@S1-A.ARPA,@MIT-MC.ARPA:FRIEDRITR%VAXJ.GATNET.MFENET@LLL-MFE.ARPA (08/02/85)
From: FRIEDRITR%VAXJ.GATNET.MFENET@LLL-MFE.ARPA I dug out my copy of George Abell's "Exploration of the Universe", to try to find out what is known about polar wandering and such. First, a couple of definitions, and an apology for such a long message, but I wanted to be as complete as possible. Celestial pole: the points at which the Earth's axis, extended out toward infinity, appear to meet the sky. For the north celestial pole, this point is near Polaris, the "north star". Celestial equator: the line around the sky that represents the Earth's equator, similarly extended. North and south poles: the points at which the Earth's rotational axis would stick out of the ground. Let's also agree not to discuss wandering of the magnetic poles; this effect, as has been pointed out, is rather large, but is not related to the originally-posed problem of astronomical alignment of ancient structures. First, let's talk about the movement of the celestial poles. The Earth, acting as a gyroscope because of its rotation, precesses. Thus, the celestial poles describe circles in the sky with a diameter of twice the inclination of the Earth's axis to its orbital plane, or 47 degrees, with a period of about 26,000 years. This effect was discovered by Hipparchus, in the second century B.C. Consequences: this motion DOES NOT affect the latitude of the Tropics of Cancer and Capricorn. This motion DOES give us two definitions of the length of a year. The "sidereal year" is the time it takes the Earth to travel 360 degrees around the Sun. However, we can also define a year as the time it takes for the Sun, starting from the celestial equator, to travel completely around the sky and come back to the celestial equator. Because of precession, this takes about 20 minutes less than a sidereal year, and is called a "tropical year". It is this definition of a year, 365.242199 days, which the calendar is tied to. The consequence of THAT is that the seasons will NOT shift with respect to the calendar over time. Spring will always begin in March, July will always be midsummer, and you can always hope for a white Christmas. (Apologies to those in the southern hemisphere.) The tropical year, not the sidereal year, determines the seasonal cycle, and the calendar keeps pace with the tropical year. Superimposed on precession is nutation, which is caused by the gravita- tional effects of the Sun and the Moon (mostly the Moon). Nutation superimposes a wave motion with a period of about 19 years on the precessional circle, with an amplitude of 9.2 seconds of arc. Finally, the plane of the Earth's orbit changes slightly, a fraction of a second of arc each year; this also adds a very slight wobble to the motion of the celestial poles. Let me point out again that NONE of these effects alters the latitude of the Tropics. Now let's talk about the movement of the north and south poles them- selves over the surface of the Earth. This has nothing to do with precession, but is a shifting of the position within the Earth of the rotational axis itself. This is called "variation of latitude", and also does NOT change the latitude of the Tropics; it just changes where on the surface of the Earth the Tropics fall. This motion seems to be comprised of two components: one, with a period of one year, is a circle roughly 20 feet in diameter. The second has a period of about 14 months, and is also a circular motion, but the diameter of the circle varies from about 10 to about 50 feet. Both of these motions are attributed to variations in the distribution of the mass of the atmo- sphere over the surface of the Earth, the former a seasonal change, and the latter random changes. On top of this is a secular drift of the poles in one direction of about 4 inches per year, as yet unexplained. Finally, NONE of these effects alter the latitude of the Tropics. I do not have any references that suggest that the tilt of the Earth's axis with respect to the Sun has changed, which is what determines the latitude of the Tropics. I would appreciate hearing about any reputable references to such a phenomenon. Terry