It's been said that we can judge the greatness of a person by how long he or she can hold up progress. And by that standard, Aristotle has to rank as one of the greatest people of all time. His incorrect ideas about astronomy held up progress for more than a thousand years. In other fields he almost doubled that number.

Aristotle held to the ancient view that Earth was the center of the universe. And "universe" was much less than what the word encompasses today. Different cultures saw patterns in the stars, patterns that we today call constellations. Different cultures saw different patterns, to a large extent, and most of us today have a hard time making out what our ancestors imagined they saw. We can all recognize the Big Dipper easily, but it is only part of Ursa Major, the Big Bear, which is much more difficult to perceive.

But ancient observers noted that some "stars" did not stay in the same place. They "wandered" and thus acquired the name planets. But they followed set patterns, and that was important. They made their way across the skies from one constellation to another. Or, as we discussed last week for Mercury and Venus, they were sometimes evening stars but at other times morning stars. And for Aristotle (and doubtless others) that meant the planets could not be at the same distance from Earth as the (fixed) stars.

So Aristotle (about 350 B.C.) pictured a universe consisting of nine spheres, each made of something some authors call "quintessence." This was said to be unchanging and transparent and perfectly spherical. This latter idea seems to derive from the notion that because the Gods created the place, they would have made it perfect and so therefore the spheres must be perfect spheres. Ptolemy, a Greek astronomer and mathematician of about 140 A.D., refined the Aristotelian concepts. Earth, it was asserted, occupied the first and central sphere. Moving progressively outward, the moon occupied the second sphere, then Mercury, Venus, the sun, Mars, Jupiter, Saturn and, lastly, the fixed stars. The planets, moon and sun thus moved around in their spheres and were observed against the background provided by the fixed stars. Later observers we'll not mention made some refinements to this idea, but it was the basic Aristotelian/Ptolemaic system that carried us into the Middle Ages.

Copernicus, the Polish astronomer, mathematician and priest, launched the first major challenge to the Ptolemaic system, using recorded visual data on planetary movements. He proposed a sun-centered, thus heliocentric, system, which proved to be much simpler than the cumbersome Ptolemaic system. But it was Galileo, the Italian astronomer and mathematician, who provided the first empirical data to evaluate the two. As earlier mentioned, it was he who in 1609 first turned the newly invented telescope to the heavens and quickly produced data shaking the Ptolemaic ideas. Critical was his observation of the phases of Venus. Venus was not just a point of light, but went through phases just like the moon.

The Ptolemaic system, by now complicatedly refined, suggested that Venus moved on an elliptical path totally between Earth and sun. This predicted that it should always appear in a telescope as a crescent. Galileo showed that the crescents appeared only when Venus was off to the side of the sun as seen from Earth, but the crescents shrunk and Venus turned virtually to a dark shadow as it passed between Earth and sun. As it passed to the opposite side of the sun, it became totally lit, a "full" Venus. Only by orbiting the sun could this pattern of nature be explained.

• Duane Jeffery is a professor emeritus of biology at Brigham Young University.