Sunspots and Transits

November 11, 2019

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Photographer: John Stetson 
Summary Author: John Stetson 

In 1612, Galileo Galilei published a pamphlet, “Letters on Sunspots,” in which he provided evidence that sunspots were on the Sun and not transiting planets. How? He observed and documented that sunspots appeared to move more slowly at the Sun’s limb; this was because of foreshortening. Of course, a transiting object would be expected to maintain its size/shape and rate of speed.

Johannes Kepler had observed a sunspot, and he thought it was a Transit of Mercury. Christoph Scheiner, a German sunspot observer, believed that sunspots were undiscovered planets transiting our Sun. That is to say that there was little consensus at the time about the nature of sunspots.

Foreshortening at our Sun’s limb can be observed in the above series of sunspot photos that were taken over three days from Southern Maine Community College, Maine. The bottom photo shows the full disk of the Sun during the last Transit of Mercury on May 9, 2016; Mercury (at lower left) can be seen along with the sunspots. These photos were taken with a 4-inch (10 cm) aperture telescope. The difference between a sunspot and a transiting planet will be clear and quite wonderful when seen through a modern telescope.

Today there’s an opportunity to watch Mercury transit our Sun. This will be one of 13 such transits during the 21st century.