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Offline Dlarah

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Why does the earth tilt?
« on: 03/02/2009 05:47:40 »
The earth is tilted about 23.5 degrees, and I was wondering why. Is it because of gravity?
« Last Edit: 11/02/2009 12:57:14 by BenV »


 

Offline Chemistry4me

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Re: Why does the earth tilt?
« Reply #1 on: 03/02/2009 05:56:25 »
Probably some big impacts that eventually tilted the axis...
 

Offline Chemistry4me

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Re: Why does the earth tilt?
« Reply #2 on: 03/02/2009 05:59:35 »
I don't think anyone really knows, so you can speculate all you want... :)
 

Offline syhprum

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Re: Why does the earth tilt?
« Reply #3 on: 03/02/2009 06:58:58 »
I think it's because it's not completely homogeneous continents rising and sinking make it unbalanced
 

Offline Chemistry4me

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Re: Why does the earth tilt?
« Reply #4 on: 03/02/2009 07:05:16 »
Does that mean that it can keep on tilting?
 

lyner

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Re: Why does the earth tilt?
« Reply #5 on: 03/02/2009 08:26:37 »
I think the real question is "why should we expect it not to tilt?"

The supplementary question would also be about how the planets' orbits lie so close to a plane.
 

Offline DoctorBeaver

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Re: Why does the earth tilt?
« Reply #6 on: 03/02/2009 10:04:57 »
As Chemistry4Me stated, the common belief is that it was caused by impacts; the most violent of these being the collision that created the moon.

The Earth actually wobbles on its axis.


graphic from http://www.geocities.com/astrologyages/precession.htm

It takes almost 26,000 years for the axis to make a complete circle (to sweep out a cone).

The supplementary question would also be about how the planets' orbits lie so close to a plane.

Probably because they were formed from a disk of matter surrounding the sun.
 

lyner

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Re: Why does the earth tilt?
« Reply #7 on: 03/02/2009 11:42:41 »
..... and why was it a disc?



The Sun would have been formed from an immense Nebula - a cloud of Hydrogen gas and dust which pulls itself together by the gravity due to its mass. This cloud would have been rotating a bit around some axis (being stationary would have been a real exception). As the Nebula collapsed, the angular momentum would have been conserved so the spin would have become faster and faster (like a skater, as she brings her leg and arms closer into her body).
Smaller and smaller - faster and faster, until the diameter of the 'blob' of matter was small enough for it to become unstable. Then, some bits flew off (the future planets) until the new Sun became a stable size for its rotation speed. The flung-out bits would all be in or near a plane (round the Sun's equator) and they would also be swirling and spinning with their axes more or less parallel with the Sun's axis. Random motions and gravity will send them bumping amongst themselves will result in the bits gathering into lumps (the planets), each one spinning with an axis roughly parallel to the Sun's axis. But they may well end up with tilted axes - like Uranus, for example, which rotates at about 90degrees.
Planets are a bit 'lumpy'; they are not of uniform composition. This means that they will probably wobble as they rotate. Large planets are molten inside and so the massive lumps can shift around inside them and, every now an then, the outside skin of the planet may move to preserve the total angular  of the planet.
Because they are not of uniform construction, they can interact with each other (gravity again) and one object's rotation can transfer to that of another object. This is why the Moon rotates at a rate which is the same as the rate that it orbits the Earth. It's also why the Earth's tides are causing the Moon's orbit to be increasing in diameter every year by a tiny bit.
 

Offline Vern

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Re: Why does the earth tilt?
« Reply #8 on: 03/02/2009 11:45:54 »
Quote from: DoctorBeaver
t takes almost 26,000 years for the axis to make a complete circle (to sweep out a cone).
Interesting; I wonder how that correlates to the ice age cycle which I think is less than that. It might be a multiple of it.
 

Offline LeeE

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Re: Why does the earth tilt?
« Reply #9 on: 03/02/2009 14:45:08 »
First of all, just to get things clear, the Earth doesn't tilt, which implies an activity; but is tilted.

Yes, the Earth does wobble on it's axis, and so the tilt does cyclically change, but only by a very small and mostly negligible amount.

The current thinking is not that the planets are spun-off from the central star, until the star reaches a state of stability, but that they form within the protoplanetary disk which forms around the protostar.

From:http://en.wikipedia.org/wiki/Protoplanetary_disk

"Protostars typically form from molecular clouds consisting primarily of molecular hydrogen. When a portion of a molecular cloud reaches a critical size, mass, or density, it begins to collapse under its own gravity. As this collapsing cloud, called a solar nebula, becomes denser, random gas motions originally present in the cloud average out in favor of the direction of the nebula's net angular momentum. Conservation of angular momentum causes the rotation to increase as the nebula becomes smaller. This rotation causes the cloud to flatten out—much like forming a flat pizza out of dough—and take the form of a disk. The initial collapse takes about 100,000 years. After that time the star reaches a surface temperature similar to that of a main sequence star of the same mass and becomes visible."

and from:http://en.wikipedia.org/wiki/Planetary_formation

"The protoplanetary disk is an accretion disk which continues to feed the central star. Initially very hot, the disk later cools in what is known as the T tauri star stage; here, formation of small dust grains made of rocks and ices is possible. The grains may eventually coagulate into kilometer sized planetesimals. If the disk is massive enough the runaway accretions begin, resulting in the rapid—100,000 to 300,000 years—formation of Moon- to Mars-sized planetary embryos. Near the star, the planetary embryos go through a stage of violent mergers, producing a few terrestrial planets. The last stage takes around 100 million to a billion years.

The formation of giant planets is a more complicated process. It is thought to occur beyond the so called snow line, where planetary embryos are mainly made of various ices. As a result they are several times more massive than in the inner part of the protoplanetary disk. What follows after the embryo formation is not completely clear. However, some embryos appear to continue to grow and eventually reach 5–10 Earth masses—the threshold value, which is necessary to begin accretion of the hydrogen–helium gas from the disk. The accumulation of gas by the core is initially a slow process, which continues for several million years, but after the forming protoplanet reaches about 30 Earth masses it accelerates and proceeds in a runaway manner. The Jupiter and Saturn–like planets are thought to accumulate the bulk of their mass during only 10,000 years. The accretion stops when the gas is exhausted. The formed planets can migrate over long distances during or after their formation. The ice giants like Uranus and Neptune are thought to be failed cores, which formed too late when the disk had almost disappeared."


The end result is that the planets in a planetary system end up orbiting in the same direction, rotating in the same direction and share the same orbital plane.

All of the planets in our solar system (now that Pluto has been relegated) still orbit in the same plane and direction but some of them have obviously had their axis of rotation knocked about a bit.  The Earth is tilted at just under 23.5 degrees away from the plane-vertical and was almost certainly caused by the collision that resulted in our moon, as DrBeaver says.  Collisions from smaller objects though are likely to have had little effect, not only because of the relative sizes but also because there would have been many more of them and would have tended to cancel each other out.  Saturn too has quite a large degree of tilt, which is why it's only possible to see the rings from Earth when it's at the right points of it's orbit; currently, it's edge-on to us.  Once again, because of it's size, the collision that tilted Saturn must have been pretty big (although it may have happened before it was it's current size, but in any case, it would still have been considerably larger than the Earth), and as Saturn's rings are aligned with it's tilt, they must have formed after it tilted.  Uranus's tilt is the greatest of all at 97.77 degrees and the collision that caused it must have been correspondingly big (although as with Saturn, it has probably grown to it's current size since the collision).  As the quote above notes, the gas giants could have migrated over long distances, so it's possible that both Saturn's and Uranus's tilt occurred from the same collision i.e. they collided with each other.  This is just pure speculation though as I don't think there's any evidence to support this, apart from a lack of any other suitable bodies remaining in the solar system.
 

Offline demadone

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Re: Why does the earth tilt?
« Reply #10 on: 03/02/2009 15:00:16 »
..... and why was it a disc?



The Sun would have been formed from an immense Nebula - a cloud of Hydrogen gas and dust which pulls itself together by the gravity due to its mass. This cloud would have been rotating a bit around some axis (being stationary would have been a real exception). As the Nebula collapsed, the angular momentum would have been conserved so the spin would have become faster and faster (like a skater, as she brings her leg and arms closer into her body).
Smaller and smaller - faster and faster, until the diameter of the 'blob' of matter was small enough for it to become unstable. Then, some bits flew off (the future planets) until the new Sun became a stable size for its rotation speed. The flung-out bits would all be in or near a plane (round the Sun's equator) and they would also be swirling and spinning with their axes more or less parallel with the Sun's axis. Random motions and gravity will send them bumping amongst themselves will result in the bits gathering into lumps (the planets), each one spinning with an axis roughly parallel to the Sun's axis. But they may well end up with tilted axes - like Uranus, for example, which rotates at about 90degrees.
Planets are a bit 'lumpy'; they are not of uniform composition. This means that they will probably wobble as they rotate. Large planets are molten inside and so the massive lumps can shift around inside them and, every now an then, the outside skin of the planet may move to preserve the total angular  of the planet.
Because they are not of uniform construction, they can interact with each other (gravity again) and one object's rotation can transfer to that of another object. This is why the Moon rotates at a rate which is the same as the rate that it orbits the Earth. It's also why the Earth's tides are causing the Moon's orbit to be increasing in diameter every year by a tiny bit.

I always thought that the consistence of the planets tells us that they were actually captured from a nearby supernova explosion. Otherwise we wouldn't have Gold or Platinum on earth, just carbon, iron and gases. But then again I might be wrong.
« Last Edit: 03/02/2009 15:15:05 by demadone »
 

Offline DoctorBeaver

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Re: Why does the earth tilt?
« Reply #11 on: 03/02/2009 15:07:19 »
demadone - The cloud of gas that condensed to form the sun & planets contained ejecta from a nova or novae. If material from novae had not been present then the planets would consist of hydrogen, helium and, possibly, a teensie-weensie bit of lithium, and nothing else. Those 3 elements are the only 3 that were formed as a result of the Big Bang. All elements heavier than lithium have been made inside stars.

Working out that the heavier elements were the results of stellar synthesis was probably Fred Hoyle's greatest accomplishment.
« Last Edit: 03/02/2009 15:10:35 by DoctorBeaver »
 

Offline LeeE

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Re: Why does the earth tilt?
« Reply #12 on: 03/02/2009 15:16:04 »
Yup - agreed, and quite a staggering thought.
 

Offline DoctorBeaver

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Re: Why does the earth tilt?
« Reply #13 on: 03/02/2009 15:31:25 »
A stroke of pure genius.
 

Offline yor_on

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Re: Why does the earth tilt?
« Reply #14 on: 05/02/2009 00:45:30 »
So it can't be John Barleycorn related then?

What can I say?
It's very dark here:)
 

Offline Chemistry4me

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Re: Why does the earth tilt?
« Reply #15 on: 05/02/2009 00:48:50 »
Who is John Barleycorn?
 

Offline Chemistry4me

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Re: Why does the earth tilt?
« Reply #16 on: 05/02/2009 01:01:14 »
"John Barleycorn Must Die is a 1970 album by English rock band Traffic"

http://en.wikipedia.org/wiki/John_Barleycorn_Must_Die

This what you're talking about?
 

Offline yor_on

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Re: Why does the earth tilt?
« Reply #17 on: 05/02/2009 11:16:35 »
John Barleycorn: An alcoholic beverage that is distilled rather than fermented
------
Just had to teas(t) it
Not that Earth tilt that much:)

http://www.johnbarleycorn.com/about.htm
 

Offline LeeE

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Re: Why does the earth tilt?
« Reply #18 on: 05/02/2009 17:08:59 »
Alcohol has to be fermented before it's distilled.  It's the fermentation process that makes the alcohol, distilling it just refines it once it's been made by fermentation.
 

Offline yor_on

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Re: Why does the earth tilt?
« Reply #19 on: 05/02/2009 19:09:47 »
Yep I know:)
That was just a definition of what we Swedes call 'sprit'

Hard liquor in the States:)
 

Offline A Davis

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Re: Why does the earth tilt?
« Reply #20 on: 11/02/2009 00:36:56 »
Your all going to hate me again. It has been calculated from the Associated Legendre Polynomial P41cosθ
it has a maximum value of 23.4deg. The solution is due to the fact that the Moon has joined with the Earth and caused the tilt, remove the Moon and the tilt is zero.
 

lyner

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Re: Why does the earth tilt?
« Reply #21 on: 11/02/2009 12:17:23 »
What's "hate" got to do with it? Is that why you post this stuff? If you want hate, it's no use conversing with phlegmatic Scientists. Go to a more nutty website if you want 'emotions'.

What values did you put into the Legendre Polynomial to get that value. Could you show your workings, please?

Did you know that Science is based on detail?
 

Offline A Davis

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Why does the earth tilt?
« Reply #22 on: 12/02/2009 00:18:49 »
The theory comes from spherical harmonics and the fact that there is a cos3θ function inside the earths geoid, there is also a cos3θ function in the Associated legendre Polynomial P41cosθ . Plot this function and it has a precession angle of 23.4 degrees. Take the theory further and the solution for mars is P72cosθ, the 2 value is due to mars having two satellites. There was a programme on Sky by National Geographic about the moon solution.
 

lyner

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Why does the earth tilt?
« Reply #23 on: 12/02/2009 07:43:36 »
So far it's numerology. What is the actual derivation of the theory? This would be groundbreaking stuff if it were true. Give us some initial equations of motion or a proper reference.
When is a satellite a satellite? We have hundreds of them.
 

Offline A Davis

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Why does the earth tilt?
« Reply #24 on: 13/02/2009 23:19:54 »
 It depends on whether you believe the universe is made from electromagnetic radiation. if you do then maxwells equations rule and the solution inside all spherical bodies are spherical harmonics governed by strattons equations. Whenever there is a join in the solution the equations change and produce different precession angles, hence the earth and mars solutions. The earth has one moon and mars has two. I can't help you much more than this you have to look at the maths.
 

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Why does the earth tilt?
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