Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/5/84; site mordor.UUCP Path: utzoo!linus!decvax!genrad!panda!talcott!harvard!seismo!lll-crg!mordor!@S1-A.ARPA,@MIT-MC.ARPA:FIRTH@TL-20B.ARPA From: @S1-A.ARPA,@MIT-MC.ARPA:FIRTH@TL-20B.ARPA Newsgroups: net.space Subject: precession of the equinoxes Message-ID: <2944@mordor.UUCP> Date: Sun, 4-Aug-85 18:48:26 EDT Article-I.D.: mordor.2944 Posted: Sun Aug 4 18:48:26 1985 Date-Received: Wed, 7-Aug-85 02:17:57 EDT Sender: daemon@mordor.UUCP Lines: 48 From: FIRTH@TL-20B.ARPA First, to set the record straight, the precession of the equinoxes was discovered by Hipparkhos in about 150 BC. The phenomenon has nothing to do with the earth's axial tilt, which does change, but only slightly. It rather concerns the direction the axis points in space, which makes a slow circle once every 22000 years or so, thereby taking the pole star from Polaris round to Vega. This of course makes no difference to the terrestrial seasons, which depend on the relative positions of earth and sun, and therefore no difference to any true solar calendar, ie one that sets its starting point with reference to something like the winter solstice. However, what does change is the relative position of sun and stars at any given season; thus, in Hipparkhos' time the vernal equinox began as the sun entered the Zodiacal sign of Pisces, whereas 2000 years earlier it had occurred when the sun entered Aries (and by about 2050 I think the sun will still be in Aquarius). This of course makes nonsense of traditional astrology; since no astrologer seems to have looked at the real sky in several millenia, the dates given in your daily paper for 'Aries' &c are wildly wrong. Interestingly, one calendar WAS subject to discombobulation by the precession of the equinoxes: the Egyptian calendar, which set the new year at the time of the heliacal rising of the star Sirius, at which time the Nile was supposed to flood. Well, between Narmer and Ptolemy I the precession had moved this date by almost 7 weeks through the solar year. For this and other reasons, Ptolemy V Epiphanes replaced the calendar with one that took the vernal equinox as the new year, keeping however the 12 'months' and the five intercalary days; it was this calendar that, on the advice of the astronomer Sosigenes, Julius Caesar introduced to Rome, and which became essentially the Julian calendar. On the motion of the poles: the magnetic poles move around all the time, but I don't think anyone believes the true poles move. There is an SF book, The HAB Theory, built arond the idea that the poles suddenly move a large distance in a catastrophic manner (or maybe the poles stay in the same place but the earth's crust moves; the book isn't clear on this). Naturally, such an event wrecks civilisations, &c, so if you think one is coming, sell your orichalcum-mining stock and build a deep shelter some place like the Andes, for convenient later discovery by E von Daeniken. The book is very long and very bad, but has a lot of fun fabricating "evidence" in support of the "theory" Robert Firth -------