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 (see Chapter 2, "Ancient Evenings: The First Watchers"). 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.

The Least You Need to Know

V Astronomy has always been associated with the keeping of time. The observed repeating motions of the sun, moon, and stars were first used to divide time into now familiar intervals: days, months, and years. Unfortunately, none of these intervals is evenly divisible into another.

V The earth's rotational axis is ever so slowly precessing, tracing out a circle on the sky. It takes the earth a long time (26,000 years) to precess once.

V Seasonal temperature variations and the difference in the length of days on the earth in the summer and winter are due to the 23V2-degree tilt of the earth on its axis. The earth is not closer to the sun in summer and farther away in winter—in fact, the opposite is true.

V Equinoxes and solstices are special days in the astronomical year. The summer and winter solstices represent the longest and shortest days of the year, and the vernal and autumnal equinoxes are times when the day and night have equal length.

V Ptolemy made a highly successful—and complicated—attempt to account for all the complexities of movement among the celestial bodies. His geocentric model of the solar system was accepted until better observations of planetary positions in the sixteenth and seventeenth centuries made it clear that the model had to be significantly revised.

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