this though just popped into my head and im sure there is a simple explanation that i dont remember but why do planets have only one rotational axis?
Any rotating object has a single rotational axis at any given moment, otherwise known as the angular velocity vector.
i guess the best way to visualize it would be to RCS rotate left... that is how a planet rotates to my understanding. now rotate up. there is a dramatic change in orientation that seems utterly chaotic (in my imagination anyway).
This is called torque free precession*, ie a changing in the direction and magnitude of the angular velocity vector. All the planets do this, but the effect is very much smaller. The effect is caused by the angular momentum vector remaining in a constant direction and magnitude (ie, it is conserved) but the object having principle moments of inertia that are both unequal and not aligned with the angular momentum vector.
to my understanding, planets begin to rotate as they are formed and only change their axis if they are impacted by something witch gives an "earthlike" offset axis. while that all makes sense i dont understand why earths rotational axis is so stable. why does the rotational axis itself not continue to rotate?
Perfectly spherical objects have no precession because their principle moments of inertia (PMIs) are all equal. The Earth is forced to be nearly spherical by its gravity so it has nearly equal PMIs and therefore very little precession. The rotation does cause a slight bulge at the equator (the planets are not perfectly rigid) which decreases the PMIs in the two of the principle axes slightly which has the effect of further reducing the precession. You can see that effect in this cool Java applet:
http://faculty.ifmo.ru/butikov/Applets/Precession.html
Lower gravity objects that have not been forced into near-spherical shapes, such as asteroids, have much more interesting torque-free precession due to their unequal PMIs.
The equatorial bulge does introduce another effect though: torque induced precession. Tidal forces on the equatorial bulge rotate the angular momentum vector, thereby rotating the angular velocity vector. This is referred to as the precession of the equinoxes. There is a good description of how that works here:
http://www.crystalinks.com/precession.html. Orbiter does simulate this effect in an approximate way - have a look at the precession values in the planet config files.
As I understand it, the rotation of the planets during their formation is due to the orbital angular momentum of the individual particles that form the planets, ie, before the planet was formed they were orbiting about a centre of mass where the planet ultimately formed. This is why the angular momentum vectors of most planets in our solar system point in roughly the same direction (the direction of the total angular momentum of the solar system) - they were formed out of the same spinning cloud of dust/debris.
am i even making sense? am i just being totally ignorant of something very simple?
Precession bends the mind. Very good question IMHO :thumbup:
Things are complicated further when a planet/moon is effected by tidal forces from multiple bodies, and also when the non-rigid nature of the body is taken into account (for example, the weather and ocean currents on Earth causes slight changes in the PMIs and therefore the direction of the angular velocity vector. You can see some of that effect in these graphs:
http://www.iers.org/nn_11252/IERS/E.../Plots__BulA/generischeTabelle__Diagramm.html).
* A note on nomenclature: Astronomers normally refer to the torque-free precession as "nutation" since the angular change in the velocity vector is typically very small compared to the torque-induced precession. They normally refer to the torque induced precession as just "precession". Typically the nutation has a much smaller period
EDIT: It took me a while to type all that while you guys were madly replying away. Forgive me for being redundant, if I was.
