In the year 1611, Galileo, with the aid of his telescope discovered that there were dark spots upon the Sun, and that those spots moved across the surface of the Sun from day to day. This was a very interesting and important discovery, though it had terrible consequences for Galileo. There was nothing to be found about sun-spots in the writings of the great Greek thinker Aristotle, and so the people who were powerful in the time of Galileo said that what he called sun-spots were due to faults in his telescope or in his eyes. Worse than this, the discovery of sun-spots was regarded as an insult to the Sun.
But since the time of Galileo we have learned to regard sun-spots as one of the most interesting things about the Sun. When large ones are present, anyone can see them by looking at the Sun through a smoked glass. If we watch them from day to day, we find, as Galileo himself found nearly three hundred years ago, that they very often travel right across the face of the Sun, from side to side, then disappear, and then appear again on the other side.
We notice also that as they reach the side of the Sun they seem to get narrower, as if we were looking at them sideways. This can only mean that the Sun spins round upon itself, and we now know that it takes rather more than twenty-five of our days to do this – that is to say, that while the Sun spins round once, the Earth spins round more than twenty-five times. The Earth’s spinning makes day and night for us, but of course, it makes no difference to the brightness of any part of the Sun, which is the source of our day.