# Moons changing their rotational periods

There is a planetary system with a big planet and two moons, Luna and Laltre orbiting around it.

Luna has a pretty similar orbiting (1 earth-day) and rotating period (slightly less than 1 earth-day). It's mass is similar to Earth's.

Laltre has a very eccentric orbit around the planet.

Is it possible that the gravitational effect of Laltre over Luna changes Luna's rotating period every now and then? If so, how or when it would happen?

Feel free to add further information regarding Luna, Laltre and their relationship that's needed to answer the question.

EDIT: Note that I only would like to know if changing the rotating period of a moon is possible. Regarding the how or when is intended to have a slight understanding of what conditions make that change possible. Maybe any conditions make it, so there would be not much more to add.

• I think you need to specify the masses and the distances of these moons. Else the answer will be a planetary dynamic dissertation. – L.Dutch Nov 20 '18 at 7:38
• @L.Dutch I understand that it might be needed to give an estimation on how much the rotating period would change, but I only ask about if such change is possible. In case distance or mass are relevant, that's what the how and when pretend to ask (simply explaining why it would be relevant). – Masclins Nov 20 '18 at 7:43
• You might be interested to read up on Epimethus and Janus, two of the moons of Saturn. Their rotational periods don’t change, which is why this isn’t an answer, but they swap places in their orbits every four years or so (Earth years, not Saturn years). planetary.org/blogs/emily-lakdawalla/2006/… – Mike Scott Nov 20 '18 at 9:35

Rotation in celestial body is a consequence of momentum conservation. The only way to change the rotational period of a body is by adding or removing rotational momentum to the body.

For a perfectly homogeneous body gravity will not be able to add or remove momentum, however for non homogeneous bodies difference of gravitational pull can exert a torque on the body, resulting in changing the rotational period.

Luna seems to be almost tidally locked with the main planet, thus it means that its shape will have a bulge pointing toward the planet, as a consequence of tidal effects.

When Laltre approaches the periapsis, if it is close enough to Luna it can exert a significant differential force on the bulge. However, due to highly eccentric orbit, Laltre velocity in that point will also be high, resulting in a short time of the force being applied.

Given the right combination of factor, Laltre can affect the rotational period of Lune, then.

Another way, much more dramatic, in which the rotational period can be affected is by having Laltre impacting Luna with a very oblique impact angle. A similar event is thought to have slowed down the rotation of Venus, for example.

Youre probably searching for sth like this:

This is possible, but the moons could and would probably crash at some point
(What could be a pretty cool story)
The effects would be a pretty rough tide

EDIT:
The period change would happen if the planets speed/slow each up/down,
because speed means more radius (up until flying away), and less speed means less radius (down until crashing the orbit center/earth)

• Thanks for the picture. That, though, would be a change in orbit, but it says nothing to how or whether the rotation period would change. – Masclins Nov 20 '18 at 8:15
• @Masclins No problem with my MS Paint skills :D The Inner moon stays longer outside, while the inner moon stays longer on the inside, is a change of period. When also considering a change of route, the time definetly changes ... I'll make another pic – user55267 Nov 20 '18 at 8:16
• @Masclins I've updated – user55267 Nov 20 '18 at 8:38