Transit of Venus through history

By Natsumi Penberthy 1 June 2012
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Three hundred years of adventures and misadventures to see the Transit of Venus helped us measure the heavens.

TODAY, THE MOVENMENT OF a small spec in front of the sun that is the transit of a planet in front of the big orb is being measured by incredibly precise instruments. The measurements help map distant galaxies in the hope of determining if there are planets with the right conditions to support life.

But before the turn to extra-terrestrials, the Transit of Venus – the journey of Venus across the face of the Sun as visible from Earth – was one of astronomy’s holiest grails, its timing used to calculate Earth’s distance from the Sun and to map our solar system.

Venus’ Transit is only visible in a pattern of 8 years, 121.5 years, 8 years, 105.5 years repeated again and again – and has only happened seven times since the telescope made it possible to see the black spec of Venus crossing the Sun. So, each event was much anticipated and brought astronomers tantalisingly closer to truly understanding the dimensions of the universe. 

Predicting the Transit of Venus

The history of observing the Transit of Venus, however, has a slightly tragic tone littered with missed moments and untimely deaths. The first of these was only 21 years after Galileo Galilei made viewing the Transit possible by inventing the telescope in 1610.

Johannes Kepler, a German mathematician, astronomer and astrologer, first predicted a Transit, which occurred on 6 December in 1631 – but because it happened after sundown for most of Europe, nobody is known to have seen it.

A few years later, Englishman Jeremiah Horrocks used Kepler’s work to predict a second pass on 4 December, 1639, and became the first person to observe part of the Transit. But he died at the tender age of 22 of unknown causes before he’d published any results. His papers were eventually published more than two decades later in 1662 by members of London-based scientific body the Royal Society.

In these works, Horrocks had used his observations to guess the size of Venus (previously thought to be larger and closer to Earth), as well as to make an estimate of the distance between the Earth and the Sun, now known as the astronomical unit (AU). His figure of 95 million kilometres was far from the 150 million kilometres (93 million miles) that it is known to be today, but it was a more accurate figure than any of the time.

Adventures to see the Transit of Venus

Another 99 years would pass before the Transit would be observed again, and this time former British Astronomer Royal Edmund Halley (now famed for his prediction of the eponymous comet) would call for expeditions to document the 1761 event.

Explorers and astronomers set out around the world to find the best place to view it. These included Englishmen Charles Mason and Jeremiah Dixon, who ended up in Cape Town after a French ambush thwarted their attempt to reach Sumatra. Mason and Dixon, however, were the only party to reach the Southern Hemisphere where the Transit is best viewed. Then on the day cloudy weather hampered their observations and they failed to provide a reliable astronomical unit. 

Tthe focus then shifted to the 1769 event, and Britain mounted an ambitious program, and parties were dispatched to the best known locations to see the event: North Cape in Norway, Hudson Bay in Canada, and Tahiti in the Pacific.

The most famous voyage was that of Captain James Cook in 1769 to Tahiti on HMS Endeavour. Captain Cook and astronomer Charles Green, used the most advanced equipment of the day – a magnetic compass, a device called an octant (precursor to the sextant) – but there was uncertainty about the time when Venus made first contact with the Sun due to something known as the black-drop effect (the ‘black-drop’ effect is the tendency for two black dots to appear to merge when placed close together, which occurs when Venus reaches the edge of the Sun’s outline).

All expeditions successfully recorded the Transit, but on the journey home more than half of them fell ill and died. Back in England, Cook wrote up the Transit observations for the Philosophical Transactions of the Royal Society, but for some inexplicable reason only used data obtained one of his four observation sites.

The Transit of Venus: a useful measure of the universe?

It was finally left to Oxford astronomer, Professor Thomas Hornsby, to produce a figure from the Tahitian data and observations made at three other sites (1763-1771). Different results obtained by other researchers, however, fuelled controversy over the effectiveness of the Transit of Venus as a valid means of determining the astronomical unit. In fact, the figure obtained by Hornsby was remarkably similar to the currently accepted value.

Further expeditions were organised in the nineteenth century to locations as far afield as Honolulu and Kerguelen in the Indian Ocean in 1874 and the West Indies, Australia and Madagascar in 1882. Each time astronomers anticipated inching closer to an accurate figure, and the events remained a focus of public fascination.

In 1874 Warwick Examiner and Times in Queensland reported that: “An error of 3,000,000 miles is believed to exist, and this error is thought to be due to a mistake in the observations made in North Lapland. This has caused astronomers and scientists of all nations look forward to the next Transit of Venus as the opportunity to determine accurately the sun’s distance from the Earth.”

In the end, although Canadian-American Simon Newcomb did try to use the Transit of Venus to produce a figure, he eventually harnessed an equation using the speed of light to come up with a distance that was accepted as the astronomical unit and our distance from the sun from 1896-1964, and has been only slightly refined since.

The next Transit can be seen on June 6, 2012. For more information see Astronomer in Charge of the AAO Fred Watson’s tips on how to view the Transit of Venus event.