By guest blogger Peter Bond

The first person to predict a transit of Venus was the German mathematician and astronomer, Johannes Kepler, who calculated that one would take place on 6 December 1631. Unfortunately, the transit was not visible from Europe and there is no record of anyone seeing it.

Jeremiah Horrocks, a young English astronomer, studied Kepler’s planetary tables and discovered, only a month in advance, that a previously unrecognised transit of Venus would occur on 24 November 1639. (Note: two calendars were used at that time. According to the Gregorian calendar, which added 10 days to the older Julian calendar, the date of the 1639 transit took place on 4 December).

Horrocks observed part of the transit from his home at Much Hoole, near Preston. His friend, William Crabtree, also saw it from Manchester, having been alerted by Horrocks. As far as is known, they were the only people to witness the event.

By the mid-17th century, the relative distances of the planets from the Sun were well known. According to Kepler’s third law of planetary motion, if Earth was at one astronomical unit (1 AU), then Venus orbited the Sun at 0.72 AU, Mars at 1.52 AU, and so on.

However, the actual distance of the Sun and planets from Earth was not known with any degree of accuracy. The best estimate of the time was that the Sun-Earth distance was 137.7 million km. (The actual figure is about 150 million km.)

Edmond Halley (of comet fame) suggested that observations of transits of Venus could, in principle, be used to find out how far the Sun is from Earth.

He suggested that observers in widely separated locations would carefully measure the time that either side of Venus’s disc first ‘touched’ one edge of the Sun, and then exited on the opposite limb of the Sun. This sequence comprised four separate events, called contacts. Each of these had to be timed with split-second accuracy at the far-flung observing sites.

Once the precise contact times were determined, a complex mathematical calculation was performed to determine the observed path of Venus across the Sun from each location. The angular difference between these paths resulted from a parallax shift. Corrections had to be made for the slight differences in contact times caused by east-west differences in longitude.

Halley died in 1742, but hundreds of scientists travelled across the world to try out his method during the transits of 1761 and 1769. Captain James Cook’s expedition to Tahiti in 1769 is one of the most famous expeditions, part of a voyage in which he discovered the east coast of Australia.

One of the unluckiest expeditions was led by Guillaume le Gentil, who set out for Pondicherry, a French colony in India. As his ship was nearing India, he learned that the British had occupied Pondicherry, so he returned to Mauritius. Unable to make proper observations of the 1761 transit, he decided to stay on the island until the next transit, eight years later.

Unfortunately, when the long-awaited day arrived, the Sun was hidden behind a blanket of cloud. When he finally arrived back in Paris in October 1771, he had been declared legally dead, his wife had remarried and all of his possessions had been divided up among his relatives!

Continue reading in part 2: the ‘black drop’ problem…