A Celestial Event.

In 1716 Englishman Edmund Halley (for whom the comet is named) described a procedure for using observations of the "transit" of the planet Venus across the face of the Sun to determine the solar parallax—an angle calculated from two positions on Earth—and hence the distance from the Earth to the Sun. Astronomers were eager to know this distance because it would enable them to calculate more accurately the distances of the other known planets from the Sun. They knew the relative distances of the planets from one another, but they could not determine accurately the solar system's size without learning the Earth's distance from the Sun. And there was a problem: transits occurred rarely; before 1761, the last transit of Venus had been in 1639. The transit was expected again in 1761 and 1769, but after that not for another 105 years. Clearly...