In the second century AD, Claudius Ptolemy portrayed the accepted view of his age that the planets moved in celestial spheres around Earth. In the Almagest, he collected tons of data from observations, which he tried to fit into his view. The Geocentric Model set the ordering of planets (supposedly based in their revolution speeds) for oncoming centuries. The order was: Moon-Mercury-Venus-Sun-Mars-Jupiter-Saturn.
The sorting of the days of the week looks like a bit counter-intuitive, especially related to Friday (Venus' Day), located between Thursday (Thor=Jupiter's Day) and Saturday (Saturn's Day). At first glance, they appear to be ordered by its relative distance ti Earth. There is no academic theory about the current order of the days in the week. However, there's a tale running through the web which assures that its source point to an algorithm employed by ancient astronomers.
Roman period astrologers brought into Europe an ancient Egyptian tradition assigning every daily hour to a planet (starting with Saturday, which seems to arise from Babylon, though it is not important here). The 7-day week is said to have originated in Babylonian times, but it is well-known that the Roman Republic Calendar and the Julian Calendar employed an 8-day count, naming the days from A to H (1) . Some scholars argue that the decline of the 8-days week started in the times of Julius Caesar, mainly by the exposure of Rome to growing religions (esp. Christianism) and the rising cult of Mithra.
In the Baths of Titus (81 AD, before Ptolemy's birth) was found a stick calendar showing a 7-day week, with days ordered in the modern fashion, and starting with Saturday. That order is still a mystery.
The tale I mentioned earlier (3) says that when the 7-day week was broadly spread in the Empire during the reign of Constantine the Great (1), seers and astrologers used the 30 days month of Egyptians to link the planets with a day. The first hour of the first day of the week was ascribed to Saturn and the following to Jupiter, Mars, the Sun, Venus, Mercury and the Moon according to the Ptolemaic order. Thus, the eighth hour of the first day was ascribed again to Saturn, and also the fifteenth and twenty second. Following this cycle for every hour and every day in the week, the first hours of the following days would be ascribed to the Sun, the Moon, Mars, Mercury, Jupiter and Venus, respectively. Therefore, each day of the week received the name of the planet to which its first hour had been ascribed. The result was the current order of days, which, as we have seen, had already been in use during the age of Titus (5 decades before).
So questions arise: Why did Ptolemy used that specific order in the first place? And when did that sorting originated and why? Which was its point?
The Almagest
In his Book IX, Ptolemy discusses the ordering of planets. He wrote (4):
"Concerning the spheres of Venus and Mercury, we see that they are placed below the sun’s by the more ancient astronomers, but by some of their successors (Plato, Eratosthenes and Archimedes among them) these too are placed above the sun, for the reason that the sun has never been obscured by them".
He notes that no transit of Mercury and Venus was ever observed (or at least the ancient did not have any account of it). This thing seems odd but let's skip it for now. He tried to explain it arguing that "it is possible that some planets might indeed be below the sun, but nevertheless not always be in one of the planes through the sun and our viewpoint, but in another plane, and hence might not be seen passing in front of it". Nevertheless, that's not what happens as we currently know. He continued:
"And since there is no other way, either, to make progress in our knowledge of this matter, since none of the stars has a noticeable parallax (which is the only phenomenon from which the distances can be derived), the order assumed by the older [astronomers] appears the more plausible".
Placing Venus below the Sun (interior orbit) explained its huge brightness increase, but failed to account for the parallax (5). Putting it above (exterior orbit) would do the contrary. Ptolemy did not have any objective (scientific) reason to choose either option. "Ptolemy did not get his final mean motions for Mercury by analysis of the two observations used in Almagest 9. Moreover, when he revised the eccentricity of his model for this planet, he chose not to recalculate the mean motions in the light of this change, but instead manipulated either the reports of those two observations or the reduction and analysis of them (or both) so as to obtain as close agreement as possible with the mean motions to which he had already committed himself". This is reported in a 2005 paper (6). In other words, he was biased.
Chaldean Astronomy
How could ancient astronomers know the speeds of planets? Some hold the idea that Babylonian astronomers had a Heliocentric view of the solar system, so they could have know the exact arrangement of planets (at least, in their epoch) (7). To back this they refer to a Hellenistic astronomer called Seleucus of Seleucia (the name seems rather dubious), that is said to have lived in the 2nd century BC. He is known from the writings of Plutarch, Aetius, Strabo, and Muhammad ibn Zakariya al-Razi. The Greek geographer Strabo lists Seleucus as one of the four most influential astronomers, who came from Hellenistic Seleuceia on the Tigris, alongside Kidenas, Naburianos, and Sudines. Their works were originally written in the Akkadian language and later translated into Greek (8). Seleucus, however, was the only one supporting the Model of Aristarchus of Samos (300 BC). According to Plutarch, while Aristarchus postulated Heliocentrism only as a hypothesis, Seleucus maintained it as a definite opinion and gave a demonstration of it (9) but no full record has been found (10). However, there is a big hindrance to that postulate: Why Greeks and Romans overlooked that knowledge and went on with a Geocentric Model?
Others, keep on with the idea that Chaldean astronomers handled a Geocentric Model already. How do the reached the planetary speed arrangement later inherited by Ptolemy? Did they make the same convoluted epicycles (to explain the retrograde motions and bright increases in the sky) and deferents (the actual displacement in the sky along the ecliptic)? It seems strange, at best. In a Geocentric view, the only way for they to calculate the period the planets spent to reach the same position in the heavens, was to measure the time between two inferior (for interior planets) conjunctions. For Venus this would take 584 days (a period called synodic year), which is obviously larger that the Sun's revolution period of 365 days around the ecliptic. Thus, they should reach the following order: Moon-Mercury-Sun-Venus-Mars-Jupiter-Saturn, according the synodic periods, instead of the Ptolemaic one.
If the Ptolemaic ordering did not arise from the synodic arrangement, and they didn't know a heliocentric model, might it be that planet configuration was different in Babylonian times?
The Ammizaduga Tablets
It's a record of astronomical observations of Venus as preserved in numerous cuneiform clay tablets dating from the first millennium BC, though they are believed to have been compiled during the kingdom of Babylonian King Amizzaduga (maybe 1500, maybe 17-1800 BC). They were recalled by Carl Sagan when attacking Velikovsky during the IAAS meeting in 1974, claiming that Chaldeans astronomers recorded Venus in its exact current position (11).
Normally after disappearing in front of the Sun in the west, Venus remains out of view, caught in the glare of the Sun, for about 8 days. After disappearing behind the Sun in the east, however, Venus remains invisible for an extended period of time. This is both because of a longer path it has to travel behind the Sun, and because the Earth keeps moving, making the disappearance even longer. Today Venus remains hidden for about 50 days (it varies somewhat with the inclination of its orbit). At the time of the Venus Tablets of Ammizaduga it varied wildly and inexplicably.
It has been published that several Babylonian dates have a gap of 2 days (which is difficult to explain by errors of scribes), or even a month apart (B32 and B42 for example), and that Egyptians lunar calendar was closer to the Jewish or Aramaic calendar than its Babylonian counterpart (12).
Large mistakes were found in several annotations. Since observations may also have been missed or delayed by bad weather, some dates may have been “corrected” and/or filled in afterwards by contemporary or later scribes [possibly corrupted observations in Table 1 are numbers 5, 9, 10, 17, 18, 26-30, 32 and 33]. This errors are acknowledged but some scientists blame atmospheric extinction effects and aerosols from Minoan and Thera (Santorini) volcanic eruptions instead of scribe errors (13).
"The differences for corrupted dates are quite large and listed in square brackets. Larger differences occur for morning last and evening first observations near superior conjunction of Venus and the Sun, when the relative velocity of Venus with respect to the Sun is small (~ 0.3o per day) so that variations in extinction may lead to large differences in the date of first or last appearance".
"By interpreting the large extinction values in Tables 1-4 for the Venus observations in the 12th and 13th year of king Ammizaduga in terms of a physical process we reduce the number of observations in Tables 1-4 that must be discarded to 8 or 9, less than 25% of the total number". Even so, a quarter part of them would still be wrong. Their conclusions state: "In this article we have shown that the results of an analysis of the Venus observations on Tablet 63 of 'Enuma Anu Enlil' in terms of a physical model of the visibility of Venus in a realistic twilight atmosphere do not allow a decisive choice between the four different Venus chronologies".
In other papers (14), scientists remark the observation of Venus setting a day after or rising a day before its modern computed date. This is attributed to much cleaner atmosphere in Ammizaduga's days. In the 'Long Chronology' of Ammizaduga, it was registered that in the 5th year, Venus remained visible 3 days longer than we calculate. Periods of invisibility 4 and 9 days longer than they should have been are found during the 10th and 15th years.
John D. Weir specifically states:
"The two potential patterns of variation from the computed values have their respective maximum and minimum periods of invisibility separated from each other by an interval of 6 months. This suggests that the cause, if it is astronomical, it is more likely to be found in some modification of the Venus orbit, than in terrestrial changes. Moreover, years 4 and 5, which had their invisibility periods the correct length, come roughly half-way between perihelion and aphelion on the Venus orbit. Year 4 comes 7 days too early, Year 5 comes 2 days later. This is also consistent with a modification in the shape of the Venus orbit".
He ends: "No solution (chronologies of ruler kingdom) will satisfy the groups of two successive 30-day months: the computed sequences of lunar months only approximate to reality, and this limitation has to be allowed for when comparing solutions".
More extreme scientists as Lynn Rose and Raymond Vaughan, claims that the data of the tablets belongs to the VIII century (750 BC), but did not offer proof (15). The Babylonians ended observations around VI century BC, and they were only continued in the Mayan culture, where the Dresden Codex states invisibility periods of 90 days (current ones are 50 approx.). Of course, it was the Mayan's fault and the observations jumped from solstice to equinox (16). In his paper 'Astronomical dating of Babylon III and Ur I', Peter J. Huber makes a detailed analysis (independent of Long or Short chronologies) of the tablets, discarding data because of huge errors. "At least 20 to 40% of the dates are grossly wrong". "That leave us with 31 of 49 preserved observations". A further filtering by specific chronologies reduced the reliability to 50% (17).
Final Remarks
So, if Babylonian's Venus observations were not accurate enough, it may be that Venus' orbit was more elliptical than today's almost perfectly circular one. Perhaps even the Earth's orbit was slightly different. That will minimise and even make the mistakes disappear. Why Ptolemy decided to use the ancient (Chaldean) astronomers ordering we might never know; but it has been shown that it had nothing to do with calculation or sky observations. The fact that ancient cultures placed Venus between Jupiter and Saturn remains elusive.
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