Ptolemy as Astronomer

Ptolemy as Astronomer

The study of the heavens brought Ptolemy a kind of ecstasy. “Mortal as I am”, he wrote, “I know that I am born for a day. But when I follow at my pleasure the serried multitude of the stars in their circular course, my feet no longer touch the Earth…”.

Ptolemy believed that the Earth was at the center of the universe; that the Sun, Moon, planets and stars went around the Earth. This is the most natural idea in the world. The Earth seems steady, solid, immobile, while we can see the heavenly bodies rising and setting each day. Every culture has leaped to the geocentric hypothesis. As Johannes Kepler wrote, “It is therefore impossible that reason not previously instructed should imagine anything other than that the Earth is a kind of vast house with the vault of the sky placed on top of it; it is motionless and within it the Sun being so small passes from one region to another, like a bird wandering through the air.”

But how do we explain the apparent motion of the planets – Mars, for example, which had been known for thousands of years before Ptolemy’s time? (One of the epithets given Mars by the ancient Egyptians was sekded-ef em khetkhet, which means “who travels backwards,” a clear reference to its retrograde or loop-the-loop apparent motion).

With the Earth the center of the Universe, there is little motivation for astronomical observations. Image: Neon Light Planet © Elena

Ptolemy’s model of planetary motion can be represented by a little machine, like those that, serving a similar purpose, existed in Ptolemy’s time. For example, four centuries earlier, such a device was constructed by Archimedes and examined and described by Cicero in Rome, where it had been carried by the Roman general Marcellus, one of whose soldiers had, gratuitously and against orders, killed the septuagenarian scientist during the conquest of Syracuse.

The problem was to figure out a “real” motion of the planets, as seen from up there, on the “outside”, which would reproduce with great accuracy the apparent motion of the planets, as seen from down here, on the “inside”.

The planets were imagined to go around the earth affixed to perfect transparent spheres. But they were not attached directly to the spheres, but indirectly, through a kind of off-center wheel. The sphere turns, the little wheel rotates, and, as seen from the Earth, Mars does its loop-the-loop. This model permitted reasonably accurate predictions of planetary motion, certainly good enough for the precision of measurement available in Ptolemy’s day, and even many centuries later.