# Can moons have moons? **URL:** \ **Category:** Factual Questions\ **Created:** [June 11, 2015, 5:44am UTC](https://boards.straightdope.com/t/can-moons-have-moons/722240 "2015-06-11T05:44:14Z")\ **Posts on this page:** 5\ **Page:** 3
**Author:** ![Balance](https://avatars.discourse-cdn.com/v4/letter/b/ccd318/32.png) [@Balance](https://boards.straightdope.com/u/Balance)\ **Post date:** [June 15, 2015, 7:32pm UTC](https://boards.straightdope.com/t/can-moons-have-moons/722240/41 "2015-06-15T19:32:15Z")
> [@nearwildheaven](#): > > I’m surprised I haven’t seen the word “planetesimal” in this thread yet. Not directly, but I was playing on it when I suggested “moonimal”. ---
**Author:** ![Goosie](https://avatars.discourse-cdn.com/v4/letter/g/b5a626/32.png) [@Goosie](https://boards.straightdope.com/u/Goosie)\ **Post date:** [June 16, 2015, 1:10am UTC](https://boards.straightdope.com/t/can-moons-have-moons/722240/42 "2015-06-16T01:10:41Z")
I remember on an old Beany & Cecil episode that Earth’s moon had a moon, and they called it the “Schmoon”. ---
**Author:** ![OldGuy](https://avatars.discourse-cdn.com/v4/letter/o/3bc359/32.png) [@OldGuy](https://boards.straightdope.com/u/OldGuy)\ **Post date:** [June 16, 2015, 2:44am UTC](https://boards.straightdope.com/t/can-moons-have-moons/722240/43 "2015-06-16T02:44:32Z")
> [@Saint\_Cad](#): > > No because the Lagrange Points also take centripetal force into account. Nitpick. You mean centrifugal force. Centripetal force is center-directed force; that’s the gravity. The L1 Lagrange point between a large mass M and a small mass m is located where the gravity pulling towards the large mass equals the gravity pulling towards the small mass plus the centrifugal force pointed away from the large mass and towards the small mass (outward from large mass because the three objects orbit around the barycenter which is between the large mass and the Lagrange point). The equation is M/(R-r)[sup]2[/sup] = m/r[sup]2[/sup] + [M/R[sup]2[/sup] - (M+m)r/R[sup]3[/sup]] The last term is the centrifugal force. ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [June 17, 2015, 5:40am UTC](https://boards.straightdope.com/t/can-moons-have-moons/722240/44 "2015-06-17T05:40:15Z")
> [@OldGuy](#): > > Nitpick. You mean centrifugal force. Centripetal force is center-directed force; that’s the gravity. > > The L1 Lagrange point between a large mass M and a small mass m is located where the gravity pulling towards the large mass equals the gravity pulling towards the small mass plus the centrifugal force pointed away from the large mass and towards the small mass (outward from large mass because **the three objects** orbit around the barycenter which is between the large mass and the Lagrange point). The equation is > > M/(R-r)[sup]2[/sup] = m/r[sup]2[/sup] + [M/R[sup]2[/sup] - (M+m)r/R[sup]3[/sup]] > > The last term is the centrifugal force. bold added Say this again.Three objects, including L. Point, orbit, so that “point” is a moving target? O . o Large L.point small ---
**Author:** ![sbunny8](https://avatars.discourse-cdn.com/v4/letter/s/c77e96/32.png) [@sbunny8](https://boards.straightdope.com/u/sbunny8)\ **Post date:** [June 17, 2015, 1:15pm UTC](https://boards.straightdope.com/t/can-moons-have-moons/722240/45 "2015-06-17T13:15:05Z")
> [@Leo\_Bloom](#): > > bold added > > Say this again.Three objects, including L. Point, orbit, so that “point” is a moving target? > > O . o > > Large L.point small Yes, the L1 point is a moving target, relative to the barycenter (or in fact relative to any inertial reference frame). But, no, it doesn’t move relative to the other two objects or the imaginary line connecting them. The two objects rotate around the barycenter and the imaginary line connecting them rotates around the barycenter and the L1 point is on this rotating line but it doesn’t move relative to the line. Although, if you want to get picky, since all orbits have some eccentricity (ellipses rather than perfect circles) the line itself stretches and shrinks with each period of the orbit, so in that sense the L1 point is a moving target. But the idea is that if your spaceship could reach the L1 point and match velocities with it then afterwards your momentum and the gravity of the two bodies would pull you along with the L1 point’s motion and wherever the L1 point moved your ship would move with it. [Previous page](https://boards.straightdope.com/t/can-moons-have-moons/722240.md?page=2)