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Why does Earth have an axial tilt

by shifty88
Tags: axial, earth, tilt
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D H
#19
Dec19-12, 11:29 AM
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Quote Quote by lpetrich View Post
Faster??? It would still orbit at very close to its original rate.
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.
mrspeedybob
#20
Dec22-12, 07:40 AM
P: 705
Quote Quote by D H View Post
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.
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?
D H
#21
Dec22-12, 10:36 AM
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Quote Quote by mrspeedybob View Post
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?
First I'll examine what radiation pressure does in our transparent and nearly empty solar system. The radial component of radiation pressure dominates over gravitation for very small particles, making the net radial force be outward. This causes these very small particles to be ejected from the system. Acceleration due to radiation pressure decreases as particle size increases. Beyond some critical size, the net radial component of acceleration will still be inward. If the only effect of radiation pressure was radial, all that this would mean is that smallish (but not too small) particles would orbit at less than a Keplerian velocity. There is, however, a tangential component to radiation pressure thanks to the finite speed of light. This is the Poynting-Robertson effect. This tangential component is directed against the motion of the orbiting particle, making small (but not too small) particles fall inward toward the Sun.

The protoplanetary disk changes things a bit, but the end result is still the same. The point at which a protostar begins emitting a significant amount of radiation spells the beginning of the end of the protoplanetary disk.
schaefera
#22
Dec24-12, 10:21 AM
P: 208
Quote Quote by xzardaz View Post
//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.
Except that you forgot about the quantization of space in this "proof"...
AgentSmith
#23
Sep27-13, 08:45 PM
P: 28
Quote Quote by Integral View Post
Because it does.
True. It has nothing to do with any fundamental laws, its the way the cookie crumbled in the history of the solar system.

NB: the crumbling cookie has great explanatory power.


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