A history of the transits of Venus

venus-transit-01-800The transit of Venus across the sun is a rare celestial alignment that, like London buses, then comes along twice in quick succession. Venus orbits the sun roughly every 225 days and its path occasional overlaps Earth’s. The transit lasts several hours with Venus visible from earth during the day as a black dot travelling across the sun’s surface. This happens roughly twice every century coming (usually) in pairs eight years apart. We’ve had it twice in the 21st century but few of us will be alive to see the next one in December 2117. There is a gap of 105.5 years if the previous transits were in June (like they were in 2004 and 2012) or 121.5 years if the previous transits were December (as in 1874 and 1882). Together they form a cycle of 243 years.

Venus has always been a bright giant of the night sky as the Morning or Evening Star closely monitored by astronomers for thousands of years. The Egyptians called it “Benin”, the heron who disappeared under the Nile only to rise again. Once Copernicus discovered Earth and other planets went around the Sun, there was a growing realisation the planets closer to the Sun (Mercury and Venus) could move between the Earth and Sun.

It took the mathematical genius of Kepler to figure out when that would happen and he correctly predicted the transit of Venus in 1631. Kepler did not live to see the transit, he died in 1630, but he realised transits were important not just as a celestial event but as a way of using parallax to determine the precise distance between the Earth and the Sun. Unfortunately it was night-time in Europe when the 1631 transit happened so no one saw it and no one in south Asia, where it was daytime, recorded it.

Kepler’s maths wasn’t completely accurate – he failed to realise the transits come in pairs and he missed the fact another was due eight years later in December 1639. That realisation fell to young English astronomer Jeremiah Horrocks barely months before the 1639 event. Horrocks was the first ever written witness to a transit of Venus on an unusually clear winter’s day. Horrocks used the transit to estimate the distance between the Earth and the Sun as 191 million kilometres, about 40 million too high, but the most accurate distance yet recorded.

By the time of the next transit in June 1761, scientists had a much better what to look for and where and when to look for it. Almost 200 astronomers trained telescopes on the transit from 100 locations with the best views in Asia. The British Royal Society sent two expeditions, despite the distraction of the Seven Years War with France. One expedition to St Helena in the south Atlantic was a flop as the day remained cloudy. The second was led by Greenwich Observatory’s Charles Mason assisted by surveyor Jeremiah Dixon. Their later work surveying the border between Maryland and Pennsylvania made them famous for the Mason-Dixon line but in 1761 Britain sent them to Sumatra to observe Venus. After their ship became involved in a skirmish with French forces, they were delayed and only made it to Cape Town. Though the transit was partially blocked by clouds they successfully timed the moment Venus moved off the Sun. Mathematicians used the data of Mason and Dixon and others across the world to work out the Earth-Sun distance to within two million kms. The war made science difficult, but it was over by the time of the next transit in June 1769.

Again, the British Government sent out an expedition with scientific goals but it also had political ends and would accidentally end up with the founding of a British colony. With the best viewing in the Pacific Ocean the Royal Society petitioned the government for funds to observe the transit noting other European nations were making similar plans. The request was an ideal excuse to fund a voyage of discovery to the South Seas and James Cook was the ideal captain to make it happen. Though not a commissioned officer he had the perfect combination of leadership, seamanship, astronomical knowledge, mapmaking and mathematics to understand the new method of finding longitude at sea.

His instructions from the Admiralty were to sail to Tahiti aboard the Endeavour to make observations on June 3, 1769. When this was completed he was to “put to Sea without Loss of Time and carry into execution the Additional Instructions contained in the inclosed Sealed Packet.” Inside the Packet were Cook’s secret instructions to find Terra australis incognita (the unknown south land) in the southern ocean. If he could not find this, he was to chart New Zealand and return to England via whichever route was most convenient.

Cook arrived in Tahiti in April 1769 via Cape Horn after an eight-month journey from Plymouth. He erected Fort Venus to accommodate 45 men, an observatory, a forge and an oven. On the day of the transit they were ready to observe with the help of two telescopes, a quadrant and an astronomical clock. To guard against cloudy weather cook sent parties to two other nearby islands. But as he observed, “not a Clowd was to be seen the whole day and the Air was perfectly clear”. They saw the “dusky shade” around the planet as it crossed the path of the Sun. They gathered excellent results although the demanding Cook saw the experiment as a failure because they missed the exact time of first contact, and the measurements his team took did not match precisely with each other.

After charting Tahiti’s islands, Cook left on July 13 to map the Society Isles before sailing south to find the terra incognita. By September he had found nothing and put off by heavy gales, he set sail for New Zealand. He circumnavigated the two islands to prove they were not part of a southern continent, concluding his mission in March 1770. The idea of going through Cape Horn in winter did not appeal so he decided to go home via the coast of New Holland and the East Indies.

The Dutch had mapped most of the north and west coast of New Holland but had found its inhabitants intimidating and there was little wealth by way of spices they could trade in Europe. The entire east coast was unknown to European navigators until Cook came. He called this strange new land New Wales before renaming it New South Wales, though it looked nothing like the principality it was named for. Cook was aware of human habitation along the coast thank to the columns of smokes but the people kept their distance and were often hostile when Cook made landfall at Botany Bay, Bustard Head and Endeavour River.

Rounding the Torres Strait, Cook proved New Holland was a different land mass to New Guinea. At Bedanug, which Cook called Possession Island, he laid claim of all of New South Wales in the name of King George III. Returning home via Batavia and the Cape of Good Hope, he landed at Deal, Kent on July 13, 1771 to the shock of many who assumed the Endeavour had been lost at sea.

The Royal Society was disappointed with Cook’s results from the transit and the blame was laid on ship’s astronomer Charles Green who conveniently died on the way home. Cook’s discovery of New South Wales was also temporarily laid aside. But Botany Bay wormed its way into the British consciousness in a way the science could not and although Cook did not live to see it, his New South Wales became a reality within two decades as a home for transported prisoners. Australia was born out of a rare celestial event, a fact remembered in Melbourne and Sydney a century later as a confident, wealthy and white Australia had the prime position to record the 1874 Transit of Venus. At a cloudless Mornington near Melbourne professor of maths William Parkinson Wilson observed the transit with great delight but the excitement was too much for him and he died two days later.

The results from 1874 and 1882 (where Europe and the US had the best viewing) showed the distance from Earth to the Sun to be around 149 million kms. The science from the 2004 and 2012 events has moved on to the study of exoplanets by measuring the dip in the planet’s brightness as it crossed the sun. With any luck, by the time of the next transit on December 10, 2117, humans – if we survive the next century – will be making plans to visit one of those exoplanets, and like Cook find a new frontier. Let’s hope they will be more gentle on its inhabitants than those who followed Cook were.


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