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The difficult task of breaking with tradition and accepting the Sun as the center of the universe began with a Polish priest and astronomer named Nicolaus Copernicus (1473–1543). He decided that the only way to make sense of the planetary orbits was to relegate Earth to the status of a planet that orbited the Sun. The movement of the stars across the sky was then explained by the rotation of the spherical Earth, while the calendar of seasons and changing constellations in the heavens were accounted for by its year‐long journey around the Sun.

Copernicus' most significant work, called De Revolutionibus Orbium Celestium (Concerning the Revolutions of the Celestial Spheres), was published shortly before his death. Curiously, this did not provoke a violent reaction by the establishment of the day, nor did it immediately lead to any major upheaval in scientific thought. Lacking enough evidence to swing the argument one way or the other, the great minds of the day were faced with an impasse.

Half a century passed before the interventions of two great scholars swung the argument in favor of Copernicus' heliocentric theory. The first breakthrough was made in 1609 by a young German named Johannes Kepler. By one of those strange twists of irony, Kepler was a pupil of Tycho Brahe, one of the leading opponents of the Copernican order. Given the unenviable task of finding an explanation for the retrograde motion of Mars (see Figure 1.3), Kepler was able to draw upon the excellent observational data recorded by his employer.

Brahe died in 1601, but Kepler continued to laboriously examine the problem before finally arriving at his eureka moment. The planetary orbits, he declared, were not circles but ellipses (regular oval shapes).⁴ Within a short time, Kepler was able to draw up the first two laws of planetary motion (see Box 1.2). His third, and probably most important law, followed in 1619.

As a result, the relative distance of each planet from the Sun could be calculated accurately. Saturn, the most remote planet known at the time, turned out to be nearly 10 times further from the Sun than Earth. Since the actual distances remained unknown, the standard unit of measurement became the astronomical unit, so Saturn's distance from the Sun was about 10 astronomical units or 10 AU.

Figure 1.5 In January 1610, Galileo Galilei used his simple refracting telescope to discover three “stars” aligned on either side of Jupiter. Over a period of several weeks, a fourth “star” appeared. As they shifted positions, Galileo correctly deduced that these were satellites. Occ. is the Latin abbreviation for “west” and Ori. stands for “east.”

(NASA)

In the same year that Kepler discovered elliptical orbits, an Italian scientist named Galileo Galilei made a simple refracting telescope, comprising two lenses at either end of a narrow tube, and began to study the heavens. Within a short time, despite the small magnification offered by his “optic tube,” he had obtained visual evidence to support the theories of Copernicus and Kepler. Galileo became the first person in history to see the phases of Venus caused by its movement around the Sun. He also observed mountains and craters on the Moon, and saw the planets as disks, rather than points of light.

Most significant of all was his discovery of four star‐like objects close to Jupiter (Figure 1.5). By watching their daily motions, he was able to calculate their orbital periods and show that they were Jovian moons (see Chapter 7). The discovery of the first planetary satellites (other than the Moon) supported theories that Earth was not at the center of the universe and confirmed that everything did not revolve around our world.

Galileo's discoveries caused a sensation, although the leaders of the Roman Catholic Church obstinately continued to support a geocentric universe. In 1633, Galileo was brought before the Inquisition and forced to recant under threat of torture.