# Possible rotation of a tidal locked object?

1. Aug 21, 2008

### Gajillian

Hello everyone!
Real quick question here,
I've been trying to figure this out on my own all day to no avail.
Sorry if it is a dumb question....

"Can an object locked in orbit, rotate on an axis if the axis is horizontal?"

If for example a planet's north pole was locked in orbit to a sun, could it rotate on it's side? (see picture)

GJ

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• ###### tidal locked .jpg
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2. Aug 21, 2008

### mgb_phys

Yes, tidal forces wouldn't effect rotation in that plane because any point on the planet stays the same distance from the sun and so there is no torque for the tidal force to act on.

(unless there is some second order precession effect I have forgotten about - whihcis why I normally don't risk answering questions on gyroscopes!)

3. Aug 22, 2008

### tiny-tim

Welcome to PF!

Hello Gajillian! Welcome to PF!

I think the more problematic question is: can an object rotate on an axis if the axis is towards whatever it's orbiting?

A planet's rotation must remain the same …

the Earth's points towards the Pole Star (more or less), except that precession moves the axis extremely slowly in a circle once every 25,000+ years …

but the star it's orbiting is always in a different direction.

Even if we superimposed a "horizontal" rotation on a basic "vertical" rotation, the "horizontal" addition would have to be constant.

Your object couldn't have a "horizontal" axis of rotation.