Spots on the Sun: Galileo and Scheiner Disagree

Spots, Letters and Presses, 1610–1613

1By 1600, the Renaissance had spread from the city-states of Italy across Europe. Scholars studied ancient Greek and Roman books, and printing presses in hundreds of towns turned out copies of old and new works. Many scholars still accepted Aristotle's ancient teaching that the heavens were perfect and never changed.

2In 1609 and 1610, Galileo Galilei of Italy turned a telescope to the sky. Within a year or two, he and several other observers noticed dark spots on the face of the Sun. Looking straight at the Sun can blind a person, so Galileo's student Benedetto Castelli found a safer way: he let the telescope throw the Sun's image onto a sheet of paper, where an observer could trace the spots day after day.

3Christoph Scheiner, a Jesuit priest who taught mathematics at the University of Ingolstadt in Bavaria, watched the spots too. He described them in three letters in Latin, the language of European scholars, to Mark Welser, a banker and scholar in Augsburg. Welser had the letters printed in January 1612. Scheiner's superiors asked him not to use his own name, so he signed as "Apelles hiding behind the picture," after a painter of ancient Greece who was said to hide behind his paintings to hear what viewers said.

4Galileo read the printed letters and answered with three letters of his own to Welser. In 1613 the Lincean Academy, a society of scientists in Rome, printed them as a book. Galileo wrote in Italian, not Latin, so merchants, artists and other readers who had never studied Latin could follow his argument.

5Art played a part as well. Galileo had studied drawing in Florence, and in 1610 he used the shading of light and shadow that Renaissance painters had mastered to show mountains and craters on the Moon. His friend, the painter Lodovico Cigoli, painted a Moon with craters beneath the feet of the Virgin Mary in the dome of a chapel in Rome, finished in 1612.

6The quarrel did not end quickly. Scheiner kept watching the Sun for years. In a long book on sunspots published in 1630, he placed the spots on the Sun itself.

Account A: Adapted from Scheiner's letters to Welser, printed in Augsburg, 1612

Source note: Scheiner wrote these letters in Latin under a false name. They were printed in January 1612, before Galileo's answer.

For several months I have watched dark spots on the Sun through a telescope fitted with colored glass. At first I suspected a flaw in my lens or my eye, but the spots appeared through different telescopes and were seen by other observers. They cross the Sun from one edge to the other in about two weeks.

I cannot believe the spots are on the Sun itself. The Sun is the brightest and purest of all bodies, and it is hard to accept that it carries dark stains. Also, if the spots were fixed to a turning Sun, the same spots would come back into view about two weeks after they left, yet I have not seen them return. I judge that the dark spots are small bodies, perhaps stars, that circle the Sun as Venus and Mercury do. We see them only when they pass in front of it.

Account B: Adapted from Galileo's letters to Welser, printed in Rome, 1613

Source note: Galileo wrote these letters in Italian in 1612, after reading Scheiner's printed letters.

The dark spots on the Sun are not stars. Stars keep their shapes, but these spots appear and vanish, grow and shrink, and join together and break apart, much like clouds. Some last only a day or two, others a month or more.

Using my student Castelli's method of throwing the Sun's image onto paper, I traced the spots day after day. Near the edge of the Sun the spots look thin and move slowly; near the middle they look wide and move faster. This is just what we would see of marks on a turning ball. So the spots are on the Sun's surface or very close to it, and the Sun turns about once a month. That the same spots do not return is no surprise, since they change and dissolve. Those who say the heavens never change must give up that belief.