One brisk wintry day in 1639, a young man named Jeremiah Horrocks — barely 20 — set up a telescope in his quarters near Preston, England and focused an indirect image of the sun onto a small card. Lack of finances had cut short his brief academic career at Cambridge, despite his intelligence, but he still had a passion for the stars, bolstered by reading the works of Johannes Kepler and Tycho Brahe.

Kepler’s writings were the reason for Horrocks’ homemade experiment. In 1627, Kepler was the first scientist to predict a transit of Venus — a very rare astronomical event during which the planet Venus passes directly between the Sun and Earth, a black dot obscuring part of of the solar disk. It’s similar to a solar eclipse by the moon — we witnessed an annular eclipse just last week! — except transits of Venus occur every 243 years. Well, it’s a bit more complicated than that, because they usually happen in pairs: the pattern is eight years, 105.5 years, eight years, and 120.5 years.