Saturn | Rings and Moonlets

Saturn is one of the favorite objects of astronomers. It has been viewed and imaged millions of times. The Cassini-Huygens spacecraft orbits Saturn and returns images of the rings, moons, and moonlets. Operated at Saturn since July 2004, Cassini yields views of the fine structure details of the rings.

Galileo thought there were three objects when he telescopically viewed Saturn in March 1610 noted in a letter to his patron Medici in July 1610. From The Galileo Project at Rice University:

I discovered another very strange wonder, which I should like to make known to their Highnesses . . . , keeping it secret, however, until the time when my work is published . . . . the star of Saturn is not a single star, but is a composite of three, which almost touch each other, never change or move relative to each other, and are arranged in a row along the zodiac, the middle one being three times larger than the lateral ones, and they are situated in this form: oOo.


U of IA Robotic Telescope | Winer Observatory | Sonoita AZ


Saturn takes about 30 yrs to orbit the Sun. The plane of the rings is tilted with respect to the plane of the orbit. From our orbit closer to the Sun, our view of the rings varies over time as illustrated in this graphic. Sometimes we see the rings from above, sometimes below, and sometimes they are edge on to us and not visible. The edge on views are the equinoxes of Saturn seen every 15 yrs.



This beautiful collection of images taken over a 6 yr interval by Alan Friedman between 2004-2009 appeared on the Astronomy Picture of the Day web site. Bookmark APoD if you haven’t yet.



Cassini completed the primary mission exploring the Saturn System in June 2008. The mission was extended to September 2010. The spacecraft is now in a second extended mission called the Cassini Solstice Mission which goes through September 2017.

Cassini has helped confirm the orbits of 62 moons of Saturn. Among many wonderful discoveries over the years about those moons, Cassini has looped close by the ring system many times and viewed them from many different vantage points. Some views were from afar and some extremely close. This image in silhouette revealed some previously unknown faint rings.



Some of the most interesting views of the rings have come during the times near equinox when the rings are illuminated at their edge. They are less visible to us from our Earth view at that time. But, Cassini is close enough to still capture their details. In these close and detailed views, there are some surprising features of shadows cast across the ring plane by small shepherd moons, moonlets, and by piles of ice in a ring. Shepherd moons clear gaps between rings and bring stray ice chunks back into rings by gravitational interactions. The fine structures of the rings look like the tiny grooves of a phonograph record. Here are some beautiful examples.


This 53 sec video shows the outer F-ring orbiting Saturn. At the 34 sec mark, watch the two shepherd moons come around the orbit. Their action as a pair keeps the smaller particles of the F-ring in a tightly confined narrow band.


The Cassini-Huygens Mission is a resounding success. Cassini will continue returning data until September 2017. Tens of thousands of images have been obtained. You can browse them at this convenient web site called CICLOPS – Cassini Imaging Central Laboratory for Operations. Or, use this search page. Type in a keyword or two and click search. Try ‘pandora’. Click on any of the results.



11 thoughts on “Saturn | Rings and Moonlets

      • Hi, Jim.
        Thank you for your note on my website, and I certainly do not want to miss out on your card!
        My address is: Melissa Pierson
        230 Burton St.
        Grayslake, Il, 60030
        I look forward to it 🙂
        Now that the dust has settled from my son’s graduation, (at Northern, and yes, we were all thinking of the shooting there)
        I can turn my attention to writing cards, too. Can you give me your’s? I think I have it but would like to be sure.


  1. The thought of “shepherd moons” keeping the rings under control is almost more than I can conceive. It’s funny, and amazing, and intriguing. You’re right that the photos are spectacular. We’re so blessed to have the technology to see these things: the people to create it, and the people to use and refine it.

    The phonograph analogy is a good one, given those “grooves.” Maybe Saturn needs a song. Instead of “Shine On, Harvest Moon,” we could have something like “Roll On, Shepherd Moons.”

    Liked by 1 person

    • Great choice of music. 🙂

      Their actions are really simple and complex. Simple because they utilize the force of gravity. Complex because they create wave structures in the band of smaller particles. Have a look at this video.

      Modeling this behaviour is challenging. Thanks for being here.


  2. Amazing. It really does look like a phonograph record, with slight variations in the grooves. The video shows what looks like pulses of larger rocks in the ring.
    Seeing the different appearance of the rings when they are at the various angles compared to us reminds me of a question I’ve been wrestling with, that maybe you can explain. Why does the moon rise sometimes to the north of east and sometimes south of east? It’s on a monthly schedule so I’m guessing it has to with its period of revolution, and likely to do with its inclination relative to the equator. Or to the ecliptic? Why is it inclined like that? and cause the moon to be in very different locations in the sky. Maybe you could write a piece about this, with diagrams – you have a knack for explaining things clearly for regular people. 🙂


    • OceanDiver, Jim can probably explain this better, but in a nut shell the moon follows the same path (the ecliptic plane) across the sky as the sun. So because of the earth’s tilt the sun rises north of east in the summer and south of east during the winter. The moon does the same but because we usually observe it at night it does just the opposite. It rises north of east during the winter and follows a long path across the sky. In summer it rises south of east and follows a short path across the southern sky.


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