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Those are horseshoe orbits in a rotating reference frame (your link does not provide the caption). Janus and Epimetheus do orbit around Saturn elliptically (exc

by 205guy 7y ago

Those are horseshoe orbits in a rotating reference frame (your link does not provide the caption). Janus and Epimetheus do orbit around Saturn elliptically (except when they switch positions) when seen from an inertial frame. https://en.wikipedia.org/wiki/Epimetheus_(moon) https://en.wikipedia.org/wiki/Epimetheus_(moon) https://en.wikipedia.org/wiki/Horseshoe_orbit https://en.wikipedia.org/wiki/Horseshoe_orbit

chmod775 7y ago
I didn't give it much thought what that gif would look like in a vacuum. I should've probably linked to this instead: https://upload.wikimedia.org/wikipedia/commons/f/ff/Epimetheus-Janus_Orbit.png https://upload.wikimedia.org/wikipedia/commons/f/ff/Epimethe... The orbits probably wouldn't look very fancy if you were observing from Saturn (besides the speeding up and slowing down), but when seen from either of the moons, that's roughly what things would seem like to you.
- chmod775 7y ago
  > The orbits probably wouldn't look very fancy if you were observing from Saturn (besides the speeding up and slowing down) Unless you were moving around the thing at precisely the right velocity I suppose. But it being a gas giant I suppose the point is kind of moot. It's not like you or a manmade device can stand touch down anywhere while still observing the sky.
  - 205guy 7y ago
    "moving around the thing at precisely the right velocity" would be the rotating frame of reference that gives you the horseshoe orbits seen in chmod775's 2 links. My point is that for people like myself, referential frames are not intuitive, and temporarily misleading. If they are unlabeled, one might even confuse them for the inertial frame, and then if one were uncritical enough, assume erroneously that these moons "bounce" back and forth like billiard balls (https://www.teachersource.com/product/newtons-kinetic-yoyo-set-of-12/physics-laws https://www.teachersource.com/product/newtons-kinetic-yoyo-s...). It took a bit of searching, but here is an animation showing how the two moons look in the inertial frame of reference. The interesting bit is how they interact gravitationally so that the faster one never overtakes the slower one, rather it causes the faster one to slow down and the slower one to speed up and pull ahead. https://youtu.be/r9PSimuA9a8?t=45 https://youtu.be/r9PSimuA9a8?t=45 Of course, this is what a knowledgeable reader grasps from the rotating frame horseshoe diagram, but I needed the inertial frame animation to get it.

by 205guy 7y ago

chmod775 7y ago

I didn't give it much thought what that gif would look like in a vacuum. I should've probably linked to this instead: https://upload.wikimedia.org/wikipedia/commons/f/ff/Epimetheus-Janus_Orbit.png https://upload.wikimedia.org/wikipedia/commons/f/ff/Epimethe... The orbits probably wouldn't look very fancy if you were observing from Saturn (besides the speeding up and slowing down), but when seen from either of the moons, that's roughly what things would seem like to you.

chmod775 7y ago
> The orbits probably wouldn't look very fancy if you were observing from Saturn (besides the speeding up and slowing down) Unless you were moving around the thing at precisely the right velocity I suppose. But it being a gas giant I suppose the point is kind of moot. It's not like you or a manmade device can stand touch down anywhere while still observing the sky.
- 205guy 7y ago
  "moving around the thing at precisely the right velocity" would be the rotating frame of reference that gives you the horseshoe orbits seen in chmod775's 2 links. My point is that for people like myself, referential frames are not intuitive, and temporarily misleading. If they are unlabeled, one might even confuse them for the inertial frame, and then if one were uncritical enough, assume erroneously that these moons "bounce" back and forth like billiard balls (https://www.teachersource.com/product/newtons-kinetic-yoyo-set-of-12/physics-laws https://www.teachersource.com/product/newtons-kinetic-yoyo-s...). It took a bit of searching, but here is an animation showing how the two moons look in the inertial frame of reference. The interesting bit is how they interact gravitationally so that the faster one never overtakes the slower one, rather it causes the faster one to slow down and the slower one to speed up and pull ahead. https://youtu.be/r9PSimuA9a8?t=45 https://youtu.be/r9PSimuA9a8?t=45 Of course, this is what a knowledgeable reader grasps from the rotating frame horseshoe diagram, but I needed the inertial frame animation to get it.