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rate determination of the sizes of planets. This removed one of the objections that Tycho Brahe had proposed concerning the vast distance to the fixed stars thatwas implied in On the Revolutions. Based on naked-eye observations, Tycho had calculated the sizes of stars that would be implied by this great distance, and found that they would have to have diameters that were of the same order of magnitude as the radius of the orbit of the Earth, something Tycho thought was ridiculous. Telescopic observations showed, however, that the apparent

10 Another person to look at the heavens through a telescope around this time, maybe even earlier than Galileo, was Thomas Harriot (see, for example, Montgomery (1999), pp. 106-13). Harriot, who was one of the leading mathematicians in Elizabethan England, was responsible for the first recorded astronomical observation in North America, made during the attempted colonization of Virginia in 1585 (Yeomans (1977)).

A century before Galileo, Leonardo da Vinci had written: 'Construct glasses to see the moon magnified' (Montgomery (1999), p. 97).

Galileo was not the first to put forward this explanation (Ashbrook (1984), p. 198).

For example, the traditional value for the angular diameter of Venus at apogee was about 3', but Galileo determined the much more accurate value of 1 (see van Helden (1989)).

diameters of the fixed stars were much smaller than had been thought previously. On the other hand, the advent of the telescope did not result in anybody being able to measure any stellar parallax, and so Copernicans had to keep revising upwards the distance to the fixed stars.

By early 1610, Galileo had built a telescope that could magnify about 20 times, and he turned his attention to Jupiter and saw four small star-like objects close to the planet. Observations over the next few nights showed that while these 'stars' moved with respect to each other and to Jupiter, they participated in Jupiter's motion with respect to the fixed stars, and he realized that they must be satellites of the planet. This discovery further undermined Aristotelianism, as it implied the existence of another centre of rotation. It also aided the Copernican cause because the Earth ceased to be the only planet with a satellite, thus lessening the force of any argument that was based on the special nature of the Earth. While Galileo's discoveries strengthened his Copernican convictions, he still chose, for the time being, not to publicize his beliefs.

He did, however, publish the results of his telescopic observations and he did this very quickly to ensure priority - within 2 months he had written up

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