Astronomer Claudius Ptolemy

Claudius Ptolemy (c. 100-170 AD) was a Roman astronomer, astrologer, geographer, musicologist, and opticologist. He worked in Alexandria, Egypt. Little else is known about his life. Ptolemy’s Almagest is the only surviving comprehensive ancient treatise on astronomy. It served as the guide for astronomers and students of astronomy for some 1300 years until Copernicus published his Revolutions of the Celestial Spheres in 1543.

The only hope for survival an astronomer who lived before Ptolemy had was to be incorporated within the Almagest. Of interest is Hipparchus’ star map that included the positions of about 850 stars. Ptolemy included Hipparchus’ data and increased the number of stars in the collection to 1,022. This resource was available for any interested person to do a comparative study of the positions of stars observed by Hipparchus (c. 190 – c. 120 BC) with their own observations.

The intellectual climate was not encouraging for anyone reporting differences between the original data and current observations. This would indicate instability in the heavens. Copernicus feared the consequences of moving the Earth from the center of the universe. He consented to publication of his Sun-centered universe only on his deathbed.

The intellectual climate had changed within certain countries after the Protestant Reformation especially England and the Netherlands. The Royal Society of London for Improving Natural Knowledge was granted a charter by King Charles II in 1663. The society’s motto is Nullius ad verbum, take nobody’s word for it. It expresses a determination to resist the domination of authority and to verify all statements by an appeal to facts determined by experiment.

Edmond Halley participated in this society and financed the publication of Isaac Newton’s Principia Mathematica in 1687. Among Halley’s many other accomplishments was his presentation of the first evidence against the solid sky in 1718. He compared Hipparchus’ positions of stars with his own observations and found three stars out of place: Procyon, Arcturus, and Palilicium (Aldebaran). If it were not for Hipparchus’ star map preserved in Ptolemy’s Almagest Halley would not have had an ancient reference to compare his observations with.


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