# Earth's Precession?

Discussion in 'Astronomy, Exobiology, & Cosmology' started by D-9, Dec 6, 2011.

1. ### D-9Registered Member

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26
Hi all, this is D-9 here. I have a question regarding Earth's precession. So what better place to ask than a science forum.

Basically, why is it that Earth rotates and revolves around the Sun in a counterclockwise fashion, while Earth precesses on its axis in a clockwise manner; IOW Earth rotates and precesses in opposite directions, why?

Also, do all objects that undergo precession rotate and precess in opposite directions, is it just the end result of the laws of physics?

3. ### arfa branecall me arfValued Senior Member

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7,696
I think precession occurs because angular momentum points in a different direction than angular velocity. The precession of the Earth's axial orientation is (I think) independent of its orbital precession around the Sun.

I don't know about a 'rule' that makes planets rotate and precess in opposite directions.

5. ### James RJust this guy, you know?Staff Member

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The direction of precession of an objects depends on the direction of the applied torque and the direction of rotation of the object. Different combinations are possible.

7. ### D-9Registered Member

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Thanks for the replies.

It sounds like it is just the way it worked out, Earth could have a precession in the same direction as rotation if the conditions were met. Do you know why Earth precesses clockwise? I mean beyond that the torque force's vector is pointing in that direction - what is putting that vector direction there? Thanks.

8. ### James RJust this guy, you know?Staff Member

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37,189
D-9:

I'd have to check what the main causes are for the precession of Earth's axis. I imagine it is mostly due to a torque from the Sun acting on the Earth because it is not spherical, but I am aware that the other planets may also play a role.

I also haven't checked that the direction of precession is opposite to the direction of spin of the Earth. Are you sure that's the case?

Maybe somebody else can help with this.

9. ### D-9Registered Member

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26
According to my astronomy textbook the precession of the Earth is caused by the Sun and the moon, the other planets have an effect but it is very small. The diagrams it has shows the precession in the opposite direction of rotation.

When they talked about sidereal days vs. solar days, the sidereal day is shorter.

My interpretation: The sidereal day is shorter because Earth is revolving around the Sun in the same direction that it is rotating, so that the Earth needs to rotate less for the stars to be in the same position as defined by a sidereal day; if the Earth is rotating in the opposite direction the solar day would be shorter than the sidereal day because the Earth has to revolve less for the Sun to be in the same position.

Textbook: Now the textbook also says that because of the precession, the sidereal year is longer than the tropical/calendar year. This is also why the solstices and the equinoxes change over the years.

My interpretation: The same logic applies to the year as does the day. In the same direction the sidereal year should be shorter as the Earth needs to move slightly more to compensate for the moving position of the Sun (by perception) as the Sun is playing catch up. While in the opposite direction the Sun should be in its place slightly sooner, which is what we observe. But since we rotate and revolve one way, and we precess another way according to my thinking, that poses a mystery, at least to me.

Not sure if that was clear at all, I would put up images but I can't until I have 20 posts. But if you go to google and type in "sidereal vs. solar day" and "rotation precession" you should get some images that resemble what I'm talking about.

D-9

10. ### James RJust this guy, you know?Staff Member

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D-9:

There's a reasonably good article on axial precession on wikipedia:

Axial precession

That's correct. I have now checked this.

Yes.

The answer, I think, is that the Sun and Moon exert gravitational forces on the Earth that attempt to align the Earth's rotational axis with the ecliptic polar axis (i.e. they "try" to eliminate the tilt of the Earth's axis).

These force produce an average torque at right angles to the rotational axis of the Earth. The direction that the rotational axis precesses is in the direction of the applied torque.

Does that help?

11. ### D-9Registered Member

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26
Thanks James R, that helps out a lot. I really wish I knew physics so I could understand the equations used, but I think I pretty much got it.