Appendix: Galileo and Sun Spots

Although sun spots had been observed with the naked eye as far back as 28 AD, Galileo and Thomas Harriot were the first to observe them with the telescope in the last part of 1610. Galileo didn't study them in detail at this time, but Christoph Scheiner, a Jesuit Mathematician, undertook a study in March of 1611. Scheiner accepted the Aristotelian viewpoint that the sun must be a perfect body and argued that the spots were due to a satellite of the sun passing in front of it. However, Galileo later argued that the spots were on the sun itself. Galileo observed the sun spots over time by projecting the telescope image onto a piece of paper and colorng the spots with a pen. Figure 1 shows one of his drawings.

Figure 1: Galileo Drawing of Sun Spots

Galileo gave several arguments for his claim that the sun spots were on the surface of the sun.

  • The spots were irregular in shape wheras planets tended to be more circular.
  • Sometimes the spots appeared for the first time inside the sun's image and disappeared before crossing the image.
  • The spots that appear near the edge of the image were thinner and moved slowly. As they moved across the image they grew in size and moved faster. This is consistent with the spots being on the sun and the sun rotating. Near the edge the spots are moving towards you and not moving much across the image. There is no reason that an orbiting satellite would produce a spot that changes greatly in speed as it crosses the image.

For a long time Scheiner was angry with Galileo for not giving him credit for his pioneering work in this area. Eventually Scheiner agreed with Galileo and gave up the Aristotelian viewpoint for Tycho Brahe's model.