Fig. 1 The circle describing the north celestial pole during a precessional cycle.
The question now arises, when was the discovery of precession actually achieved. Standard scientific point of view states the following:1. Precession was first discovered in 117 BC by Hipparchus of Rhodes.
2. Precession was never discovered in pre-Columbian cultures. In other words it was not known in any place in the Americas before Columbus.
In spite of this, the very opposite idea that all archaic civilizations discovered precession very early has been around for a long time and was stated in an authoritative way by Giorgio de Santillana and Hertha Von Dechend in their famous book Hamlet’s Mill (1983). Although an extremely interesting book, and worth reading, Hamlet’s Mill, however, cannot be of any help when discussing the basic issue of the discovery of precession, since all the ‘proofs’ recorded in the book cannot be considered as true scientific proofs. Indeed the authors report an (albeit impressive) amount of occurrences of similar images, same numbers, similar situations in a great number of cosmological myths around the world. Although it is well known that myths have actually been used to sometimes convey a technical language, without independent verification of assertions in contextual environment it is impossible to accept ‘images’ and ‘numbers’ as ‘proofs’.
The aim of the present paper is, therefore, to discuss which hints do we have about discovery of precession before Hipparchus in ancient cultures in order to stimulate further research in this field.
2.0 Astronomical Data
It would have been nearly impossible, even for a very experienced naked-eye astronomer in ancient times, to discover precession in the course of his own life using only his own observations, due to the extremely slow nature of the phenomenon with respect to the length of a human life. It is, however, sufficient to have astronomical data collected during, say, two or three centuries, like the height of transit of a bright star, and to trust in them, to become aware that ‘something is happening’ in the sky with a very low, but measurable, velocity (this is exactly what happened to Hipparchus: he collected a great quantity of astronomical data over more than 800 celestial objects coming from the Alexandria observatory and based his discovery on such data). I shall thus, of course, be concerned here with what I have called the discovery that ‘something is happening’. This means that I am not speaking about the possible discovery of the actual mechanism and/or of the length of the precessional cycle (although this discovery is not a priori excluded) but rather of the observation of a discrepancy of specific visual data which, from now on, I will call precessional effects. Typical examples may include the observation of the ‘Precessional Era’, namely the fact that the sun at the spring equinox rises in different places within a constellation and finally changes constellation every 2000 years, or the observation of the change in the declination of heliacal rising of a star.
2.1 Babylonian culture
There are many examples of ancient cultures that kept written track of astronomical data for centuries. First of all, of course, the Mesopotamian cultures (usually referred to, collectively, as Babylonian). We have astronomical data collected by Babylonian astronomers on clay tablets which contain observations which are more precise than one minute of arc. Since it is nearly impossible to obtain such an accuracy with the naked eye, it was probably obtained with the first spyglasses ever invented (Pettinato 1998). One example of a Babylonian star catalogue is the famous Mul-apin. Probably written around 1000 BC, it contains astronomical data which can be traced back in time up to 2048 BC. The content includes:1. A list of 71 celestial objects (constellations, single stars and the five planets) divided in three “courses” (Enlil, Anu and Ea).
2. A list of heliacal rising of many stars.
3. A list of simultaneous rising/settings of couples of stars.
4. A list of time delays between the rising of the same stars.
5. A list of simultaneous transit/rising of some other couples of stars.
It is difficult to believe that astronomers possessing data so accurately did not notice the effect of precession, for instance on heliacal risings. However, no written record citing the phenomenon explicitly has been discovered so far.
2.2 The Indo-Savrastati Culture
The history of the Indian civilization has been plagued until twenty years ago or so by the foolish and anti-historical idea of the so called Arian invasion. The basis of this idea was that civilization was brought to India by indo-European people, the Arians, around 1000 BC. After the discovery of the 2500 BC towns of Harappa and Moenjo-daro, the Arians started to be considered warriors and invaders, but the idea remained that the fundamental books of the Hindu religion, the Vedas, were conceived after this invasion. Today we finally know that the Arians simply never existed and that the Indian civilization (traditionally associated with the sites of Harappa and Moenjo-daro, but actually much more spread than the area individuated by these two cities) developed between two rivers, the Indo and the Savrastati rivers (Feuerstein, Kak, and Frawley 1995). The Veda contains explicit reference to the latter river, which was however dry at about 1900 BC, and thus the books (actually memo-books learned by memory by Brahmins) are at least as old as that period.
Together with this new approach to the Veda, in recent years a new approach to what we can now call Vedic astronomy emerged (Kak 2000).
In Vedic astronomy a fundamental role is played by the five visible planets, the sun and the moon, identified with seven fundamental deities. However, to keep track of their motions, 27 astronomical objects were used, the naksatras, asterisms/constellations used to divide the ecliptic in equal parts, in each one the sun “resting” about 13 and ⅓ days. Naksatras occur in ordered lists. For instance, one can identify (using modern names) the Pleiades, alfa-tauri (Aldebaran), beta-tauri, gamma-gemini, beta-gemini (Pollux), delta-cancri, Hydra, Regulus, and so on. Interestingly enough, lists of naksatras belonging to different periods contain the same objects but begin at different points. The starting point is individuated by the sun at the spring equinox, and this means that Vedic astronomers were almost certainly aware that the Sun was ‘changing naksatra’ with a velocity of more than one naksatra per roughly one millennium (25,776 ÷ 27).
2.3 Egypt: Middle and New Kingdom Astronomical Data
The study of ancient astronomy in Egypt has been plagued for many years by the influence of the most important scholar in the field, Otto Neugebauer, who stated in several occasions view such as ‘Egypt did not contribute to the history of mathematical astronomy’ (Neugebauer 1969, 1976). But it just suffices to read the information contained in the monumental books by Otto Neugebauer himself and by Richard Parker on ancient Egyptian astronomical texts (1964) to realise how such an assertion is far from being true. Another serious problem generated by the negative influence of Neugebauer is the idea that astronomy was not present in the Pyramid Age (Old Kingdom). In fact the Neugebauer-Parker book begins with the Middle Kingdom (we shall see later that also this assertion is clearly false).
Much of the confusion arises from the fact that we do not have any Egyptian text of explicit astronomical nature, a thing that, in my opinion, is probably due to the fact that papyri were simply not part of the funerary items, and almost only such items are being recovered. In any case, it is obvious that Egyptian astronomers did actually keep track of many astronomical data. This is readable from those “astronomical texts” which were used in funerary contexts and are written in Middle Kingdom sarcophagi and in many New Kingdom tombs, such as the famous tomb of Semnut, architect of the Queen Hatshepsut, and many of the Ramesside tombs of the King Valley.
In the Middle Kingdom, the so-called decanal lists were used. Decans were 36 stars (or groups of stars) whose heliacal rising (the day of the first rising before dawn after a period of conjunction with the sun, i.e. invisibility) occurred in subsequent “weeks” (the Egyptian week was made out of 10 days). In this way, the calendar was divided into decans (36 × 10) plus 5
epagomenal days associated to special decans as well (the calendar I am speaking about is the so-called religious or Sothic one, based on heliacal rising of Sirius which therefore was the first of the decans).
It was shown by Neugebauer and Parker that possible decans must lie in a band south of the ecliptic (decanal band) but they considered explicit identification of decans to be impossible. This is untrue and, in fact, today we do have a quite clear picture of which stars the decans represented (Belmonte 2001a,b). Decans were used to keep track of time during the night as well. This is proved by the so called Star Clocks in which hours during the night are counted associating the last hour of the first day with the decan which has heliacal rise in that day. After one “week” the rising of this decan shifted back in time to signal the previous hour, and another decan signals the last hour, and so on 12 times. Of course each hour had a non-fixed length. One can say that for us one hour has a fixed length and that the night has a variable length in the course of the year, but for the Egyptian it was the opposite (our 24 hour division of the day comes from the 12+12 Egyptian division added to the fixed length Babylonian division of hours).
In the New Kingdom the decans were observed at the meridian transit rather than at rising, but the way of keeping track of stellar events was similar. This is evident in the so called Ramesside star clocks. In a Ramesside star clock a man (an assistant of the astronomer, or perhaps a statue) is seen behind a list of 9 columns and 13 levels. Levels are associated with hours of the night, columns with parts of ‘the reference man’, and spots signal the transit or position of stars during the night. The framework was changed each 15 days. I will not enter into further details on the problems of interpretations of such texts. The point I want to stress here is, that such astronomical devices, although depicted in the tombs (as ‘guides to the soul during the night’) were almost certainly copied from scientific sources (the reader can, if he likes to, add quotation marks to the word ‘scientific’ but I will not do so). In fact, already in the Middle Kingdom Egyptian astronomers were able to keep accurate track of 36 stellar objects taking into account their motion (hour of rising, period of invisibility and so on) and therefore they should have selected such properties from a huge amount of observational data. It is absolutely certain that one can discover a precessional effect in the heliacal rising of a star with data accurate to ½ of degree in, say, three centuries. This led Pogo (1930) and Zaba (1953) to propose that precession was probably discovered very early in Egypt. It is, in addition, worth mentioning that several authors have proposed, in order to explain the curious arrangements of the constellations in the famous round picture of the sky known as the Dendera Zodiac, that it could contain a reference to the precessional movement of the north pole (see for example Trevisan). The Zodiac is however dated to the first half of the last century BC, and therefore after Hipparchus’ discovery. Again, we do not have any explicit records which can be associated unambiguously to the discovery of a precessional effect.
Fig. 2 Examples of Ramesside star clocks.
2.4 Mesoamerica
As is well known, the Maya kept track of astronomical data in a written and extremely accurate way (Aveni 2001). Unfortunately, only four Maya ‘codices’ survived the auto da fe during which the bishop of Yucatan, Diego de Landa, condemned all the heretic books. Such codices contain data about eclipses, Venus and Mercury. The Data is so precise (for instance, the Venus table in the Dresda codex is based on tens of years of observations) that the ability of the Maya astronomers in taking extremely accurate measures is beyond any doubt. However one cannot discover precession using the motion of the sun, of the planets and of the moon, and we do not possess any record of star observations by the Maya (the unique exception possibly being in the so called Paris codex, which is still not fully understood).
3.0 Astronomical Alignments
So far, we have discussed possible textual evidences. There is, however, another possibility to keep track of celestial motions and to leave astronomical data to successors as a heritage, namely the construction of stellar alignments. Following their accuracy during a few centuries one can easy discover precessional effects (I am using here an abuse of notation calling ‘stellar’ the alignments pointing to stars different from the sun).
3.1 Egypt: Orientation of Temples
The pioneer in the studies of the astronomical orientation of temples in Egypt was Norman Lockyer (1894). In his book he studied the orientation of many temples, but I shall discuss in details here only the case of the two main Theban temples, Karnak and Luxor, because it suffices for our purposes.
These two temples have a millenary history and were embellished and enlarged several times. In particular, different pharaohs in different epochs added further galleries in the direction of the main axis of both temples. If one looks at the plan of the Karnak temple, it is clearly seen that the temple was always enlarged maintaining strictly the original direction of the main axis. It was shown by Lockyer that this direction is that of the setting sun of the summer solstice. The work of Lockyer was criticised because hills at the horizon would have prevented the light of the setting sun from penetrating the gallery, and today we actually know that observations were performed at the other end of the temple in a chapel which - being on an axis parallel with the temple - is obviously oriented to the winter solstice sunrise (Krupp 1983, 1988). In any case, solstice alignment of the temple is certain, and of course, since precession does not effect the apparent motion of the sun, so any enlargement was added in the same direction.
Fig. 3 Plan of Karnak and Luxor temples.
The other main temple of Thebes, today called Luxor temple, is instead aligned to the stars. This is pretty clear because the axis was slightly deviated no less than four times, every time on the occasion of a subsequent enlargement which took place over the centuries. Unfortunately, although we do have several descriptions of the alignment ceremony of temples to the stars, called by the Egyptians Stretching of the Cord, we do not have a clear picture of how the ceremony actually took place. For instance, in many cases it is said that the alignment occurred towards the Mes constellation, i.e. the Big Dipper/Plough which the Egyptian saw as a Bull’s Foreleg, but we do not know exactly to which star it was made. It is as yet unclear therefore to which star or asterism the Luxor temple was aligned (Lockier proposal, alfa-lyrae or Canopus, is, as far as I know, still to be confirmed). In any case, the slight deviations in the temple axis clearly point to the discovery of a precessional effect.
3.2 Egypt: orientation of pyramids
It is very well known that the main pyramids of the fourth dynasty (the main three at Giza and the two Snefru pyramids at Dashur) were oriented to face the cardinal points with a high degree of precision. The deviation of the east side from true north is in fact the following:
Meidum -20′ ± 1.0′; Bent Pyramid -17.3′ ± 0.2′; Red Pyramid -8.7′ ± 0.2′; Giza 1 (Khufu) -3.4′ ± 0.2′; Giza 2 (Khafre) -6.0′ ± 0.2′; Giza 3 (Menkaure) +12.4′ ± 1.0′.
The precision achieved by the pyramid builders was so high that it is absolutely certain that the orientation method used was based on stars and not on the measurement of shadows (recently, the French mission directed by M. Valloggia has determined the orientation of the pyramid at Abu Roash [Mathieu 2001], probably constructed by Djedefre who ruled between Khufu and Khafre, to be - 48.7′, but this error is so out of stream with respect to the others that it points to a different, perhaps solar, orientation ceremony).
The stellar methods which have been proposed in the past, e.g. the observation of rising and setting of a bright star on an artificial horizon, are not affected by precession. However, as already noticed by Haack (1984), the data strongly point to the existence of a time-dependent font cause of systematic error and this font is certainly precession. This problem induced Kate Spence (2000) to propose a method of orientation - “the simultaneous transit” - which consists in observing the cord connecting two circumpolar stars, namely Kochab (
b UMi) and Mizar (z UMa) when it is orthogonal to the horizon. Due to the precessional motion of the earth axis the cord does not always identify the true north: it has a slow movement which brought it from the left to the right of the pole in the 25th century BC. Plotting the deviation from north against time, Spence shows that the corresponding straight line fits well with the deviation of the pyramids i.e. to true north if the date of ‘orientation ceremony’ occurred for the Giza 1 pyramid in 2467 BC ±5y (although no written evidence of orientation ceremony exists for the old kingdom pyramids, the ‘Stretching of the Cord’ foundation ceremony is actually already present in the Old Kingdom stele called ‘The Palermo Stone’). If one, in turn, accepts the method as the one effectively used, the graph plotted can be used to calibrate the dates of construction of all the fourth dynasty pyramids, which turn out to be around 80 years later than usually accepted.
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