Galileo,Venus and the Sun-Centered Theory
Scientist's Study of the Planet Boosted the Heliocentric Theory
Aug 6, 2009
In September or October 1610 Galileo began viewing Venus with his telescope in a systematic, scientific way. He observed the planet in the evening and by the middle of November he noticed that the size and shape of the planet gradually changed as the nights passed. By the end of the year, he had made drawings of these changes, which we now call the phases of Venus.
Significance of the Phases of Venus
When Galileo observed the planet in late 1610, he saw it as a small, fully illuminated sphere. As the evenings passed, it gradually became a half-sphere and then a crescent. As Venus changed from sphere to crescent, it seemed to grow larger. Galileo correctly thought that this apparent growth of the planet meant that it was coming closer to earth. And the fact that it went through a variety of phases meant that it orbited the sun.
If instead Venus were orbiting the earth, it would never be seen here on earth as a completely lit sphere because the sun would be beyond the orbit of Venus. In this position Venus would receive diminished sunlight, which would severely limit the variety of its phases. (In the heliocentric theory of the universe, the relative positions of the celestial bodies discussed here as seen from earth are the earth, the sun and Venus. In the earth-centered theory, the order is the earth, Venus and the sun.)
When Galileo observed Venus go "from gibbous (more than half illuminated) to crescent (less than half illuminated)...this was proof that Venus was shining by reflected light and that it (had ) passed from beyond (or on the far side of) the sun to closer to the sun," said the article Venus Through the Galilean Telescope. In other words, Venus orbited the sun.
Aristarchus, Ptolemy and Copernicus
Galileo's studies of Venus strengthened the sun-centered theory championed by the Polish astronomer Nicolas Copernicus (1473—1543). (Incidentally, Copernicus is said to have known that Aristarchus, the Greek who lived in the third century B. C. had said that the sun was the center of the universe). Copernicus rejected the ancient belief advocated by (Claudius) Ptolemy, the ancient astronomer and mathematician of Greek, Berber and Roman ancestry who lived in Alexandria, Egypt. Ptolemy theorized that the earth was the center of the universe and that the sun and the five other known planets orbited it.
Copernicus took up the heliocentric mantle, saying that the earth was not the center of the universe. Moreover, not only did he say that the sun was the hub of the universe but also that the earth was not stationary but moved around the sun. Since the telescope was needed to more closely observe the planets to prove these theories and since Copernicus died before its invention, it was up to Galileo to present the proof that a planet could orbit the sun.
Decades Pass Before The Heliocentric Theory Begins to Gain Acceptance
However, Galileo offered no studies to show that Mars, Jupiter and Saturn – the planets farthest from the sun – could not orbit the earth, said the website 100 Hours of Astronomy. It would be decades before the earth-centered theory began to fall out of favor, particularly helped by the scientific work of Englishman Issac Newton (1642—1727).
Sources:
Fermi, Laura and Gilberto Bernardini. Galileo and the Scientific Revolution. New York: Basic Books, Inc. 1961.
Further Reading:
Galileo's Discoveries Using Telescopes, 1609-10
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