A tiny dot crosses the sun

On this day in 1631, the French astronomer Pierre Gassendi shone the sun's image through a telescope onto a sheet of paper and saw a small black dot creep across it. It was the planet Mercury, passing between the Earth and the sun, just as Johannes Kepler had predicted.

Why it mattered

It was the first time anyone had watched a planet cross in front of the sun. It also showed that Kepler's new way of working out where the planets would be was right.