Cassini team. I love your work! (& I bet you do too! ;-) )

  8. Plutonium being from Pluto

    @ 4. Nick G Says:

    Why is it that objects seem to orbit on the same plane? Looking at saturn, the solar system, and the universe it seems that most orbits are restricted to a few degrees difference instead.

    Saturn & the Solar system yes – as they form dust and gas and stuff generally tends to flatten into the one plane.

    The cosmos more broadly though -not so much.

    Spiral (& barred spiral) galaxies have the disk and spiral arms which are sorted of flattened by a similar~ish (?) process although the stars are randomly oriented I gather, so that we see some stars like Vega pole on and others like Regulus (or was it Achernar?) appear to tumble end over end in the sky as we see them. There’s also the different layers of younger Thin disk population, older Thick (more scattered) disk, and the spherical Galactic halo matrix which contains the rest of our Galaxy and from which stars such as Arcturus and Kapteyn’s stars – and the globular clusters belong.

    Then there’s the additional complications of the warp of Gould’s belt and other galactic structures which may be related to gas clouds colliding from the halo into the disk, eg. Symths/ Smith’s cloud & may be due to gravitational interactions such as encounters with the Magellanic clouds and other small galaxies.

    Then we’ve got elliptical galaxies which don’t have planes at all and the intricate soap bubble strands and voids and bubbles and “Great Walls” and so on of the universe at the vastest scale.

  9. Messier TidyUpper

    Then we’ve got elliptical galaxies which don’t have planes at all

    Except there’s Centaurus A or NCG 5128 which looks like its got a dark band of dust around its middle – probably having swallowed or in the process of swallowing a spiral galaxy that it will merge entirely with.

    See : http://en.wikipedia.org/wiki/Centaurus_A

    Also, (& its surprising that a Plutonean like “Plutonium being from Pluto” didn’t mention this) many ice dwarf planets and planetoids in the outer solar system plus comets have very inclined – and tilted orbits indeed. For examples the ice dwarfs such as Pluto, Eris, “Buffy” (2004 XR-190), Sedna &, most of all, the retrograde orbiting Centaur, “Drac” (2008 KV42) all have extremely odd and not in the “solar plane” orbits.

    See : http://en.wikipedia.org/wiki/2008_KV42 & http://en.wikipedia.org/wiki/File:Eris_Orbit.svg

  10. Spectroscope

    @1. Larian LeQuella Says:

    Is ‘Cassini’ another program that was supposed to have a limited life, and has now wildly exceeded that lifespan (i.e. like Spirit and Opportunity)? All i can say is keep the funding going! This is great stuff!

    It is indeed. Agreed. :-)

    I think Cassini‘s mission has already been extended once at least but I don’t think it is, as yet, anywhere near beating the Mars Exploration Rovers unexpected longevity record. :-)

    (Raises beer to Spirit & Opportunity & Cassini & hopes all three amazing machines keep up their marvellously great work! :-) )

  11. Nigel Depledge

    The BA said:

    This is simply too cool: the shadow of Saturn’s rings moving across the face of its tiny moon Janus

    Wait, what?

    “The face” singular of Janus? Janus, of course, famously has two faces, so you’ve obviously made the whole thing up and it’s just a Hollywood backlot.

    [/parody]

  12. Pluto’s orbital inclination is only 17 degrees, much less than that of other outer solar system bodies such as Eris, which has an orbital inclination of 42 degrees. Objects like Eris, which are in the scattered disk zone of the Kuiper Belt, have much more eccentric orbits than those in other parts of the Kuiper Belt. Also, the ice dwarfs large enough to be spherical may be more rocky than icy. Pluto is estimated to be about 70-75 percent rock.

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