and he had soon (by autumn 1609) built himself a telescope with a magnification factor of about 8. He did not (as is sometimes said) invent the telescope,

^ Quoted from Pannekoek (1961). 8 Quoted from Pannekoek (1961). Galileo's preferred name was a perspicillum, and Kepler approved, so it is perhaps surprising that this did not last; for the explanation, see Rosen (1947).

and the original instruments he used probably were rather less effective than a modern pair of binoculars, but he was the first person to use such a device to make a systematic survey of the the heavens, with devastating consequences for man's view of the Universe.10

Not unnaturally, Galileo's first target was the Moon,11 and when he looked at it he found it was not perfectly smooth and spherical as a heavenly body should be, according to Aristotle, but instead had shadows that moved as the angle of the Sun varied. He noticed that the border between the light and dark parts of the Moon was not perfectly straight. Near this border, in the dark portion, he observed light patches that grew larger and gradually merged with the light part of the surface as the Moon got fuller. He concluded that these were the tops of mountain ranges. Thus, the Moon was seen to be like the Earth in many ways, and not some pure crystalline object. He explained also the appearance of a slight illumination in the dark part of a thin crescent moon (sometimes referred to as 'the old moon in the new moon's lap') as arising from 'Earthshine', the

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