Why Halley did not discover proper motion and why Cassini did

TL;DR

This paper reevaluates Halley's 1717 claims of stellar proper motion, showing his conclusions were based on measurement errors, and highlights Cassini's work as the first solid evidence for proper motion.

Contribution

The paper demonstrates that Halley's data did not contain significant evidence for proper motion and clarifies Cassini's role in providing the first reliable evidence for it.

Findings

Halley's measurements were dominated by errors, not proper motion.

Cassini's analysis provided the first credible evidence of stellar proper motion.

Modern analysis refutes Halley's original claims based on measurement errors.

Abstract

In 1717 Halley compared contemporaneous measurements of the latitudes of four stars with earlier measurements by ancient Greek astronomers and by Brahe, and from the differences concluded that these four stars showed proper motion. An analysis with modern methods shows that the data used by Halley do not contain significant evidence for proper motion. What Halley found are the measurement errors of Ptolemaios and Brahe. Halley further argued that the occultation of Aldebaran by the Moon on 11 March 509 in Athens confirmed the change in latitude of Aldebaran. In fact, however, the relevant observation was almost certainly made in Alexandria where Aldebaran was not occulted. By carefully considering measurement errors Jacques Cassini showed that Halley's results from comparison with earlier astronomers were spurious, a conclusion partially confirmed by various later authors. Cassini's careful study of the measurements of the latitude of Arcturus provides the first significant evidence for proper motion.