It is virtually impossible for one single person to discover the precession of the earth's axis by means ofobservations, even over many decades, since the phenomenon changes far too gradually. Nevertheless, it is sufficient to have precise information, for example, regarding the position of the heliacal rising of a star over the space of 200 or 300 years, or the position of the equinoctial point over an analogous period of time, to notice that something is happening: the shifting of the rising point of the star, or the fact that the sun is rising against a background of different stars. Hipparchus worked in this way: he catalogued an enormous number of astronomical observations (on over 800 heavenly bodies), based on data taken from the observatory of Alexandria, established 150 years previously, and probably also using data from even further back, of Babylonian or Egyptian origin. It is quite reasonable, therefore, to suppose that this phenomenon could be tracked, in exactly the same way, by anyone who kept meticulous and accurate recordings of astronomical observations over many years. For instance, the Babylonians' observations ofthe heliacal rising ofstars were accurate on the order of an arc minute (Chapter 5). Thus in a very short space of time (a few dozen years) astronomers could have become aware of the precessional shift. In India, observations ofequinoctial constellations went on for thousands of years. It is certain that Vedic astronomers knew that the stars related to the rising of the sun at the equinoxes change over the centuries, because their lists of heavenly bodies started off with these stars and it was noticed that they changed from century to century (Chapter 5). Observations of the Decanal stars in Egypt had begun during the Middle Reign or possibly before (Chapter 4), although they were not as accurate as those of the Babylonians. They stretched over at least 2000 years, a long enough period to show macroscopic precessional effects; the theory that the Egyptians discovered the precession by watching the Decans was first put forward by Zaba (1953). The Mayas unquestionably had sufficient technical ability to discover precession quite readily (Chapter 8); in addition, the occurrence of the solstices at the intersection of the ecliptic with the Milky Way, a fact of fundamental importance in their cosmology, lasted—again due to preces-sion—for a relatively short period of time. However, while we know a great deal about their planetary and lunar observations, we know little about the study of stars or the existence of star archives or catalogues among Mayan astronomers. The deciphering of the Codex of Madrid or the discovery of some new codex in future may be of help in assessing this issue.
Another aid in discovering precession may have been the reuse of stellar alignments bequeathed by predecessors, noticing that these would have become inaccurate. Some examples follow:
1. The central axis of the temple of Ggantja I in Malta was probably oriented on the Cross/Centaurus group of stars. Some centuries later, a second temple was created, absolutely analogous with the first, but with its axis shifted more to the south, almost certainly to take into account precession (Chapter 3).
2. The orientation of the Sardinian Nuraghi toward the stars of the Cross-Centaurus group appears to change over time in accordance with the precessional shift of these stars (Chapter 3).
3. In various Egyptian temples, we can see deviations in axis direction occurring during rebuilding or amplification work. The only logical explanation for at least some of these divergences is that they derive from the need to take into account the precessional shifting of the stars that their alignments pointed to (Chapter 4).
4. The Medicine Wheel of Moose Mountain shows that the alignment with the star Fomalhaut was "adapted'' over the years owing to the precessional shifting of its rising point (Chapter 7).
I believe that in each of these cases it is extremely likely that the astronomers realized that something was going on, very slowly, in the sky. And yet, without any explicit sources to provide support, we cannot be certain of how they interpreted this, nor can we rule out a priori that they simply put the disparity down to an error in their predecessors' measurements. The question of the discovery of precession is thus quite elusive, and, despite a great deal of clues, we lack any unequivocal evidence that it took place before Hipparchus. As a consequence, although I am not keen at all on it, I must admit that many clues indicate that precessional effects had been discovered by numerous astronomers of the past, but that the discovery was kept hidden, and that only an elite of initiates were privy to this arcane knowledge.
Indeed, also some extremely interesting and curious evidence dating from the age just after Hipparchus's supposed breakthrough seems to point in this direction.
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