# Why does Earth have an axial tilt

1. Dec 4, 2012

### shifty88

Why does Earth have an axial tilt?

I thought it may be due to the suns mass and the gravitational effect on space.
But Mercury and Venus doesn't have a tilt so I concluded that cant be a plausible explanation.

2. Dec 4, 2012

### Mordred

One of the theories I've read on this is that at one time the planet was struck by a smaller planet which later became our moon, which is also acting as a stabilizer currently

3. Dec 4, 2012

### Integral

Staff Emeritus
Because it does.

4. Dec 4, 2012

### Staff: Mentor

And given that there are 360 degrees in a circle, the odds were 360:1 against the tilt rounding to zero.

5. Dec 4, 2012

### mheslep

Perhaps the OP question might have been better phrased as

1. If it is true that the accretion and formation of first the sun and then the planets occurs via a rotating disk over time, i.e. the spinning disk forming a spinning sun/planets, then
2. Why don't all the rotational axis of the planets align better with that of the sun?
3. Do post formation impacts explain all the mis-alignments?

*I don't know to what degree 1. is true.

6. Dec 4, 2012

### Staff: Mentor

That's assuming that axial tilt is a uniformly distributed random variable. It's not good to use the principle of indifference when it doesn't apply. Axial tilt is not uniformly distributed, based on perhaps overly simplistic models of planet formation, and also on a sample size of [strike]nine[/strike] eight (the planets in our solar system).

The answer given by Mordred is the current best guess as to why the Earth has an axial tilt that is not zero but also is apparently quite stable.

7. Dec 4, 2012

### Staff: Mentor

Good point. Since Earth and the solar system formed from a cloud of rotating matter, you'd expect them to rotate and revolve in the same direction, with aligned axes.

....But not exactly.

[I could always change the precision criteria, though... ]

8. Dec 5, 2012

### Jobrag

And given that there are 360 degrees in a circle, the odds were 360:1 against the tilt rounding to zero.

Do all cultures divide a circle into 360 degrees?

9. Dec 5, 2012

### Staff: Mentor

Dunno, but mine has other ways of dividing it as well.

10. Dec 5, 2012

### Staff: Mentor

Per the naive concepts of planetary formation, yes. (Naive = almost everything from 10 years ago or more.) Per these theories, planets formed by sweeping up smaller stuff. As soon as the planetesimal had gained some non-negligible mass it would orbit a bit faster than the smaller stuff around it, gobbling up stuff and clearing a path through that stuff. The collisions with that smaller stuff would have slowed the planet down and made the planetesimal migrate sunward a bit into a fresh batch of stuff to be gobbled up. This created a torque on the planet because of the greater density of stuff-to-be-gobbled in the sunward direction as opposed to the anti-sunward direction. Per these theories, it was this torque rather than conservation of angular momentum that gave the planets their spin.

Explaining Uranus and Venus is a bit difficult with these theories because they don't fit. They don't fit at all. The standard explanation is that something big hit them. It turns out that this hand wave solution may not be needed the case with Venus; what hit Venus might well have been just been chaos theory, triggered by perturbations from Jupiter and the Earth. (Per this chaos-based explanation of Venus's axial tilt, the Earth escaped the chaos thanks to the stabilizing influence of the Moon.)

Explaining the very weird orbits of the hoard of exoplanets discovered in the last ten years is even harder. The simple, naive theory suggests that planets should naturally be in circular orbits. A number of those exoplanets are in anything but circular orbits. The last ten years of discoveries coupled with improved simulations of our own solar system make this simple theory appear to be a bit too naive.

In any case, the current hypothesis regarding the formation of the Moon is also regarded as explaining why the Earth has it's own somewhat anomalous axial tilt.

11. Dec 5, 2012

### SHISHKABOB

I think I hear engineers divide it into 2pi radians ;)

12. Dec 5, 2012

### Whovian

And some mathematicians. The Radian, as it turns out, for some purposes, is a lot more convenient. For instance, the length of a circular arc is (the angle in radians)*(the radius of the arc), no constant needed. It's also nice for differentiating and integrating trigonometric functions.

I think the whole "360 degrees in a rotation, 60 seconds in a minute, 60 minutes in an hour" deal comes from the Babylonians' base 60 number system. And, actually, 60's a fairly beautiful number in Number Theory. (In calculus, the pretty one's e.)

13. Dec 5, 2012

### shifty88

Interesting replies guys, thanks for helping me clear that up and understand that a bit better.
Another thought popped into my mind when reading them

Did planetary formation begin prior to the ignition of the sun or afterwards?

14. Dec 17, 2012

### Philosophaie

One possible explanation is the theory that the Earth was struck by a planetoid which later became the Moon. The Earth never settle back into its original no Axial Tilt because the Moon and Earth form a Barycenter. A Barycenter is when two bodies have a mutual center of mass compared to the sun distance. This creates a "wabble" in the orbit explaining why the Earth has kept its Axial Tilt

15. Dec 17, 2012

### Staff: Mentor

I would say that it was when the gas cloud that existed before our solar system began to collapse that the planetary formation started.

16. Dec 18, 2012

### xzardaz

//offtopic
To continue that:
Let the angle be λ and half* a circle be β (0≥λ≥β and β≠0)
The mesuring unit for λ and β must be the same - doesn't matter radian, degrees or other - see almost all used

If we chose some λ, the chance C that λ is some constant a (0≤a≤β) is 1/(all possible choices for λ)
If we assume that λ∈ℝ, then all possible choises for λ are ∞ (all the real numbers between 0 and β) ⇔ C=1/∞ ⇔ C→0 for any a for any β for any mesuring unit.

So for a=0° C→0. The same is for a=(the actual tilt of the Earth's axis).

* if you do a full rotation, there will be two points at wich the axis will lie on one line - just in opposite direction (so if the initial rotation is clockwise and we rotate the axis by half a circle the rotation will be counter clockwise). That's why β is half a circle.

17. Dec 19, 2012

### lpetrich

Faster??? It would still orbit at very close to its original rate.

This protoplanet would still clear the space around it, because the material there either orbits a bit faster or a bit slower, giving it a chance to run into the protoplanet.

Turning planetary theory upside down: Nine new exoplanets found, some with retrograde orbits relative to their stars' rotations.

Checking on Exoplanet Orbit Database | Exoplanet Data Explorer, the champion eccentricity is for HD 80606 b: 0.9340.

This is for a planet at least 4 times Jupiter's mass orbiting a star not very much different from the Sun. A planet with an orbit with an eccentricity close to what a comet's orbit typically has.

Its semimajor axis is 0.45 AU, making it much like the numerous "warm Jupiters" and "hot Jupiters" that have been found, planets that are too close to have formed with their likely compositions.

18. Dec 19, 2012

### BobG

Actually, the Babylonian base 60 number system comes from 360 degrees in a circle. 360 was the important part and 60 happens to work very well with 360.

There's 360 degrees in a circle - and how many days in a year? Each night, the stars shift approximately 1 degree (slightly less). 360 is pretty close to 365.25, but 365.25 would be a horrible number to build a numbering system around.

But, to answer the original question, only civilizations that came into contact with Babylon, or came into contact with someone who had come into contact with Babylon, use 360 degrees in a circle. That winds up being a pretty big percentage of civilization, but not everyone.

19. Dec 19, 2012

### Staff: Mentor

Faster.

A tiny little planetesimal will orbit at essentially a Keplerian rate. For an Earth-sized object, the velocity is 5 centimeters/second faster than that for a test particle. For a Jovian body, it's a difference of several meters per second.

This is admittedly a tiny effect. There is a much larger effect, though. The gas and dust in the protoplanetary disk orbits at less than a Keplerian rate. The density of that gas and dust decreases with increasing distance from the central plane and with increasing distance from the protostar. For stuff in the central plane, that density gradient makes for an outward pressure. This outward pressure counters the centripetal force of gravity, making the unincorporated gas and dust orbit a bit slower than Kepler's law would indicate.

20. Dec 22, 2012

### mrspeedybob

What about radiation pressure? A smaller object will have a higher surface area to mass ratio making radiation pressure more influential for smaller objects. How does this effect compare to the ones you mention?