Although the planet Venus passes between the Earth and the sun roughly every 584 days, when viewed from the Earth it normally passes either above or below the Sun. Just occasionally however, it passes directly between the Earth and the Sun, appearing as a small black dot against the Sun’s bright disc. These passages over the face of the Sun are known as transits and occur in pairs eight years apart. Each pair of transits is separated by more than a century. Since the telescope was invented in the early 1600s, just eight transits of Venus have occurred. They happened in the years:
1631 & 1639
1761 & 1769
1874 & 1882
2004 & 2012
The next transit of Venus will be in December 2117.
In 1663, the Scottish mathematician James Gregory, outlined in his Optica Promota (Prop. 79. Problema.), an idea that amongst other things would allow astronomers to measure the distance of a planet from the Sun by using simultaneous observations made from two widely separated places on the Earth (click here for an english translation). This was important, because at the time of his proposal, there was a great deal of variation in the existing measurements of the Earth’s distance from the Sun.
Subsequent measurements based on observations of Mars at opposition in 1672, put the distance from the Earth to the sun at somewhere between 41 and 85 million miles – a result that left much to be desired.
In the solar system not only does Venus pass in its orbit between the Earth and the Sun, so too does the planet Mercury. In Mercury’s case, the transits are more frequent, occuring some 13 or 14 times each century.
Having timed a transit of Mercury in St Helena in 1677 with some precision, Edmond Halley, built on Gregory’s work and discussed the possibility of using transits of Mercury or Venus to determine the solar parallax and hence the distance from the Earth to the Sun. Of the two planets, he believed that the geometry of Venus’s transits was much more likely to produce accurate results and promoted the idea in papers presented to the Royal Society in 1691 and 1694.
In 1716, Halley, who became Astronomer Royal in 1720, published a third paper giving suggestions for the locations of where the transits of 1761 and 1769 should be observed. Knowing that that he would be dead by the time of the next transit occured, he urged others to take up the challenge. This they did and in the 18th and 19th centuries, huge resources were devoted to sending international expeditions to the far flung corners of the Earth.
The Royal Observatory, Greenwich played a part in all four of the British18th and 19th century transit expeditions. It had a significant organisational role in those of 1769 and a major organisational role in those of 1874.
Of the eight, transits to have occurred since the start of the 17th century, only the transit of 2004 was visible in its entirety from the UK. Although by then there were better methods for determining the solar parallax, the transit caused great interest amongst the public. A public viewing event attended by over a thousand people was organised by Graham Dolan at the old Observatory site in Greenwich. Amazingly, the weather was glorious and the transit was visible from beginning to end.
1761 transit expeditions
1769 transit expeditions
1882 transit expeditions