# How do you define a planet's rotation?

1. Jul 16, 2012

### skiller

Hi,

I'm more of a maths man but I'm quite keen on astronomy, although without any formal training. I hope I've put this question in the right place and isn't too dumb!

What defines the rotation of a planet, ie its DAY? Surely you need some sort of point of reference to measure how much it rotates. (Never mind how to define exactly its axis of rotation!)

For gas planets, such as Jupiter, Uranus and Neptune, what are we looking at for our points of reference? Doesn't the gas just flow all over the place?

For planets like Mercury, Venus, Earth and Mars, we obviously have solids to observe as reference points; but technically, they aren't really fixed at all either are they? Over the course of the history of the Earth, for example, our continents have moved significantly.

I'm probably wrong, but the "DAY" of a planet actually means nothing. Please correct me if I am!

2. Jul 16, 2012

### Anti-Crackpot

By that logic one could argue that distance has no meaning on a boat out of sight of land since the water is moving all over the place ;-)

3. Jul 16, 2012

### DaveC426913

I don't know how they do it officially, but...
While they do flow, an average can be calculated. If you choose a frame of reference in which all your measurement points have the least relative motion, you can take that as the average rate of movement of the planet's surface.

Continents move on the order of micrometers per day. So, if you chose a specific spot to measure the day by, your results would have a margin of error of a couple of micrometers over 25,000 miles per day. How close do you want it?

Closer? So choose several points on different continents (which are not moving the same direction as your first point) and average them. The average is tantamount to the Earth's rotation rate as a whole.

4. Jul 17, 2012

### Staff: Mentor

This is significant on the sun, where the poles rotate at a different rate compared to the equator. The gas planets are easier, you can define wind relative to the dense inner regions. Subtract wind, and you get the rotation.

5. Jul 17, 2012

### Philosophaie

The gas planets are spinning at a constant rate.

The only factor is the wind and the tidal force of the methane and air composition.

6. Jul 17, 2012

### DaveC426913

Not sure you're getting the gist of the OP's question. If they are made of shifting gasses under constant movement, how can we determine what that rate is?

7. Jul 17, 2012

### Philosophaie

Why do you not take the Vernal Equinox direction of Earth and assign an instantaneous point on the Gas Giants core then the wind, the atmosphere and Weather are not a concern.

8. Jul 17, 2012

### DaveC426913

How do you see the core to know what it's doing?

9. Jul 17, 2012

### Philosophaie

A look inward must be the answer. The center of gravity of these gas planets is our point of reference. No wind or atmosphere clouding our view of no discernible landscape.

The liquid metal core is the center of mass. The place where all the gravity is centered.

10. Jul 17, 2012

### skiller

Maybe a single boat cannot measure distance, but two certainly can, by having a measuring stick between the two. (I don't see what your analogy has to do with this, tbh!)
Yep, I thought as much; so it's just approximations of average positions of "stuff" really, is it?

Thanks for the replies, people!

11. Jul 18, 2012

### DaveC426913

And how do you determine the rate of rotation of a point? Let alone a point you cannot see.

And how do you observe the rate of rotation of this core?

12. Jul 18, 2012

### Philosophaie

At the previously mentioned reference point the wind and the tidal forces can be measured. The speed can the be calculate.

13. Jul 18, 2012

### DaveC426913

We were talking about gas giants. How do you establish a reference point and assert that it is stationary, such that you can measure with respect to it?