When Night Falls

Even if today’s astronomers knew nothing of the solar system, they would likely reject the Ptolemaic model on the grounds of its unnecessary complexity. And for all of its intricacy, the Ptolemaic model was not a particularly accurate predictor of astronomical phenomenon. Indeed, the errors in the model became more glaring when better data on planetary positions (from Tycho Brahe and others) became available.

A love of elegant simplicity also characterized the classical world, but Ptolemy’s era was already falling away from classical elegance and toward the cobwebbed mysteries that so appealed to people in the Dark Ages, when complexity and obscurity, not simplicity and clarity, were taken as the hallmarks of truth. Besides, Ptolemy’s model, while highly imperfect, agreed pretty well with actual observation; it kept Aristotle safely on his pedestal, and it let humankind stay right where the Church said that God had intended: at the center of everything.

There were others who came after Aristotle and before Ptolemy, most notably Aristarchus of Samos (310–230 B.C.E.), who actually proposed that the earth and the planets orbit the sun. But Aristotle, Ptolemy, and common sense drowned out such voices that, for some thirteen centuries, few wanted to hear. For the light of classical learning had been dimmed, and the spirit of scientific inquiry muffled (at least in the West) in a long age of orthodoxy and obedience.

Ptolemy’s Picture

Why didn’t the world’s astronomers just toss out Aristotle’s geocentric model of the solar system?
There are at least three reasons.

• First, and most important, the geocentric picture appeals to common sense. On a given day, as we watch the sun and the stars rise and set, we do not feel like we are in motion. There are, for example, no great winds whipping at us as one might expect if the earth were tearing through space.
• Second, human beings are egocentric creatures. We tend to see ourselves as being at the center of things. Extend this egocentric tendency into the heavens, and you have an explanation for our species’ geocentric tendency. If, individually, we feel ourselves at the center of things, we also feel this collectively, as a planet.
• The final reason? Because Aristotle was Aristotle. His opinions were regarded with awe for centuries. Few dared—few even thought—to question his teachings. Instead, generations of European astronomers wrestled with Aristotle’s model in an effort to show how it was actually correct.

The most impressive of these wrestling matches was fought by Claudius Ptolemaeus, a Greek astronomer better known as Ptolemy, active in Alexandria during C.E. 127–145. Drawing on the work of the Greek astronomer Hipparchus (who died after 127 B.C.E.), Ptolemy developed the geocentric model into an impressively complex system of “deferents,” large circular orbits centered on the earth, and “epicycles,” small circles whose centers traveled around the circumferences of the deferents.

Some 80 deferents and epicycles came into play, along with several other highly complex geometric arrangements, which allowed Ptolemy to account for many of the perplexing planetary motions actually observed.

Aristotle Lays Down the Law

Fortunately for the hard-pressed astronomers of yore, total eclipses of the sun are relatively rare events. More immediately, they had nightly occurrences they couldn’t explain.

They could watch the sky all night and see the stars glide predictably across the sky, as if affixed to a moving celestial sphere. Likewise, the moon made its traversal with perfect regularity. But there were (so far as the ancients could see) five heavenly bodies that didn’t behave with this regularity. Mercury, Venus, Mars, Jupiter, and Saturn looked pretty much like stars, but, in contrast to stars, they wandered across the sky and so were called by the Greeks planetes, or “wanderers.” While the planets always remain near the ecliptic, they have strange motions. Relative to the stars, planets seem to slow down and speed up, moving (from an earthbound observer’s perspective) generally eastward (prograde motion), but sometimes westward (retrograde motion) with respect to the background stars.

How did the ancients explain this strange observation? The great Greek philosopher Aristotle (384–322 B.C.E.), tutor of Alexander the Great, formulated a picture of the solar system that put the earth at its center with all the other heavenly bodies orbiting around it. The orbits were described as perfect circles. Now, Aristotle was a smart man. He provided observational evidence that the earth was spherical rather than flat. Moreover, his descriptions of the orbits of the moon and the sun accorded very well with what people actually observed. As far as the stars went, the celestial sphere idea explained them well enough. The problem was the planets. Aristotle’s simple geocentric (earth-centered) model did not explain why the planets varied in brightness, why they moved at varying speeds, and why their motion was sometimes prograde and sometimes retrograde.

The Sun Goes Dark, the Moon Becomes Blood

The ancients had another, far more dramatic, celestial irregularity to contend with. On rare occasions, the moon would gradually dim and turn a deep red for a time, only to reemerge in its full glory after a short time. On even rarer occasions, daylight would fade as a great shadowy disk stole across the sun

As we saw in the previous section, eclipses were events that could cause great fear, and governments and rulers put tremendous pressure on astronomers (or soothsayers or astrologers or whatever they called their official sky watchers at the time) to come up with dependable ways of predicting when eclipses would occur. Here was yet another set of celestial events that certainly weren’t random, yet, without a complete understanding of how the various parts of the solar system were put together, they were hard to predict accurately.

Time on Our Hands

Why did the ancients concern themselves about things moving in the sky when they were stuck here down on Earth? Chalk it up in part to human curiosity. But their interest also had even more basic motives.
You’re walking down the street, and a passerby asks you for the time. What do you do?
You look at your watch and tell him the time.
But what if you don’t have a watch?
If you still want to be helpful, you might estimate the time, and you might even do this by noting the position of the sun in the sky.

The ancients had no wrist watches, and, for them, time—a dimension so critical to human activity—was measured by the movement of objects in the sky, chiefly the sun and the moon. What, then, could be more important than observing and explaining the movement of these bodies?

Heavens on the move

The next time you’re outside doing yard work in the sun, put a stick in the ground, call it a gnomon, and watch the motion of its shadow. Believe it or not, you have made a simple sundial, which was one of the earliest ways that human beings kept track of time. In fact, keeping time was one of the two major reasons that early civilizations kept a close watch on the skies.

The other reason, of course, was to use the motions of the planets through the constellations to predict the future for the benefit of kings and queens and empires. Well, the first practice (keeping track of time) has continued to this day. The U.S. Naval Observatory is charged with being the timekeeper for the nation, using technology a bit more advanced than a stick in the ground. The second practice (predicting the future) is also alive and well, but astronomers have turned those duties over to The Psychic Network—at least for the time being.

In the days before movies, television, video games, and the Internet, the starry sky (untouched by city lights and automobile exhaust) was truly the greatest show on Earth. Generations of sky watchers looked and imagined and sought to explain. Common sense told many of these early watchers that they were on a kind of platform overarched by a rotating bowl or sphere that held the stars. We have seen that in various cultures, other explanations surfaced from time to time. It doesn’t matter right now whether these explanations were right or wrong (well, many of them were wrong). What matters is that the explanations were, to many astronomers, unsatisfying. None of the explanations could account for everything that happened in the sky.

For example, if the stars were all fixed in this overarching bowl, how did the planets break free to wander among the stars? And they didn’t wander randomly. The planets were only found in certain regions of the sky, close to the great circle on the sky called the ecliptic. Why was that? The sky is filled with thousands of bright points of light that move, and none of the ancient explanations adequately explained all of these movements.