On Thu, 11 Jul 1996, Rory Sellers wrote: > "Precession of the equinoxes" means the earth's rotational axis is > precessing like a tilted top (one revolution every 25,000 years or so.) > Can someone help me with the name of the axis (vector, whatever) that > the earth's axis precesses _about_?
It is the pole of the ecliptic. > And the real question: is this > "central" axis perpendicular to the ecliptic? (I.e. throughout the Great > Year, is the earth always tilted 23.5 degrees from the ecliptic?) Yes and no. In first approximation this axis is perpendicular to ecliptic as long as the bodies which try to tilt the earth's axis perpendicular to the ecliptic. But there is the moon whose plane of revolution around the earth is inclined by about 5 degrees to the ecliptic. Due to the gravitational force of the sun the moon itself makes a precessional movement which overlays the earth's precession. This is called nutation and has a period of 18.6 years. The effect of the nutation for the position of the pole of the ecliptic is only about 10 arc sec in maximum (although it is not said that the nutation affects the position of the pole of the ecliptic). Due to the precession the obliquity of the ecliptic is not constant. E.g. 7500 B.C. it was 24 deg 14' and in the year 12000 A.D. it will be in minimum with 22 deg 37'. Of course this affects a sundial which is made for "eternity" as long as it shall give a mean time only. It is possible to calculate the analemma curves (8-shaped lines) for other centuries. But then you also have to obeye the changing of the eccentricity and the perihel position of the earth's orbit around the sun (I could imagine it will give a nice movie). Formulae for the calculation of this you can find in: Jean Meeus, Astronomical Algorithms, Willman-Bell. - Daniel Roth, Arbeitskreis Sonnenuhren, Germany
