# Why the earth is rotating along its axis?

1. Jan 7, 2006

### paglren

I'm searching for a theory that explains why the Earth is rotating on its axis and how long ago it began to do it.
I should like to know, also, if rotation is common to all planets or there are some bodies that don't rotate. Are there any bodies that have their rotation axis parallel to their orbit?

2. Jan 7, 2006

### LURCH

Uranus has an axis almost eprpendicular to its orbit.

3. Jan 7, 2006

### .:JimmY:.

Well you might have read or heard about How earth was created, it says small circular discs/ plates came closer and joined together because of gravitational force.
Just assume at some point in a very early phase of the planet it was not spinning and all of sudden a large peice of planetary dics was pulled in because of the gravitational force and it hit earth at some angle and with some velocity- make a good guess what it might have done to the earth- it made it spin.

Last edited: Jan 7, 2006
4. Jan 8, 2006

### Tide

Venus' axes of rotation and revolution are antiparallel.

5. Jan 8, 2006

### paglren

Hi Lurch what you say means that Uranus is more perpendicular than Earth?
My question might be misinterpreted:
Are there planets or bodies in general that have rotation axis perpendicular to revolution axis?

6. Jan 8, 2006

7. Jan 8, 2006

### mathwurkz

Correct me if I am wrong, but I recall that the moon is a body that does not rotate on an axis so one side of the moon always faces Earth.. Hence, you might have heard of the saying, "dark side of the moon" which we never see due to the lack of rotation.

8. Jan 8, 2006

### WarrenPlatts

Actually, Venus revolves around the Sun in the ordinary east-west direction in a near perfect circle. Similarly, Venus's axis of rotation is nearly perfectly perpendicular to its orbital plane--the direction of rotation is retrograde (backwards), not antiparallel, having a sidereal period of about 243 days.
The moon DOES rotate such that the period of rotaion is equal to its period of revolution. If it didn't rotate we'd be able to see the so-called dark side (which is fully lit up during the new moon phase).

9. Jan 8, 2006

### G01

No. The moon does rotate on its axis. It has a rotation period that is exactly equal to its period of revolution. Because of THIS FACT, we can only see one side of the moon. It finishes one revolution at the same time it finishes one rotation, so the same side of the moon will always face us. Think about it.

10. Jan 8, 2006

### mathwurkz

I was on Earth when I made that statement. I guess I should have taken my spacecraft and observed the moon's rotation from somewhere in outer space.

11. Jan 8, 2006

### SpaceTiger

Staff Emeritus
Its spin angular momentum vector is antiparallel to its orbital angular momentum vector, which also means that its spin axis is perpendicular to the orbital plane. Both yours and Tide's descriptions are correct.

Last edited: Jan 8, 2006
12. Jan 8, 2006

### SpaceTiger

Staff Emeritus
Even in the coordinate system (perspective) of a stationary earthbound observer, the moon rotates. If it didn't, you'd see different parts of it as it moved across the sky.

13. Jan 9, 2006

### Chronos

The ecliptic plane hints at why orbital axes are what they are. A planet or two got flipped a bit along the way. That is not hard to explain, and as a bonus... requires no dark matter. Newtonian dynamics work very well at short distances.

14. Jan 9, 2006

### Tide

Warren,

The axes of rotation and revolution are antiparallel - that is exactly what retrograde rotation means.

15. Jan 9, 2006

### LURCH

Sorry, meant to say "Parralel to its orbit". Uranus' north pole points almost strait at the Sun.

16. Jan 9, 2006

### DaveC426913

Only at the height of summer and winter! :rofl: In autumn and spring it doesn't point at the Sun at all! :rofl:

17. Jan 9, 2006

### LURCH

Dave,

Are you sure about that? I heard it stated on a discovery channel special about the Voyager probes (or was it pioneer?) That Uranus is laying sideways, with one poll constantly pointed toward the sun, and the other always pointing away. Since then, I have heard the same statement made by several astronomers on educational television programs, and found it on several web sites as well. But I have always wondered whether this was something we actually knew for certain, or had we only observed Uranus on one brief occasion? Perhaps the poll only points directly at the sun at that one point in the planet's orbit. If one poll always pointed to the sun, then the planet has to have two rotational axes; one the axis of what we think of as normal planetary rotation, and the other a much slower rotation that is tidally locked to the sun.

The statement is made frequently, and quite dogmatically, by people who really should know, but I've never heard anyone explain it in detail.

18. Jan 9, 2006

### DaveC426913

According to nineplanets.org:
"...At the time of Voyager 2's passage, Uranus' south pole was pointed almost directly at the Sun..."
This would imply that the alignment changes with time.

19. Jan 9, 2006

### vincentm

Hello,

Why is Uranus tilted in such a manner?

20. Jan 9, 2006

### LURCH

No one really knows, but the most popular theory is that the planet was struck by on other planet-sized body at sometime in the distant past.

Last edited: Jan 9, 2006