A transit of Venus, when the planet crosses the face of the sun as a tiny black dot, will not happen again until 2117. This episode explains the strange 243-year rhythm of pairs eight years apart separated by gaps of 121.5 and 105.5 years, why Venus's 3.39-degree orbital tilt makes most conjunctions miss, and how the near resonance of eight Earth years with thirteen Venus years creates the pattern. We follow Kepler's predictions, Jeremiah Horrocks projecting the sun onto paper in a 30-minute break in the English clouds in 1639, and his estimate of 59.4 million miles to the sun built on flawed mystical assumptions.

Edmond Halley's 1716 proposal turned the transit into a global triangulation experiment for the astronomical unit, launching the 1761 and 1769 expeditions: James Cook at Point Venus in Tahiti, Jean-Baptiste Chappe d'Auteroche dyin ...