History Online - Astronomy
[an error occurred while processing this directive]Ancient and Medieval Chronology
Halley and Almagest
It is traditionally believed that the stars' proper motion was first discovered by Halley in 1718. Speaking of this fact, Kulikovsky says:
"Comparing the positions of Arctums, Sirius and Aldebaran with their positions in the Hipparchus catalogue, E. Halley (1656-1742) discovered their proper motion, and that their ecliptical longitudes changed by 60', 45', and 6' in the last 1850 years, assuming that the catalogue was already dated as having been written in the 2nd century B.C." (namely, 1718 + 132 = 1850 years-A.F.) ([110]).
The first related question then is how he could have discovered Aldebaran's proper motion. It is because Aldebaran has shifted in the assumingly elapsed time (allegedly, ca. 2,000 years) by 6', as we now know on the basis of modern knowledge. However, the division value both in the Ptolemy and Hipparchus catalogues was 10'! It is useless to discuss the phenomenon if this value does not exceed the accuracy of a measuring device, let alone the fact that the actual accuracy of measurements was much worse than 10'. How then could Halley have discovered Aldebaran's proper motion if it had shifted by only 6' in 2,000 years? Another question arises: What motion did Halley ascribe to Arcturus and Sirius? Kulikovsky reports:
"In 1738, Cassini (1677-1756) precisely determined Arcturus' proper motion, having compared his measurements with Richet's observations (7-1696) sixty years before" ([110]).
Therefore, Halley did not determine Arcturus' proper motion "precisely". Nor could he have determined Sirius' motion when it moved at a slower rate.
The natural and, to some degree, unexpected question arises: Did Halley, at least in principle, discover the stars' proper motion?
It will be appropriate to inform the reader that Halley was not at all the first one to pose the question whether stars were moving, the problem having been animatedly discussed in the 15-16th century. Moreover, within the framework of traditional chronology, this problem was investigated as early as antiquity, or approximately 2,000 years before Halley. Tradition assumes that this important problem was discussed by Ptolemy who came to the ancient conclusion that all the stars were stationary (which we know is wrong). Thus, posing the problem cannot be ascribed to Halley.
Why did astronomers not compare the positions of the stars in the sky to the Almagest for the purpose of discovering their proper motion? As a matter of fact, the very idea of such a computation, or comparison, also dates back to Ptolemy and, therefore, was not at all new to a medieval astronomer. Such attempts would have been natural and could have led to the conclusion that the stars were moving: At least, the anomalous scattering in Ptolemy's star positions could have been taken as the stars' proper motion. We have already noted that the Almagest's accuracy is not consistent with the catalogue division value. Arcturus' and Sirius' motion could have been determined by the early 17th-century astronomers (100 years before Halley), as well as by Halley himself, on the basis of Tycho Brahe's catalogue, its accuracy being 1' and dating to 1582-1588 if we compare it with the Almagest.
