Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: annie123 on 24/07/2019 05:23:17

Title: why are some heavenly bodies tilted on their axis and not others?
Post by: annie123 on 24/07/2019 05:23:17
Why is the earth tilted on its axis and not the moon? Are the other planets tilted in the same way as the earth? ANd what are they tilted in relation to  besides the sun ?
Title: Re: why are some heavenly bodies tilted on their axis and not others?
Post by: alancalverd on 24/07/2019 08:45:18
Assume that heavenly bodies are formed by the gravitational accretion of random dust.

If two particles approach each other, but not in a straight line, there will be some angular momentum about their barycentre (mutual center of gravity). So the clump will spin, as angular momentum is conserved.  The axis of the spin is perpendicular to the plane in which they finally approached, and is independent of any other factor. So as a planet grows, it will develop a spin vector all of its own.

Now consider two large particles: a planet and a much larger star. What looks like planetary capture is actually the same phenomenon: two large particles orbiting around their barycenter, but the barycenter is close to or even inside the star, so we tend to consider the planet(s) to be orbiting around the star. But the plane of orbit has no necessary relation to the spin of either body.   

Our moon does not appear to spin when seen from earth because it seems to be a chunk of Earth thrown off by some explosion or collision, rather than an independently-formed  accretion.   Other moons do spin, with spin axes oriented sort-of-close to their planet, and other planets spin and orbit in pretty random orientations relative to ours.
Title: Re: why are some heavenly bodies tilted on their axis and not others?
Post by: Halc on 24/07/2019 11:51:59
Our moon is tide locked with Earth, and thus has lost most original spin/tilt it might have had.
Looking it up, the moon seems to have an inclination (tilt) of about 1.54 relative to the ecliptic and an inclination of its orbit of about 5.15.

The inclination is typically fairly random and the odds of it being exactly zero is pretty remote.
Title: Re: why are some heavenly bodies tilted on their axis and not others?
Post by: evan_au on 24/07/2019 15:17:36
Quote from: alancalverd
Our moon does not appear to spin when seen from earth because it seems to be a chunk of Earth thrown off by some explosion or collision
Uranus has an axial tilt of 82.
Some suggest that this may have been due to a significant impact on the planet.

Quote
other planets spin and orbit in pretty random orientations relative to ours.
It is thought that stars and their planetary systems form from an accretion disk.
- This accretion disk ends up with the net angular momentum of the part of the gas cloud from which they formed.
- So the star and the planets end up with roughly the same orbital axis.
See: https://en.wikipedia.org/wiki/Orbital_inclination#Observations_and_theories

In turn, the planets and their moons form from this same disk, and are likely to have a spin axis parallel to the planetary system as a whole.
- However, over time, influences of other planets can tilt the spin axis quite a lot; some astronomers suggest that the axis of Mars has changed significantly over time
- The Earth's axis has been somewhat stabilised by the large Moon, but it still changes a bit due to the Milankovitch cycles.
See: https://en.wikipedia.org/wiki/Axial_tilt#Solar_System_bodies