History Online - Astronomy

Imagine that we are 16th-18th-century astronomers. Only the following two points of view are possible.
(1) Assume that we are of Scaliger's and Petavius' opinion and associate the second year of Emperor Antonine Pius' rule, the date of the Almagest's observation time, with A.D. 138, in which case we should discover the stars' proper motion, resorting to the allegedly ancient, about 1,500-year-old catalogue, at least proceeding from Arcturus, the brightest star in the sky. However, these natural attempts were not fixed by the traditional history of astronomy in the 15-16th century, though they would have to make us conjecture that Arcturus was moving.
(2) Assume that we regard the Almagest as a comparatively recent document, say, of the 10th-15th century, or as a source with unknown dating, in which case the attitude toward the book would be quite different. If this catalogue is assumed to be medieval, then the Almagest's inaccuracy, which is well known to the professional astronomers, and, moreover, the large division value would simply not permit us to make such a recalculation (which could not be made either if the catalogue date was unknown).

Since the history of astronomy reports nothing of the 16th- and 17th-century astronomers' attempts to discover the stars' proper motion on the basis of the Almagest, we are forced to conclude that they did not regard it as a sufficiently ancient document.

Thus, a serious 16th- or 17th-century scientist should have made the conclusion that the accuracy of the coordinates in the Almagest did not permit him to discover the stars' proper motion if he regarded it as a medieval source. On the other hand, if the Almagest had been regarded as an ancient document, say, dating back to the 2nd century A.D., then the idea itself of using it to discover the stars' proper motion would have been very simple. It should be remembered that the problem of the stars' proper motion was important in ancient times. It seems completely improbable for the idea to be new in Halley's time.

We shall now try to explain why the conclusion regarding the proper motion of certain stars (e.g., of Arcturus or Sirius) could be made in Halley's time, though more or less precise values for their rate could not be found.

The first precise star catalogue to the accuracy of 1' was made by Tycho Brahe in 1582-1588. Arcturus' and Sirius' displacements in the 100 years after Halley were about 3' and a little more than 2', respectively. With a precise catalogue of the star positions as was available in the 18th century, Arcturus' and Sirius' proper motion could already be suspected, though the catalogue's accuracy could not have as yet permitted one to determine the rates of motion. It turns out that such a catalogue was indeed available in the early 18th century. It was made by Flamsteed and was actually employed by Halley even before its publication (with Halley having taken some incomplete version from Isaac Newton, who was just investigating chronology).

Thus, in our opinion, Halley compared Flamsteed's and Brake's catalogues, and made. the conclusion regarding the existence of Arcturus', Sirius', and Aldebaran's proper motion.

Meanwhile, Aldebaran's motion indicated by Halley obtains the natural explanation: He made use of the incomplete variants of Flamsteed's catalogue in which the Aldebaran information was given erroneously. Flamsteed regarded his catalogue as not yet ready for publication at that time. It is known that Halley made Aldebaran's position more precise for the purpose (in his letter to A. Sharp on September 13, 1718; see [109]).

Why then did Halley refer to Ptolemy's Almagest as the basis for his computations, and not to Tycho Brahe's? Obviously, in Halley's time, the traditional Almagest's date, A.D. 138, "calculated" by Scaliger and Petavius had already been canonized. To give his discovery more weight, Halley referred to the Almagesi, and not to Tycho Brahe, for the simple reason that the star displacements discovered by him then looked more impressive. Calculating Arcturus' displacement from Brahe's catalogue, Halley obtained only three minutes, compared with the nominal accuracy of one minute in the former catalogue, whereas, repeating the procedure for Ptolemy's catalogue of (allegedly) A.D. 138, he naturally obtained a more substantial shift of about one degree. It is apparent that Halley compared the shift with the Almagest's nominal accuracy of ten minutes, neglecting the problem of the actual accuracy of the coordinates of the Almagest's catalogue.

This reasoning makes us think once again that the Almagest had not yet been regarded (in the 16th century) as an ancient, about 1,500-year-old document and its "antiquity" was assumed known and canonized only early in the 18th century.

Each of the above facts can also be separately "explained" within the traditional chronology framework, but, taken all together, they most probably indicate that the Almagest was written between the 10th and the 16th century. Still, in contrast to Morozov, I do not at all believe that the Almagest was fabricated. More than that, in my opinion, at any rate, its first edition is an original created in the 15th-16th century for immediate scientific purposes.