@S1-A.ARPA,@MIT-MC.ARPA:FRIEDRITR%VAXJ.GATNET.MFENET@LLL-MFE.ARPA (07/30/85)
From: FRIEDRITR%VAXJ.GATNET.MFENET@LLL-MFE.ARPA I probably won't be the first or only one to point this out, but the precession of the equinoxes has NOTHING to do with the fact that only one century year out of four is a leap year. The reason the Gregorian calendar adopted this artifice is strictly due to the fact that the year is really 365.2422 days long, and not 365.25 as would be required if every fourth year were a leap year, including the century years. However, the respondent was right in saying that the precession of the equinoxes is probably the phenomenon the original questioner is referring to. I don't know how precisely this is calculated, or even how precisely it CAN be calculated, due to the complicating factors of solar and lunar gravity, as well as that of the planets, and the fact that the interior of the Earth is not a rigid solid. The precession of the equinoxes is the same precession seen in any other gyroscope, except for the above factors. Terry
@S1-A.ARPA,@MIT-MC.ARPA:FIRTH@TL-20B.ARPA (08/05/85)
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 -------
gadfly@ihuxn.UUCP (Gadfly) (08/07/85)
-- > First, to set the record straight, the precession of the equinoxes > was discovered by Hipparkhos in about 150 BC... > > 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. Well, just to set the record straight (and *not* to get into any discussion on the worth of astrology), astrologers have long been aware of said precession, and define "Aries" as that slice of the celestial sphere between 0h and 2h RA, "Taurus" between 2h and 4h, etc. The actual stellar background to these slices is considered irrelevant. Galileo was also an astrologer, you know, although apparently not quite as good at it as he was at science. He is supposed to have seen in the stars a long and prosperous life for some duke who employed him. The duke died two weeks later. -- *** *** JE MAINTIENDRAI ***** ***** ****** ****** 07 Aug 85 [20 Thermidor An CXCIII] ken perlow ***** ***** (312)979-7753 ** ** ** ** ..ihnp4!iwsl8!ken *** ***
@S1-A.ARPA,@MIT-MC.ARPA:Lynn.ES@Xerox.ARPA (08/08/85)
From: Lynn.ES@Xerox.ARPA Contrary to previous statements made here, the precession of the equinoxes IS related to the Gregorian Calendar. Without precession, the seasons would reoccur every 365.2564 days (a sidereal year), and thus occasional EXTRA leap years would be needed to cause the average year to be more than 365.25 days. With precession, the seasons reoccur every 365.2422 days, and thus occasional dropped leap years are needed (3 out of every 4 century years are dropped). In other words, the earth reaches the point where its pole points most nearly toward the sun (beginning of summer) slightly earlier due to precession (movement of the direction of that pole). Another way of viewing it is that our seasons cycle 25,799 times while we make 25,800 trips around the sun, the difference of one being that our axis rotated once in that time period due to precession. /Don Lynn