Famous Astronomers: The Life of Ptolemy

As we look at some of history’s greatest astronomers, we begin with Ptolemy. The career of Ptolemy is one of the most remarkable in the history of human learning. While other astronomers may have done more for science than Ptolemy ever accomplished, no other discoverer has had authority on the movements of the heavenly bodies for longer than the 14 centuries during which Ptolemy’s opinions reigned supreme.

The doctrines Ptolemy lay down in his famous book, “The Almagest,” prevailed for ages. No substantial addition was made in all that time to the undoubted truths that this work contained. No important correction was made of the serious errors with which Ptolemy’s theories were contaminated. Ptolemy’s authority of knowledge over all things in the heavens and many things on the Earth was, for this period of time, invariably final. 

Though every child may now know more of the actual truths of the celestial motions than ever Ptolemy knew, the fact that his work exercised such an astonishing effect on the human intellect for some 60 generations shows that it must have been an extraordinary production.


Unfortunately, we know very little of Ptolemy’s personal history. He was a native of Egypt and, though it has been sometimes conjectured that he belonged to the royal families of the same name, there is nothing to support such a belief. The name “Ptolemy” appears to have been a common one in Egypt in those days.

The time at which Ptolemy lived is fixed by the fact that his first recorded observation was made in 127 A.D. and his last in 151 A.D. The only other personal fact known about this famous astronomer is that he seems to have lived in or near Alexandria or, to use his own words, “on the parallel of Alexandria.”

Ptolemy is, without doubt, the greatest figure in ancient astronomy. He gathered up the wisdom of the philosophers who had preceded him. He incorporated this with the results of his own observations and illumined it with his theories.

His speculations, even when they were, as we now know, quite erroneous, had such an astonishing verisimilitude to the actual facts of nature that they commanded universal assent. Even in these modern days we frequently find lovers of paradox who maintain that Ptolemy’s doctrines are actually true.

Ptolemy’s Observations:
In the absence of any accurate knowledge of the science of mechanics, philosophers in early times were forced to fall back on certain principles of more or less validity, which they derived from their imagination as to what the natural fitness of things ought to be. There was no geometrical figure so simple and so symmetrical as a circle, and, as it was apparent that the heavenly bodies pursued tracks which were not straight lines, the conclusion obviously followed that their movements ought to be circular.

There was no argument in favor of this notion, other than the merely imaginary reflection that circular movement, and circular movement alone, was “perfect,” whatever “perfect” may have meant.

During this time, many also believed that the heavenly bodies could have any other movements save those that were perfect. Assuming this, it followed, in Ptolemy’s opinion, and in that of those who came after him for 14 centuries, that all the tracks of the heavenly bodies were in some way or other to be reduced to circles.

Ptolemy succeeded in devising a scheme by which the apparent changes that take place in the heavens could, so far as he knew them, be explained by certain combinations of circular movement. Because this seemed to reconcile celestial movement with “perfect” circular movement, it’s no surprise that Ptolemy’s theory met with the astonishing success that attended it.

The Earth as the Center of the Universe
What Ptolemy saw in the movements of the stars led him to the conclusion that they were bright points attached to the inside of a tremendous globe. The movements of this globe that carried the stars were only compatible with the supposition that the Earth occupied its center. The imperceptible effect produced by a change in the locality of the observer on the apparent brightness of the stars made it plain that the dimensions of the terrestrial globe must be quite insignificant in comparison with those of the celestial sphere. The Earth might, in fact, be regarded as a grain of sand while the stars lay upon a globe many yards in diameter.

So tremendous was the revolution in human knowledge implied by this discovery that we can well imagine how Ptolemy failed to make one further step. Had he made that step, it would have emancipated the human intellect from the bondage of 14 centuries of servitude to a wholly monstrous notion of this Earth’s importance in the scheme of the heavens. The obvious fact that the sun, the moon, and the stars rose day by day, moved across the sky in a glorious never-ending procession and duly set when their appointed courses had been run demanded some explanation.

In Ptolemy’s opinion, the fixed nature of stars preserved their mutual distances from year to year, proving that the sphere that contained those stars was fixed and revolved completely around the Earth once every day. He would thus account for all the phenomena of rising and setting consistently with the supposition that our globe was stationary.

Ptolemy knew that the Earth was a gigantic object, but, large as it may have been, he knew that it was only a particle in comparison with the celestial sphere. Yet he apparently believed (and certainly succeeded in persuading other men to believe) that the celestial sphere did actually perform these movements.


Ball, Robert S. (1895). Great Astronomers.