IN CONTEXT
Astronomy
1543 Nicolaus Copernicus makes the first complete argument for a Sun-centred (heliocentric) Universe.
1609 Johannes Kepler proposes a system of elliptical orbits – the first complete description of planetary motion.
1663 Scottish mathematician James Gregory devises a way to measure the exact distance from Earth to the Sun using observations of the transits of Venus in 1631 and 1639.
1769 British explorer Captain James Cook observes and records the transit of Venus in Tahiti in the South Pacific.
2012 Astronomers observe the last transit of Venus of the 21st century.
Planetary transits offered an opportunity to test the first of Johannes Kepler’s three laws of planetary motion – that the planets orbit the Sun in an elliptical path. The brief passages by Venus and Mercury across the disc of the Sun – at the times predicted by Kepler’s Rudolphine Tables – would reveal whether the underlying theory was correct.
The first test – a 1631 transit of Mercury observed by French astronomer Pierre Gassendi – proved encouraging. However, his attempt to spot the transit of Venus a month later failed owing to inaccuracies in Kepler’s figures. These same figures predicted a “near miss” for Venus and the Sun in 1639, but English astronomer Jeremiah Horrocks calculated that a transit would in fact occur.
At sunrise on 4 December 1639, Horrocks set up his best telescope, focusing the Sun’s disc onto a piece of card. Around 3.15pm, the clouds cleared, revealing a “spot of unusual magnitude” – Venus – edging across the Sun. While Horrocks marked its progress on the card, timing each interval, a friend measured the transit in another location. By using the two sets of measurements from the different viewpoints, and by recalculating the diameter of Venus relative to the Sun, Horrocks could then estimate Earth’s distance from the Sun more accurately than ever before.
"I received my first intimation of the remarkable conjunction of Venus and the Sun…it induced me, in expectation of so grand a spectacle, to observe with increased attention."
Jeremiah Horrocks
See also: Nicolaus Copernicus • Johannes Kepler