The sun-centered universe

There were few noteworthy advances in astronomy between the seventh and fifteenth centuries. The old philosophical ideas of Aristotle became settled into an academic tradition akin to religious belief. Science and religion were so closely linked that astronomers and other scientists became circumscribed by theology, and the earth remained firmly at the center of the universe.
In the sixteenth century, the revolution in astronomy that was to lead to the scientific era of the eighteenth century was instigated by Nicolaus Copernicus, whose interest in astronomy was so keen and perceptive that his name has become immortalized through his work and writings. He postulated—in spite of fierce opposition—that the earth does not occupy the central position in the universe, butthat in common with the other planets, it orbits around the sun. This heliocentric model was regarded as heretical and morally wrong by the ecclesiastical authorities. It was because of this opposition that the few astronomers who had begun to question the old geocentric theory of the universe did so rather secretly, out of fear of being imprisoned for their unorthodox and, therefore, unacceptable views.
Brahe, Galileo, and Kepler
At that time, astrology continued to be indulged in by most ordinary people. The result was that the accurate forecasting of planetary positions and occultations became a matter of great practical importance and personal prestige. In calculating this astronomical information, however, it was found that the system of epicycles, devised to account for planetary motion, grew to be too complicated and became full of accumulated errors. This confusion acted as a spur to Copernicus and encouraged him to work on his heliocentric ideas, which he persevered with despite threats of persecution.
Following the death of Copernicus in 1543, three notable astronomers entered the public eye: Tycho Brahe, Galileo Galilei, and Johannes Kepler. Between 1560 and 1640, they made a great impact on the progress of astronomy. Brahe was a tireless observer and one of the most methodical astronomers in history. Like Kepler, he was a contemporary of Galileo, but he did not have the advantage of Galileo’s telescope and made a vast number of naked-eye observations using only a giant quadrant. He compiled numerous charts of the planets and discovered a supernova in 1572. He could not, however, reconcile his observations with his orthodox religious views, which still required belief in a geocentric universe.
Galileo and the telescope
Galileo will always be remembered as the person who, in 1609, first made serious use of the telescope to study the heavenly bodies. The first telescope, however, was probably made by Lippershey, in Holland, in 1608, after experiments with spectacle lenses. Galileo astonished the scientific world by his observations of the moon, with the telescope making the lunar mountains and craters clearly visible on the surface.
The telescope also revealed thousands of stars that had not been visible to the naked eye. It showed the planets as definite disks, and made visible the phases of Venus. It revealed for the first time the fact that the sun is spotted with blemishes (sunspots), which move across its surface. Among the most spectacular new sights were the rings of Saturn and the four main moons orbiting around Jupiter.
The most significant result of these epoch-making discoveries was the fact that they exposed the misconceptions of the old geocentric model of the universe. Despite being compelled by the church to denounce his own ideas as false, Galileo, nevertheless, remained an ardent supporter of Copernican theories. Against all opposition, there was a growing acceptance of the new heliocentric model of the universe, with the newly termed solar system at its heart.
Kepler, using information recorded by Brahe, demonstrated that the planets (including Earth) did not move in circles, as Ptolemy’s followers declared, but in ellipses. He also showed that a planet’s speed varies as it orbits around the sun. Summarized in Kepler’s laws of planetary motion, his calculations further discredited the old geocentric model and paved the way for a scientific explanation of the planets’ orbits. They also established the principles that allowed Isaac Newton to develop his theories of universal gravitation later that century.

Copernicus championed the theory that the sun lies at the center of the universe (which we now call the solar system). He assigned circular orbits to the planets in the order (out from the sunl Mercury, Venus, Earth, Mars, Jupiter, and Saturn. He acknowledged that they do not, however, move in perfect circles and devised a system of epicycles to account for this. Each planet was assumed to move in a circle superimposed on its large circular orbit round the sun.
Tycho Brahe made most of his thousands of astronomical observations using a large quadrant. Similar instruments (but smaller) became adapted for navigational purposes, such as this example equipped with filters for taking sightings of the sun.