Halley's 1716 method derives the Sun's distance from transit-of-Venus timing differences observed with only telescopes and common clocks

In a 1716 paper in the Philosophical Transactions (vol. 29), "A New Method of Determining the Parallax of the Sun," Edmond Halley proposed that the transit of Venus across the solar disk could be used to fix the Sun's distance. Because Venus's parallax is "almost 4 times greater than that of the sun," observers at widely separated points on Earth see the planet trace slightly different chords across the disk, producing "very sensible differences" in the measured duration of the transit. From those duration differences, Halley argued, the solar parallax — and with it the scale of the whole solar system — could be recovered "even to a small part of a second of time," using "any other instruments than telescopes and good common clocks."

The method's defining feature is its economy: it converts an angular-distance measurement into a timing measurement, so precision rests on accurate clocks rather than accurate angle-reading. That is why the 18th-century transit expeditions carried precision regulator clocks as core scientific instruments — for example the John Shelton regulators the Royal Society dispatched for the 3 June 1769 transit, the object at the head of this note's hop chain.

Halley never observed a transit under his own protocol. The transits fall in pairs separated by ~8 years (1761 and 1769), and Halley died in 1742; he designed the full observational program decades in advance for others to execute after his death — an early instance of internationally coordinated "big science" before the term existed. The same "timing from separated observers" logic is carried into 21st-century exoplanet detection in claim-scharf-2007-names-halley-transit-venus-as-exoplanet-transit-parallax-ancestor.