those of the sun. The moon is slower by about one hour per day. The stars remain almost the same on successive nights, but slowly it becomes obvious that they, too, are slowed in their movements compared to the sun. Thus, the stars of summer are different from those visible in the winter. In fact, it takes a full year for the stars to return to their previous position, an interval of time that defines our year.

The ancient observers realized that not all stars move in unison. Although most move in majestic unity, a few others are "wanderers"—appearing now with one group of stars and a week later somewhere else. The majority were called "fixed stars," the wanderers were called "planets."

During the late Middle Ages, and especially in the Renaissance, beautiful brass models known as orreries were made to show the relative positions and movements of the sun, planets, and moon as they circled the earth. As the center of the universe, the earth was a sphere in the center of the orrery. The other celestial bodies were positioned on rings of metal, each moving by clockwork at its own rate. The fixed stars required a simple solution—they could be considered stuck in an outermost shell, also moved by clockwork.

The problem with orreries—and with the theories of the cosmos then prevailing—was that they had to become successively more complex as more became known. Careful observations of the movements of the stars and planets greatly complicated the hypotheses used to account for those movements. This growing complexity stimulated some of the leading astronomers of the 16th and 17th centuries, including Copernicus, Kepler, and Galileo, to make even more precise observations of the movements of the heavenly bodies. Astronomers used these measurements to demonstrate that the age-old human explanations of the heavens were incomplete. In the process, they replaced a complex and confusing explanation with a simple one: the sun, rather than the earth, is at the center of a "solar system," and the earth revolves around it. That simple step—a bold departure from past thinking due mainly to the insights of Copernicus (1473 to 1543)—dramatically changed the picture of the then known universe.

This concept of heliocentricism initially ran counter to the positions of religious authorities. The view of Christianity over most of its history, based on a literal interpretation of the Bible, was that the earth is the center of the universe around which the celestial bodies revolve. Copernicus dedicated his book describing the theory of heliocentricism, De revolutionibus orbium coelestium, to Pope Paul III and promptly died. That saved him the troubles that were to beset Galileo (1564 to 1642), whose astronomical observations confirmed the views of Copernicus. Galileo was told to abandon his beliefs, and he later was tried by the Inquisition and sentenced to the equivalent of house arrest. The Church held that his views were dangerous to faith.

Continued study and ever more careful measurements of the movements of the planets and sun continued to support the heliocentric hypothesis. Then, in the latter half of the 17th century, Isaac Newton (1642 to 1727) showed that the force of gravity—as measured on the earth—could account for the movements of the planets given the laws of motion that Newton derived. As a result of the steady accumulation of evidence, the theological interpretation of celestial movements gave way to the naturalistic explanation, and it is now accepted that night and day are the consequences of the rotation of the earth on its axis. Today, we can see for ourselves the rotation of the earth from satellites orbiting the planet.

Like biological evolution, the theory of heliocentricism brought order and new understanding to an otherwise chaotic and confusing aspect of nature. It also had great practical applications, in that the exploration of the world by European seafarers used the more accurate understanding of celestial mechanics to assist in navigation.

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