Mercury on the Sun: Gassendi Tests Kepler's Prediction

On November 7, 1631, 395 years ago, Pierre Gassendi in Paris became the first person known to watch a planet cross the face of the Sun. Johannes Kepler had used his laws of planetary motion to predict that Mercury would pass in front of the Sun, but he died before the day came. Gassendi projected the Sun through a telescope onto paper and saw a tiny dark dot, so much smaller than expected that he first took it for a sunspot. The theory told him when to look, and his looking showed the theory where to improve. Theory and observation make each other stronger.

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