I wrote: > > ... I think > > the processes which change the eccentricity and obliquity of the > > Earth's orbit work on a much slower time scale than the precession of > > the equinoxes, so that we can still use the same Equation of Time > > 13,000 years from now. Does anybody know for sure about this?
Luke Coletti <[EMAIL PROTECTED]> replied: > You are right regarding the relative small changes in the values of > Obliquity and Eccentricity over the period of a precession cycle. However, it > is the phase relationship of the two effects and not their values that play > the > dominant role in the variation of the Equation of Time over this period. The > Analemma will indeed look considerably different 13,000 years from now. My mistake. The magnitude of the obliquity and eccentricity change very, very slowly, but the phase is determined by the precession of the perihelion (also very, very slow) and the precession of the equinoxes, with the 25,860 year period we are discussing here. Putting it another way, dialists (not only those using astrolabes) do care about the stars because they care about the perihelion, which is fixed relative to the stars. The Millennium Clock will either need to calculate the changing Equation of Time, or else average its sighting of the sun over several years, which I presume to be more difficult. I would anyway rather see the clock designed for the astronomically significant cycle of 25,860 years than for a numerological cycle of 10,000 years. If the mechanism of the clock inherently contains the precession period, that is one more reason to make the display correspond. The 10,000 time frame was chosen, among other reasons, because that is the length of time since the development of agriculture and technology. On the other hand, there were some damn good painters active 25,860 years ago. Art Carlson
