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 dying of yellow fever in Baja California, and Guillaume Le Gentil losing eleven years, two transits, and his legal existence. We cover the black drop effect that spoiled the timings, Mikhail Lomonosov’s 1761 discovery of the Venusian atmosphere and its 2012 vindication with antique refractors, and how the 2004 and 2012 transits became rehearsals for detecting and analyzing exoplanets, including Hubble bouncing sunlight off the moon.

• Why Venus laps Earth every 584 days yet transits only at the orbital nodes
• How Horrocks reached a record-setting distance estimate with the wrong premise
• The human cost of the eighteenth-century expeditions that measured the astronomical unit
• What causes the black drop effect and why it frustrated precise timing
• How a spotty sun in 2012 helped astronomers filter noise when hunting distant planets