Transit of Venus

On the basis of his calculations, Kepler predicted that a transit of Venus would occur on 6 December 1631. However, the phenomenon was not scientifically recorded until 24 November 1639, after Jeremiah Horrocks (1619-1641), an English cleric and astronomer, reworked Kepler's calculations and concluded that transits of Venus occurred in pairs spaced eight years apart roughly every 120 years.

It takes about 7 hours for Venus to pass across the Sun. Horrocks was able to view the transit for only about 30 minutes, but in that time he was able to take a series of readings from which he could calculate the Solar Parallax (the distance between Earth and the Sun) with greater accuracy than had previously been achieved. Horrock calculated the distance was about 56,000,000 (90,123,000 kilometres).

In 1663, James Gregory, a Scots mathematician and astronomer, suggested that a more accurate measurement of the Solar Parallax could be gained from observations of the transit of Venus made from various widely separate geographical locations.

Edmond Halley (1656-1742), a leading English astronomer particularly interested in improving navigation techniques, came to think likewise.

Halley initially thought observing the more frequently occurring phenomenon of the Transit of Mercury was a practical means of determining the Solar Parallax. This he did in October 1677 from the south Atlantic island of St. Helena, only to find on returning to England that cloudy weather had prevented comparative observations being recorded at Greenwich and other European observatories.

However, Halley came to see the transit of Mercury was not an accurate means of calculating the Solar Parallax because of the planet's size and relative proximity to the Sun. No matter how far apart two or more sets of observation were taken, the difference in readings would be very small and thus subject to a high degree of observational error.

In 1716 Halley published a greatly refined version of a paper originally read before the Royal Society in 1691, entitled 'a new Method of determining the Parallax of the Sun, or his Distance from the Earth'. In the paper he championed the idea of a scientists of various nations observing the 1761 and 1769 transits of Venus in as many parts of the world as possible. This, he argued, would result in a 'certain and adequate solution of the noblest, and otherwise most difficult problem' of accurately establishing the distance between the Earth and the Sun.

Halley's work was largely the inspiration for one of the aims of James Cook's first voyage being the observation of the Transit of Venus from the newly encountered island of Tahiti.

During the observations of the 1761 and 1767 transits, the silhouette of Venus appeared with a curved surface as it passed across the Sun. This phenomenon, which came to be known as the 'Black Drop' effect, introduced inaccuracies into calculations of the amount of time it took the planet to traverse the Sun. But despite 'Black Drop' hindering observational accuracy, the measurements taken in 1761 and 1769 allowed the French astronomer Joseph Jerôme Lalande (1732-1807) to estimate the Solar Parallax to be 153,000,000 '1 kilometres in 1771.

On the basis of modern radar-based measurements of planets, the distance between the Earth and the Sun is estimated to be 149,597,870.691'0.030 kilometres.

• O'Conner and E.F. Robertson, Longitude and the Acad'mie Royale, 1997, http://www-gap.dcs.st-and.ac.uk/~history/HistTopics/Longitude1.html. [ Details ]