Science history is never dull, that's why I love reading and writing about it.
What Is the Transit of Venus?
Twice in approximately 125 years the planet Venus visibly passes in front of the sun and this moment can be viewed from Earth. Venus appears as a black disc on the sun's face. A Transit of Venus is a privilege to witness because only Venus and Mercury have orbits within Earth's orbit so they are the only ones that earth's inhabitants can hope to see.
According to NASA a visible Transit of Venus occurs only when Venus is in "inferior conjunction" with the sun. This means that it must pass between the Earth and the sun as it crosses through Earth's orbit.
Earth Years and Venus Years
It has been calculated that 5000 years ago the Transit of Venus occurred in May and November. At the moment transits are around the 7th June and the 9th December. NASA predicts that the winter and summer solstices in 1500 years' time will also mark the transits of Venus.
We know that earth years are 365.2 days and Venus has a year that lasts for less time, 224.7 days, so this is how the date of the next transit is predicted. The two Transits of Venus occur eight years apart thanks to the difference in Earth and Venus' years.
When King George III, a keen scientist, ordered Captain James Cook to set sail to the South Pacific in 1768 the last Transit of Venus had occurred in 1761.
Captain Cook 's Endeavour Drops Anchor
Lieutenant (not Captain yet) James Cook's H.M.S. Endeavour set sail for the recently discovered George the Third's Island in the South Pacific in August 1768. Cook was travelling towards largely uncharted territory and he was concerned that he'd lose most of his 94 passengers and crew to illness (scurvy was a primary concern) or storms and sea life attacks.
The Endeavour dropped anchor at King George the Third’s Island on the 13th April 1769. Cook and his fellow travellers arrived ahead of time, probably feeling relieved that they'd survived eight months and the several thousands of miles of their sea voyage. With no accurate timepieces or navigation equipment, the accomplishment of reaching the destination at all must have felt like a miracle.
Today, we call the George the Third Island Tahiti but in 1769 Cook established that its true name was Otahite (Otaheite) and the ruler was King Tarroa. Botanist Joseph Banks researched the flora and fauna on the island. A fort and portable observatory were constructed. Cook named the adjacent group of islands The Society Islands in honour of The Royal Society of London which partly funded the expedition.
Venus and the Solar System's Size
Cook's other mission on the island was to accurately assess the size of the solar system through the Transit of Venus' data. Georgian scientists only knew of six planets. They didn't have Pluto, Neptune or Uranus charted. They could estimate how far away a planet was from the sun and roughly establish Earth's proximity but the calculations offered no indisputable answers.
In 1716 Edmund Halley of Comet fame worked out that Venus' transit was the key to knowledge so he proposed measuring the start and stop times of Venus across the sun's surface as it crossed because this would inform him exactly how far away Venus was from Earth and the Earth from the sun. Hailey died before he could test his theory with a visible transit.
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Sun Damaged Data
The Transit of Venus occurred on the 3rd June 1769. The telescopes that the British had taken with them to Tahiti were inadequate for the task and the sun was blindingly strong.
Cook recorded that "we very distinctly saw an atmosphere or dusky shade round the body of the planet which very much disturbed the times of the contacts, particularly the two internal ones." In other words, the sunlight blasted away any chance of accuracy. The ship's astronomer Charles Green recorded data that was 42 seconds different from Cook's measurements.
There was hope because readings were being taken at 76 other locations around the globe so Cook and Green's figures weren't the only ones to work from.
George III Predicted the Next Transits of Venus
George III was the first British royal to study science. He was introduced to the subject by his assistant preceptor (tutor) George Lewis Scott who was a mathematician, literary figure and a member of the Royal Society. George made full use of the astronomical observatory at Kew and he became adept at calculations. His paper about the Transit of Venus in 1769 and his predictions for the 1874 and 2004 transits are impressive.
The Royal Society members used the data provided from the 1769 transit to draw their conclusions for papers and letters including James Horsfall's letter.
James Cook paid subsequent visits to the South Pacific region and in August 1777, his final expedition there, he carved “Georgius tertius Rex Annis 1767, 69, 73, 74 & 77” on a cross to commemorate the British visits made during the previous decade of discovery.
2012, 2117 and 2125
The 2012 transit lasted for 6 hours and 40 minutes. If you're interested in watching the next Transit of Venus sadly it's not until the 10th December 2117 and the following one is in December 2125. Humans alive today won't see it but at least we can read about 1769 and Venus' subsequent transits as we discover more about the solar system and our place within it.
- Captain James Cook and the Royal Society | Royal Society Publishing
- James Cook and the Transit of Venus | Science Mission Directorate
The best reason to watch the 2004 transit of Venus across the Sun is history.
- NASA – Catalog of Transits of Venus
This is NASA's official Catalog of Transits of Venus.
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
© 2022 Joanne Hayle