# Question regarding the rotation and orbit directions of the moons Phoebe and Triton **URL:** \ **Category:** Factual Questions\ **Created:** [November 29, 2021, 12:23pm UTC](https://boards.straightdope.com/t/question-regarding-the-rotation-and-orbit-directions-of-the-moons-phoebe-and-triton/955128 "2021-11-29T12:23:58Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![Octagon](https://avatars.discourse-cdn.com/v4/letter/o/2acd7d/32.png) [@Octagon](https://boards.straightdope.com/u/Octagon)\ **Post date:** [November 29, 2021, 12:23pm UTC](https://boards.straightdope.com/t/question-regarding-the-rotation-and-orbit-directions-of-the-moons-phoebe-and-triton/955128/1 "2021-11-29T12:23:58Z")
I’ve read contradictory information online concerning Saturn’s moon Phoebe and Neptune’s moon Triton. I’m trying to find out if: 1. Triton’s rotation and orbit around Neptune are retrograde in direction or just its rotation. 2. Phoebe’s orbit and rotation around Saturn are retrograde in direction or only its orbit? I had assumed not long ago that only one moon, Triton showed a true retrograde motion. Phoebe also shows true retrograde motion. It’s just that different sources online are not always clear as to whether it is in terms of orbit or rotation or both. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [November 29, 2021, 2:15pm UTC](https://boards.straightdope.com/t/question-regarding-the-rotation-and-orbit-directions-of-the-moons-phoebe-and-triton/955128/2 "2021-11-29T14:15:00Z")
I have too much time on my hands. The orbital parameters of both Triton and Phoebe have retrograde orbits. The inclination of the orbit of Phoebe is 173º to the Ecliptic and 152º to Saturn’s equator. The inclination of Triton’s orbit is 130º to the Ecliptic. So the axis of the orbital plane of each points down, representing retrograde orbits. However when it comes to the direction of rotation, that seems harder to find. The document you want is: > **[WGCCRE2009reprint.pdf](https://astropedia.astrogeology.usgs.gov/download/Docs/WGCCRE/WGCCRE2009reprint.pdf)** This defines the axis of rotation of the solar system’s bodies. Critical is the definition of the body’s prime meridian. Termed _W_. That is defined with two components, the second provides the offset as a function of time. If the second component of W is negative, the body rotates in the opposite direction to the other bodies in the solar system. The answer is that Triton has a retrograde rotation, but Phoebe does not. ---
**Author:** ![Octagon](https://avatars.discourse-cdn.com/v4/letter/o/2acd7d/32.png) [@Octagon](https://boards.straightdope.com/u/Octagon)\ **Post date:** [November 29, 2021, 2:55pm UTC](https://boards.straightdope.com/t/question-regarding-the-rotation-and-orbit-directions-of-the-moons-phoebe-and-triton/955128/3 "2021-11-29T14:55:03Z")
Thanks Francis\_Vaughan for answering in terms of rotation. In terms of orbit: Triton does not have a retrograde orbit but Phoebe has.(???) ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [November 29, 2021, 3:03pm UTC](https://boards.straightdope.com/t/question-regarding-the-rotation-and-orbit-directions-of-the-moons-phoebe-and-triton/955128/4 "2021-11-29T15:03:34Z")
Triton has a retrograde orbit as well as Phoebe. Triton has a retrograde rotation, but Phoebe does not. ---
**Author:** ![Octagon](https://avatars.discourse-cdn.com/v4/letter/o/2acd7d/32.png) [@Octagon](https://boards.straightdope.com/u/Octagon)\ **Post date:** [November 29, 2021, 3:07pm UTC](https://boards.straightdope.com/t/question-regarding-the-rotation-and-orbit-directions-of-the-moons-phoebe-and-triton/955128/5 "2021-11-29T15:07:39Z")
(PDF) What can Triton’s retrograde orbit tell us about how the … [https://www.researchgate.net](https://www.researchgate.net) › publication › 258557705\_… "What can Triton’s retrograde orbit tell us about how the Giant Planets(and others) were constructed?Miles OsmastonThe White Cottage, Sendmarsh, Ripley, Woking, Surrey GU23 6JT, United Kingdom ([miles@osmaston.demon.co.uk](mailto:miles@osmaston.demon.co.uk))Of the 166 known satellites of the four Giant Planets (GPs) the outer ones have mixed prograde and retrogrademotions but, of the 56 which orbit their planet at 4 Gm or less, Triton, the 7th largest moon in the solar system,is the only retrograde one and it does so at one-tenth of that distance. " Spacecraft Nears First Stop In Historic Saturn Tour - NASA [https://www.nasa.gov](https://www.nasa.gov) › home › jun › HQ\_04186\_cassini " Phoebe’s retrograde orbit means it goes around Saturn in the opposite direction of the larger interior Saturnian moons. Previous ground-based observations have shown water ice present on its surface." ---
**Author:** ![Octagon](https://avatars.discourse-cdn.com/v4/letter/o/2acd7d/32.png) [@Octagon](https://boards.straightdope.com/u/Octagon)\ **Post date:** [November 29, 2021, 3:08pm UTC](https://boards.straightdope.com/t/question-regarding-the-rotation-and-orbit-directions-of-the-moons-phoebe-and-triton/955128/6 "2021-11-29T15:08:36Z")
Thanks again Francis \_Vaughan. Much appreciated! ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [November 29, 2021, 8:19pm UTC](https://boards.straightdope.com/t/question-regarding-the-rotation-and-orbit-directions-of-the-moons-phoebe-and-triton/955128/7 "2021-11-29T20:19:03Z")
It should be noted that Jupiter also has a number of retrograde satellites. They’re all very small and fairly far from Jupiter. There are three families which are based on their orbital parameters. The families are thought to have originated in the breakup of larger bodies that were captured from solar orbit. The breakups were likely caused by collisions. > **[Moons of Jupiter | Irregular satellites](https://en.wikipedia.org/wiki/Moons_of_Jupiter#Irregular_satellites)** > > The irregular satellites are substantially smaller objects with more distant and eccentric orbits. They form families with shared similarities in orbit (semi-major axis, inclination, eccentricity) and composition; it is believed that these are at least partially collisional families that were created when larger (but still small) parent bodies were shattered by impacts from asteroids captured by Jupiter's gravitational field. These families bear the names of their largest members. The identificat... ---
**Author:** ![Octagon](https://avatars.discourse-cdn.com/v4/letter/o/2acd7d/32.png) [@Octagon](https://boards.straightdope.com/u/Octagon)\ **Post date:** [November 30, 2021, 4:15am UTC](https://boards.straightdope.com/t/question-regarding-the-rotation-and-orbit-directions-of-the-moons-phoebe-and-triton/955128/8 "2021-11-30T04:15:51Z")
Thanks dtilque. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [November 30, 2021, 4:44am UTC](https://boards.straightdope.com/t/question-regarding-the-rotation-and-orbit-directions-of-the-moons-phoebe-and-triton/955128/9 "2021-11-30T04:44:22Z")
One thing I should have mentioned. It’s virtually impossible for a satellite to form in a retrograde orbit, so all of them got that way through some unusual occurance, usually a capture from solar orbit. Although there was a hypothesis that postulated that Pluto originated as a satellite of Neptune. It had a close encounter with Triton which resulted in Pluto escaping to solar orbit and Triton in a retrograde orbit. But then the rest of the Kuiper Belt was discovered and that idea was tossed in favor of Triton being a captured Kuiper Belt Object.