Early planetary schemes

Behind all the complex hypothetical machinery to move planets through the sky lay a simple aim to predict the positions of planets and the sun and Moon in the foreseeable future in order to cast horoscope, make forecasts and construct a calendar. In achieving these aims, it, did not, matter that the solution appeared to be contrived. So long as it saved the phenomenon (hardly any of them did !), that was enough.

Although the early astronomers of Egypt and Babylonia had been keen observers of the apparent motions of the Sun, Moon and planets, they did not use geometrical models of their paths through the sky. With the Greeks, the emphasis changed from a strictly observational role to one that required rational explanations for observed changes in the sky. They began to make geometrical models of planetary motions with some attempt being made to fit the appearances. The earlier attempts were not too successful from the observer’s point of view; that is, the observed positions of I planets and the positions calculated theoretically did not agree very closely. However, a model was developed by the second century AD which satisfied the observations with a fair degree of accuracy and which was accepted, with minor changes in the con¬stants from time to time, until the sixteenth century. This model was developed by Claudius Ptolemaeus, or Ptolemy as he is usually called, an astronomer from the school of Alexandria.

Hipparchus, (active c. 160-126 BC), was the author of the first systematic star catalogue. His researches were to culminate in the ‘Great Synthesis’ of Ptolemy, or. ‘Almagest’, as it is commonly called from its Arab translation. Ptolemy’s work is a masterpiece of complicated geometrical computation, and its success relied heavily on previous planetary observations, many of which came from Hipparchus. Ptolemy attempted only to explain the appearances, not the actual orbit in space; his model was a geo¬metrical device to permit correct predictions of celestial positions against the background of fixed stars from an observed fixed position on a stationary Earth.

The ‘Almagest’ treated each planet separately, and, although the mathematical scheme for each had the same general pattern, there was no connection between them to produce an integrated system. It was concerned only with apparent position; the actual size of the planetary orbit was of no consequence – Ptolemy assumed the accepted order of the celestial bodies adopted by the ancients. The general structure of the celestial sphere is described with the Earth at the centre, and the path of the Sun, the ecliptic, defining the reference plane. The motion of the Sun is taken as the reference body for describing the positions of the other planets, which can then be demonstrated fairly simply. Since the motion of the Sun is not uniform, the centre of the circle is displaced very slightly from the Earth and the resultant true position of the Sun calculated with respect to the Earth. The motion of the planets required two extra features, the equant and the epicycle.

In the period following Ptolemy’s death, (c. 151 AD), astronomers accomplished little: compilations and commentaries replaced observations, so the Ptolemaic theory remained unchanged. Only in the eighth and ninth centuries with the birth of Arab schools in Baghdad. Cairo and the Arab West (Spain and Morocco) did astro¬nomical observations once more become important. New astro¬nomical tables based on the principles laid down in the ‘Almagest’ were drawn up and minor improvements made to the theory, which basically remained the same. Ptolemy became known to the West about the middle of the twelfth century through the books of Al-Battani and Al-Farghani. The ‘Almagest’ was first translated into Latin by Gerhard of Cremona in about 1175. Around the same time, translations of Aristotle’s works on scientific subjects became available and, as with his well-known works on logic, they exerted considerable influence over mediaeval thought. A new set of astronomical tables, the ‘Alfonsine Tables’ published in 1252, was to last for the next two centuries. By this later date, discrepancies between the tabulated and observed positions, particularly for the planet Mars, became unacceptably large. In Europe, and in particular in Vienna and Prague. there was a concern on the part of the catholic church to rationalize the dates of the Easter festival. The reliability of current tables had to tested and more positions obtained. A closer study of the ‘Almagest’ was necessary and two astronomers, George von Puerbach (1423-61) and his pupil Johann Muller (or Regiomontanus) (1436-76) were invited to investigate the problem. .This resulted in the Nuremberg ‘Ephemerides’. the most detailed and accurate astronomical infor¬mation then available. The next change was to be fundamental.

In a book published in 1543, ‘De Revolutionibus Orbium Coelestium’, Nikolaus Copernicus (1473-1543), a Polish canon, for the first time offered an alternative to the ‘Almagest’ for the serious astronomer. Analysing a vast amount of data, as had Ptolemy, he reconsidered the whole problem of planetary motions with the Sun rather than the Earth at the centre of the system. From the modern standpoint, it makes little difference which reference point one uses, but the relocation of the centre played a significant role in the history of astronomy. In his scheme for inner and outer planets Copernicus, like Ptolemy, believed in the necessity for uniform, circular motion; the mathematical techniques in both the ‘Alma¬gest’ and ‘De Revolutionibus’ are essentially the same. Copernicus got rid of the five planetary epicycles but he had to add a large number of small epicycles to remove eccentrics. What he did provide was a uniform planetary system in which all the planetary distances were linked to the Sun-Earth system. From a historical point of view, he also re-opened the whole question of the Earth’s mobility and demonstrated that accurate planetary predictions could be made from a heliocentric system. The Copernican revolu¬tion in fact lay in the after-effects of ‘De Revolutionibus’ rather than in the contents of the text itself. The removal of the Earth from the centre, within a thorough mathematical treatment of planetary orbits, had consequences which Copernicus, steeped in mediaeval traditions, could not anticipate, and the introduction of a rotating Earth was another major conceptual advance.

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