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 Transit of Venus through the years

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PostSubject: Transit of Venus through the years   Thu 07 Jun 2012, 00:27

Watching a tiny black speck on top of a larger white ring, as I did while viewing the Transit of Venus, it is hard to understand just what significance this has had through the past 400 years, but it has had great astronomical importance as well as ensuring nautical and geographical discoveries.

The relative distance of planets from the sun was known earlier from Kepler’s work. But not the absolute distance, and Edmond Halley proposed the parallax measurement using the transit of Venus would allow this is to discovered. Parallax means seeing things from two different angles and you notice this if you focus on an object with one eye and then with the other. (And I often entertain myself by what must be seeing things with separate eyes, I think. I think of it as a kind of double vision that I can control or that comes when I am tired and not focussing properly – was useful in church; I could try and see how far apart I could get people’s heads twice.)

Anyway in 1716 the transit of Venus gained that importance and explorations began from that. http://www.exploratorium.edu/venus/question4b.html Using four points of contact by Venus, distances can be calculated by trigonometry. But the transit needed to be seen from different places for this to work. Halley proposed seeing the transits from Norway, Hudson Bay and the Molucca Islands, but he died long before the transit in 1761. His idea didn’t, and 120 people from 9 nations and went off to look (in China, Siberia, Newfoundland, South Africa, Sweden, Turkey, the Rodriguez Islands, St Helena, Britain, and mainland Europe) , but bad weather conditions prevented any real knowledge being obtained, though the one at the Cape of Good Hope was quite successful. Luckily, and for reasons I don’t understand, the transit comes round again in other 8 years before disappearing for the next 105 or 121 years. This time 76 different points on the globe were used for measurements, Norway, Mexico, Philadelphia and 8 locations in Russia. The discovery of Tahiti in 1767 with longitude and latitude known made it an ideal place for one observation, and James Cook and the Endeavour went sent off there. The Royal Society also sent people off to the North Cape and Hudson Bay. This lot was disappointing too, not because of the weather or efforts, which included building a viewing platform to ensure the telescopes could be held steady, but because of a haziness now known to be due to turbulence in the earth’s atmosphere. However estimates of the distances turned out to be remarkably accurate.

Viewing Venus seems to be attended with lots of tragedy and bad luck. The first young men, friends Jeremiah Horrocks and William Crabtree, who set up to watch the 1639 transit, arranged to meet afterwards to compare their (abortive) attempts – Crabtree got so excited he only got a few quick sketches made and Horrocks interrupted his careful viewing to take a church service, only to find that Venus had already arrived when he got back – but Horrocks, aged 22, had already died, and Crabtree died three years later.

And despite official cooperation between France and England for the viewings, this didn’t always go to plan. A ship leaving Portsmouth in 1761 was fired on by a French frigate with the loss of 11 men. Apart from death Guillame le Gentil had perhaps the worst luck. He set off to watch the 1761 transit at Pondicherry, India, (encountering a hurricane and suffering dysentry on the way) which was in the meantime taken by the English, so he only saw it from a rolling sea, and set off to Mauritius to wait the next eight years exploring the Indian Ocean. He intended to watch from Manila but was sent back to Ponticherry for the next transit where an unexpected cloud impeded viewing (weather was perfect in Manila). And then he went home to France where his heirs had decided he was dead and were dividing his estate and his wife had remarried.

Here in New Zealand the Transit of Venus has historical value because Cook’s hidden instructions while on the trip to view Venus’s transit were to look for a great southern continent beyond Australia, which he duly did, though only finding a small southern country (as he expected), and mapping that and spending time learning of the flora fauna and people (with the interpretative help of the Tahitian Tupaia, who died on the way back to Britain. Joseph Banks, on board the Endeavour, was keen to keep him as a pet. "Thank heaven I have a sufficiency and I do not know why I may not keep him as a curiosity as well as some of my neighbours do lions and tigers at a larger expense than he probably ever put me to."

I see the recent transit being described as just of interest only, but elsewhere see it has value as a means of checking out Exoplanets (ones outside the Solar System).

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PostSubject: Re: Transit of Venus through the years   Thu 07 Jun 2012, 09:49

I must just add to this that the suggestion of a trigonomical method to calculate the absolute distances using multiple observations of the transit was first proposed by the mathematician James Gregory in his 1663 work Optica Promota and Halley used this.

The relevance of the current transit to exoplanets is in using it to check the accuracy of spectographic readings of the atmosphere of Venus against known data and so aiding corroboration of readings from those distant bodies being turned up by projects like Keppler.

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