By Phil Plait | August 13, 2009 6:30 am

Quick! Guess the planet!

Saturn with no rings

OK, if you guessed Saturn then a) you’re smart, 2) you read the title of this post, c) you read the "alt text" of the image, δ) you saw that bare hint of ring in the corner of the picture, or 5) you had a one in 8 (or possibly 9) chance of getting it right.

But it is Saturn. It’s the equinox! This image, taken yesterday, is a raw one from Cassini. In other words, it hasn’t been processed to clean up bad pixels and take into account other weirdnesses of the digital detector. So you can’t do quantitative science on it, but what you can do is look at it and see how the shadow of the ring on the cloudtops is a thin, thin line. That means the Sun must be shining straight along them, like a tree casting no shadow at noon.

And that is exactly what’s going on. The ring particles orbit Saturn exactly above its equator, and that means that if you were standing on Saturn’s equator (or floating, I suppose) you’d see the Sun precisely overhead at noon. That’s pretty much the definition of "equinox". It’s true on Earth, too.

The images are coming in from Cassini fast and steady, and it’ll be a few days before they can get all processed and pretty. So for now, check out more raw images from the magnificent machine, and as spring settles in on Saturn’s northern hemisphere, we’ll see lots more coming from Cassini.

CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (25)

Links to this Post

  1. Raw images of Saturns Equinox « Techsteak | August 14, 2009
  1. Funny. Saturn here looks like some kind of painted shell which you could take apart, forming halves.. :)

  2. Bigfoot

    No matter how many times I hear it, I’m always amazed to see evidence of how perfectly thin Saturn’s rings are.

    Fantastic image.

  3. bilhouse

    So does that mean that I could stand an egg on its end if I was on Saturn right now? :)

  4. jedipunk

    equinox? I always thought the equinox was one of the two times a year when the day and night was equally split into 12 hours. (I guess that would put the sun overhead at noon?)

  5. jedipunk – the sun is overhead at noon on the equinox only if you’re standing on the equator (and more specifically, you’re standing on the particular point on the equator where the equinox happens at local noon).

    I guess the strictest definition of equinox would be when the imaginary line of the equator passes through the imaginary line of the ecliptic (the path the sun appears to move on through the sky).

    But they’re really planes, aren’t they? Not lines. Someone else take over. My head hurts.

  6. “like a tree casting no shadow at noon”

    umm, may i suggest that a flagpole is a better analogy?


  7. Asimov fan

    … or 5) you had a one in 8 (or possibly 9) chance of getting it right.

    Or more than nine, let’s see we got :

    1. Mercury
    2. Venus
    4. Mars
    6. Jupiter
    7. Saturn
    8. Ouranos
    9. Neptune
    10. Pluto
    11. Eris
    12. Haumea
    13. Quaoar
    14. Makemake
    15. Sedna

    … & maybe Nibiru too? Or not! 😉


    “… he had left out a planet. It was not his fault; everyone leaves it out. I leave it out myself when I list the nine planets, because it is the four-and-a-halfth planet. I’m referring to Ceres; a small but respectable world that doesn’t deserve the neglect it receives.”

    – Page 63, chapter 5 “The World Ceres” in ‘The Tragedy of the Moon’ by
    Isaac Asimov, Mercury Press, 1973

  8. Andre

    Who said that the equator is an imaginary line?

    On Saturn it doesn’t look imaginary at all!

  9. Spectroscope

    Its NOT ring-less, its just hard to see its ring! 😉

    Being pedantic here, but actually all the gas giants (plus maybe Pluto & Saturn’s moon Rhea) actually have rings – its just that Saturn’s rings are by far the brightest and most spectacular.

    Still a great post & image though. :-)

  10. Spectroscope

    @ 7 Andre :

    Ah – but is it exactly on the equator or offset to the north /south / askew somehow? 😉

    (Honestly, I’m not sure myself but I suspect the former.)

    Also those little kinks and braids in the rings – do they have an effect on the shadow or are they too small in scale to be noticeable?

  11. Petrolonfire

    So … Could they stand eggs on their long ends on Saturn today!? 😉


  12. Galileo would be standing by his optik tube, scratching his thinning hair and wondering what happened to the titan’s children.

    (Obscure astro-reference o’ the morning.)

  13. Mostly, the sun is not at the zenith at noon. No matter where you are on earth, this can happen at most twice a year, which day depending on your latitude. In particular, this never happens in any of the 50 States except Hawaii. Sorry for the snottiness, but I firmly believe that a basic astronomy course is essential to a rounded education.

  14. like a tree casting no shadow at noon

    Sorry, but as I don’t live in Syene, there are, indeed, shadows at Noon.

    ( Bonus points for getting that reference without using an online search tool. :-) )

  15. Dunc

    Nice numbering system for your options there Phil… But you missed option ζ) you spotted the ring shadow on the equator.

  16. Ed Falk

    Wait. The rings are *exactly* on Saturn’s equator? Why would that be? What’s the coupling force? It’s not like the Moon’s orbit is exactly on the Earth’s equator, for instance.

  17. Shoeshine Boy

    You forgot Roman Numeral II as well :)

  18. DrFlimmer

    Any folks living on Saturn, have the ultimate chance, now, to correct any misdrawings of their equator. Just in case it went to the side in some place, now they can push it back to its correct position. Go, Saturnians, time for work!

    (Anyone here who knows “Käpt’n Blaubär”? He once was the equator-controller on earth, with precisely the task described above 😉 )

  19. Flying sardines

    @ 12 Kuhnigget :

    I think I know the one you mean.

    Galileo Galilei : “Does Saturn (the planet like the mythical god) still eat his children?” 😉

    Or very similar words.

    @ 18 Dr Flimmer :

    Umm … Not personally, no. Or at all actually but I’ll take your word for it. 😉


    On Saturn the ocean is just like the sky
    Dense yellow and cloudy where sardines can fly!
    The clouds are faint banded
    Like Jupiter mellowed down
    But the pressure depths below us are hotter than pie!

    There’s hydrogen plenty,
    Some helium too.
    But the water enables the fish to come too!
    On Saturn its murky its warm and its dark
    But in Saturn’s great ocean there’s no sign of shark.
    The transition is even as water turns air
    So sardines have the freedom to soar with great flair! :-)

    – Flying sardines overtired doggrel of the night.

  20. M31

    Ken B, So how far is Syene from Alexandria in stadia anyway?

  21. M31:

    Ken B, So how far is Syene from Alexandria in stadia anyway?

    That I don’t know, but I could look it up.

    Did you recognize the name and context, or did you have to look it up? :-)

  22. I recognized the context. Although I heard the name spoken in Carl Sagan’s voice. 😉

  23. Elwood Herring

    Jonathan Lubin @13: “… I firmly believe that a basic astronomy course is essential to a rounded education.”

    I entirely agree, especially when I read about significant percentages of people thinking the Sun goes round the Earth, or get it right but think it orbits once a day etc.

    … AND it would help to combat the idiocy of astrology (When I tell people I am interested in astronomy I nearly always get the response “Oh really? What star sign are you then?” Aaargh!)

  24. Rich P

    Elwood Herring Says: I nearly always get the response “Oh really? What star sign are you then?” Aaargh!

    Just tell them what star was directly overhead at your place and time of birth. (mine is alpha Triangulum) Any good sky map can give you that. Then you can leave them to wonder what you are talking about, or teach them a little about astronomy.


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