Sun rotated with a period of about 1 month. The behaviour of these spots can be used to make a very convincing case for the motion of the Earth, though Galileo's argument in the Dialogue is uncharacteristically obscure. Sunspots are concentrated near the solar equator, and Figure 7.3 shows the typical motion of a sunspot as seen from the Earth at four equally spaced times of the year. When observing a sunspot from the Earth, the daily rotation of the Earth plays a negligible role (except, of course, that we can only make observations during the day!) and the observed motions are consistent with a Sun that rotates on an

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