@S1-A.ARPA,@MIT-MC.ARPA:king@Kestrel (07/30/85)
From: king@Kestrel (Dick King) > > A book I was reading recently mentioned that over time the >angle of the plane of the ecliptic changes. The consequence is that >the latitude of the tropics also move. While this makes sense, the >book also stated that there is no formula which describes the motion >over time. Is this really true? The context was that certain >archeological sites are solstice oriented and could be accurately >dated if it was known in what year a Tropic was at X latitude. Just >curious. > > jim@tycho > Wrongo. The phenomenon you're referring to is called the "precession of the equinox" and the values have been calculated *very* precisely. Roger Bacon first pointed out the phenomenon is the 13th Century, and showed that if the Julian calendar were not changed, then sometime in the 30th Century Easter would occur in midsummer (the rate of precession is about .75 days/century). The solution he proposed was the one adopted in the Gregorian calendar, in which Leap Years are not held in century years and are held every 400th year: so there was no leap year in 1900, there will be one in 2000, but there won't be one in any of 2100, 2200, 2300. wrongo. Precession of the equinoxes does indeed occur, having the effect you describe, but it would not affect archaeology. PofE describes changes in the portion of Earth's orbit that corresponds to given seasons, so different constellations would be visible in the winter night's sky in different millenia. The phenomenon referred to in the original submission also occurs; at times the axial tilt has varied from 20.6 degrees to 22. (If my figures are wrong, forgive me.) This would render obselete things like Stonehedge that can detect the first day of winter. If the axial tilt is low at the moment, the monument's first day of winter point may not be approached. If it is high, it may be exceeded (and hit twice, shortly before and after the actual solstice). -dick