All the heavenly phenomena I have mentioned so far can be appreciated from one day to the next, or at least within timeframes of months (for example, the sun moves between the two solstices in 6 months), years (for example, the cycle of Venus), or at most a few dozen years (for example, the cycle of lunar standstills). There is, however, another phenomenon that evolves over times that are much longer than the average human life: precession.
The earth, like all rigid bodies, is subject to three rotational movements. The first is the revolution around its axis in 24 hours. The earth's axis, however, rotates around the perpendicular to the ecliptic, completing a revolution every 25,776 years, and it also undergoes a slight "swing." These two movements are known as precession and nutation, respectively. The effects of nutation are negligible for the purposes of archaeoastronomy, while knowledge of precession is essential for reconstructing the skies as ancient peoples saw them. To visualize the precessional motion of the earth, the easiest way is to arm oneself with a spinning top (preferably a real one, or else a simulation program on a PC), place one end on the floor, and watch its movement after spinning it and leaving it with a certain inclination with respect to the vertical. It can be seen that the axis of the top, just like the earth's axis, begins to circle around the vertical while the top spins around the axis.
Precession has a significant effect on the observation of heavenly bodies, an effect that is extremely slow in terms of the human life span, though. First, the North Pole shifts against the background of the stars. In fact the North Pole is none other than the ideal intersection between the prolongation of the earth's axis and the heavenly sphere. Since the axis describes a cone, the pole describes what we see (or rather would see, if the change were not so slow as to be almost imperceptible) as a circumference (not quite closed in reality, owing to certain perturbations, which I shall not go into). It should be stressed that it is not the background of stars that move, but the polar axis that points to different regions of the sky; what we call the Pole Star, for instance, is the star that the heavenly north pole is near today.
Just as the North Pole shifts, so too does the South Pole, and this results in some constellations being, at certain times, completely invisible from a certain latitude (because they are too far south) and then becoming visible again with the passing of time. It is evident at this point that precession also has an effect on the apparent motion of all the other stars, which vary, in particular, in terms of azimuth of the rising point and altitude at the meridian. On the other hand, precession has no effect on the apparent motion of the sun and, in particular, on its solstitial points, which are only subject to a small displacement (appreciable over thousands of years) due to the slight variation that the plane of the ecliptic is subject to.
The effect of precession on the zodiacal constellations, that is, the constellations that the sun has "as a backdrop,'' can be visualized as a slow rotation of the backdrop itself; since there are 12 zodiacal constellations, if
the sun rises against ("in") a particular constellation on a particular day of the year, after about 2200 years (25,776 years of precessional cycle divided by 12, the number of zodiacal constellations, gives 2148 years), it will rise on the same day in the background of the constellation located to the left of the original one. In particular, the constellation against the background of which the sun rises at the vernal equinox defines what is usually called a "zodiacal age'' (currently we are at the end of the Age of Pisces and passing into the Age of Aquarius). Once again, however, I stress that we are speaking here of a well-defined, very slow but observable physical phenomenon using a traditional terminology; curiously, the so-called signs of the zodiac used in horoscopes, which (according to modern astrologers) should enable our destinies to be forecast, are actually well out-of-date, exactly because of precession.
Mysteries and Discoveries of Archaeoastronomy
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