Spindown rates of planets

[ajtribick]

Is there any way of calculating how long tidal braking would take to slow down a planet from one rotation period to another? Example if I have a planet spinning on its axis once every sixteen hours, how long would it take to slow down to once every twenty hours?

[Evil Dr Ganymede]

Yes, there is. Unfortunately, it's rather complicated - so I'll just point you to chapter four of my PhD thesis. (5.2MB download, see sections 4.1 and 4.2 specifically for a general treatment)

The solar tides are pretty constant, since the planet's orbital distance presumably isn't changing, so they're easier to cope with. The lunar tides are very much variable since the moon's distance is changing all the time.

[granthutchison]

The solar tides are pretty constant, since the planet's orbital distance presumably isn't changing, so they're easier to cope with.

Hmm, your link doesn't bring anything up but an empty page, for me. Am I right in thinking that in this situation the orbital distance does change slightly in order to preserve angular momentum, but because the orbital angular momentum is so much larger than the change in rotational angular momentum, the change in orbital distance is negligible? Or is something going in that preserves the orbital distance unchanged?

Grant

[Evil Dr Ganymede]

Hmm, your link doesn't bring anything up but an empty page, for me.

Hmm. Try right-clicking and saving the file that way. It's probably coming up empty because it's spending ages downloading the file - the link definitely works though

Am I right in thinking that in this situation the orbital distance does change slightly in order to preserve angular momentum, but because the orbital angular momentum is so much larger than the change in rotational angular momentum, the change in orbital distance is negligible? Or is something going in that preserves the orbital distance unchanged?

Well, OK - to clarify, it does change a bit, but it's a really really small change that isn't worth considering here .

Though of course, nowadays we have all these ideas of planets spiralling in and out through gas drag and other things, but I didn't consider that sort of thing in my thesis.

[granthutchison]

Hmm. Try right-clicking and saving the file that way. It's probably coming up empty because it's spending ages downloading the file

Ah, the file came down like a rocket that time - seems to be a problem with how long it takes my browser to open Acrobat for a big file.

Grant