John Horn wrote: > >> Something I read before: >> >> A serious problem with Venus is the lack of a decent >> Moon. This makes Venus's rotational angular momentum >> precess wildely, going to flat spin, spin backwards, >> or even no spin at all, due to orbit-spin coupling >> [the orbital angular momentum is much bigger than >> the rotational angular momentum, so a tiny transfer >> from one to the other causes a disaster in spin] >> >> BTW, Mars has the same problem. > >That sounds very interesting. Any chance of restating that in words a >non-rocket scientist could understand? :-) > No.
But I will try anyway A planet that rotates defines something called "angular momentum", that is a vector that points to the direction of the North Pole [a hemisphericist convention, discriminating those that are from the Southern Hemisphere :-/] and whose magnitude increases the faster the planet spins [in other words, the smaller the day, the bigger the angular momentum] [it also increases with the mass of the body and something else that reflects the distribution of the mass relative to the axis - but I will not detail, as I promised to be simple]. The orbit also generates another angular momentum, again a vector, that is perpendicular to the plane of the orbit, and that increases with the speed of the planet, its mass, and its distance from the Sun. As you might guess, the total angular momentum of an isolated system - the sum of all those spin and orbital momenta of all bodies - is a constant vector. As it happens, the orbital angular momentum is **much** bigger than the spin angular momentum, so if there is some way to transfer one angular momentum to the other, then a small chance in the orbital angular momentum causes a huge disaster in the spin angular momentum. The Moon is a stabilizing factor, because it forces the Earth's spin angular momentum to keep close to its own orbital angular momentum. Alberto Monteiro _______________________________________________ http://www.mccmedia.com/mailman/listinfo/brin-l
