The sun is much bigger than the planets

(written by lawrence krubner, however indented passages are often quotes). You can contact lawrence at:, or follow me on Twitter.

This is a great story about the moment when we humans first learned that the planets were small, and the sun was very big:

Gassendi’s most important observation was made when Mercury passed in front of the sun on November 7, 1631. Only Mercury and Venus can be observed from the earth during their solar transit. Similar passages took place in the months of May and November, around the 7th and the 9th of the month. For Mercury, one could expect this phenomenon to repeat every 7, 13, or 46 years. It was a rare observation.

The transit of Mercury on November 7, 1631 was the first to have been scientifically predicted and observed. It was difficult to know precisely when the event would take place. The tables astronomers possessed at the beginning of the seventeenth century were quite unreliable. In the ephemerides that Kepler had calculated for the years 1629 to 1631, on the basis of the Rudolphine Tables of 1627, he had added a note, titled Admonitio, indicating that Mercury would transit the sun on November 7, 1631. Following Kepler’s death in 1630, his son Jacob Bartsch had the note republished as an offprint. Gassendi had read Kepler’s note, but Peiresc had not. In a long letter written on July 9, 1631, Gassendi filled in the missing details. In Paris at the time, Gassendi did not expect to see much from a northern latitude. Trusting in Peiresc and his colleagues, he was counting on clearer skies in Provence.

November arrived, and with it the time when Mercury was to traverse the solar disk. Neither Peiresc nor Gaultier, nor any other member of the Provençal group saw anything. In fact, it was Gassendi in Paris who, alone in France, made the observation!

Gassendi took great pains with his observation. Since he could not look directly at the planet, he had the idea of projecting its image onto a sheet of paper. On November 5, he began his vigil, despite steady rain throughout the day. On the following day, he saw the sun briefly through a downpour. But on the 7th, the sun could be seen intermittently. Mercury was already visible on its surface, though Gassendi had difficulties in recognizing it due to its small size. Directly thereafter, he published an account of his observations in a pamphlet entitled Mercurius in sole visus (Mercury Seen in the Sun).30

Gassendi may have been shocked by the relatively tiny size of Mercury, but his observations confirmed Galileo’s predictions that the planets were much smaller than they seemed, and, indeed, smaller than astronomers had previously thought. Above all, Gassendi’s observations reinforced the authority of Kepler’s Rudolphine Tables, and in a more general sense, confirmed the validity of the new astronomy. It also obliged astronomers to reexamine the question of stellar and planetary diameters, and thus their distance with respect to the earth and the sun.

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