Ever since the time of Galileo, the history of astronomy has been punctuated by a series of “great debates”—disputes between competing theories of the organisation of the universe which observation and experiment using available technology are not yet able to resolve one way or another. In Galileo’s time, the great debate was between the Ptolemaic model, which placed the Earth at the centre of the solar system (and universe) and the competing Copernican model which had the planets all revolving around the Sun. Both models worked about as well in predicting astronomical phenomena such as eclipses and the motion of planets, and no observation made so far had been able to distinguish them.
Then, in 1610, Galileo turned his primitive telescope to the sky and observed the bright planets Venus and Jupiter. He found Venus to exhibit phases, just like the Moon, which changed over time. This would not happen in the Ptolemaic system, but is precisely what would be expected in the Copernican model—where Venus circled the Sun in an orbit inside that of Earth. Turning to Jupiter, he found it to be surrounded by four bright satellites (now called the Galilean moons) which orbited the giant planet. This further falsified Ptolemy’s model, in which the Earth was the sole source of attraction around which all celestial bodies revolved. Since anybody could build their own telescope and confirm these observations, this effectively resolved the first great debate in favour of the Copernican heliocentric model, although some hold-outs in positions of authority resisted its dethroning of the Earth as the centre of the universe.... [Read More]