I just learnt a new term, obliquity, also called axial tilt. Its the angle between an objects rotational axis and its orbital axis. The International Astronomical Union (IAU) defines the north pole as that which lies on the north side of the invariable plane of the Solar System while NASA defines the north pole with the right hand rule. ----- I prefer NASAs way ofdefinition. Over the course of an orbit, the angle of the axial tilt does not change, and the orientation of the axis remains the same relative to the background stars. However, because the ecliptic (i.e. the Earths orbit) moves due to planetary perturbations, the obliquity of the ecliptic is not a fixed quantity. At present, it is decreasing at a rate of about 47 per century. ---Obliquity of Ecliptic was be calculated analytically and by numerical simulation.--- All four of the innermost, rocky planets of the Solar System may have had large variations of their obliquity in the past. Mercury and Venus have most likely been stabilized by the tidal dissipation of the Sun. The Earth was stabilized by the Moon, as above, but before its capture, the Earth, too, could have passed through times of instability. Mars obliquity is currently in a chaotic state; it varies as much as 0° to 60° over some millions of years, depending on perturbations of the planets. The obliquities of the outer planets are considered relatively stable. Some authors dispute that Mars obliquity is chaotic, and show that tidal dissipation and viscous core-mantle coupling are adequate for it to have reached a fully damped state, similar to Mercury and Venus. en.wikipedia.org/wiki/Axial_tilt
Posted on: Sat, 28 Jun 2014 05:32:45 +0000
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