Hello All!   
      
   James Cook and the Transit of Venus    
      
   Every ~120 years a dark spot glides across the Sun. Small, inky-black, almost   
   perfectly circular, it's no ordinary sunspot. Not everyone can see it, but   
   some who do get the strangest feeling, of standing, toes curled in the damp   
   sand, on the beach of a South Pacific isle....   
      
   City odors drifted in from Plymouth, across the ship, shoving aside the salt   
   air.  Sea gulls fluttered upward, screeching, as the sails snapped taut. The   
   wind had changed and it was time to go.    
      
   On August 12, 1768, His Majesty's Bark Endeavour slipped out of harbor, Lt.   
   James Cook in command, bound for Tahiti. The island had been "discovered" by   
   Europeans only a year before in the South Pacific, a part of Earth so poorly   
   explored mapmakers couldn't agree if there was a giant continent there or not.   
   Cook might as well have been going to the Moon or Mars. He would have to steer   
   across thousands of miles of open ocean, with nothing like GPS or even a good   
   wristwatch to keep time for navigation, to find a speck of land only 20 miles   
   across. On the way, dangerous storms could (and did) materialize without   
   warning. Unknown life forms waited in the ocean waters. Cook fully expected   
   half the crew to perish.    
      
   http://science.nasa.gov/media/medialibrary/2012/06/02/endeavour1_med2.jpg   
      
   The Endeavour. Credit: HMB Endeavour Foundation. It was worth the risk, he   
   figured, to observe a transit of Venus.    
      
   "At 2 pm got under sail and put to sea having on board 94 persons," Cook noted   
   in his log. The ship's young naturalist Joseph Banks was more romantic: "We   
   took our leave of Europe for heaven alone knows how long, perhaps for Ever,"   
   he wrote.    
      
   Their mission was to reach Tahiti before June 1769, establish themselves among   
   the islanders, and construct an astronomical observatory. Cook and his crew   
   would observe Venus gliding across the face of the Sun, and by doing so   
   measure the size of the solar system. Or so hoped England's Royal Academy,   
   which sponsored the trip.    
      
   The size of the solar system was one of the chief puzzles of 18th century   
   science, much as the nature of dark matter and dark energy are today. In   
   Cook's time astronomers knew that six planets orbited the sun (Uranus,   
   Neptune, and Pluto hadn't been discovered yet), and they knew the relative   
   spacing of those planets. Jupiter, for instance, is 5 times farther from the   
   Sun than Earth. But how far is that . in miles? The absolute distances were   
   unknown.    
      
   Venus was the key. Edmund Halley realized this in 1716. As seen from Earth,   
   Venus occasionally crosses the face of the sun. It looks like a jet-black disk   
   slowly gliding among the sun's true spots. By noting the start- and stop-times   
   of the transit from widely spaced locations on Earth, Halley reasoned,   
   astronomers could calculate the distance to Venus using the principles of   
   parallax. The scale of the rest of the solar system would follow.    
      
   But there was a problem. Transits of Venus are rare. They come in pairs, 8   
   years apart, separated by approximately 120 years. Halley himself would never   
   live to see one. An international team did try to time a Venus transit in   
   1761, but weather and other factors spoiled most of their data. If Cook and   
   others failed in 1769, every astronomer on Earth would be dead before the next   
   opportunity in 1874.    
      
   http://science.nasa.gov/media/medialibrary/2012/06/02/cook_med2.jpg   
      
   Portrait of Cook, oil on canvas, Nathanial Dance, 1735-1811. Credit: National   
   Library of Australia. Cook's expedition is often likened to a space mission.   
   "The Endeavor was not only on a voyage of discovery," writes Tony Horwitz in   
   the Cook travelogue Blue Latitudes, "it was also a laboratory for testing the   
   latest theories and technologies, much as spaceships are today."    
      
   In particular, the crew of the Endeavor were to be guinea pigs in the Navy's   
   fight against "the scourge of the sea"--scurvy. The human body can store only   
   about 6 week's worth of vitamin C, and when it runs out seamen experience   
   lassitude, rotted gums, hemorrhaging. Some 18th century ships lost half their   
   crew to scurvy. Cook carried a variety of experimental foods onboard, feeding   
   his crew such things as sauerkraut and malt wort. Anyone who refused the fare   
   would be whipped. Indeed, Cook flogged one in five of his crew, about average   
   in those days, according to Horwitz.    
      
   By the time Cook reached Tahiti in 1769, he'd been sailing west for 8   
   months--about as long as modern astronauts might spend en route to Mars. Five   
   crewmen were lost when the ship rounded stormy Cape Horn, and another   
   despairing marine threw himself overboard during the 10-week Pacific passage   
   that followed.  Endeavor was utterly vulnerable as it angled toward Tahiti.   
   There was no contact with "Mission Control," no satellite weather images to   
   warn of approaching storms, no help of any kind. Cook navigated using   
   hourglasses and knotted ropes to measure ship's speed, and a sextant and   
   almanac to estimate Endeavor's position by the stars. It was tricky and   
   dangerous.    
      
   Remarkably, they arrived mostly intact on April 13, 1769, almost two months   
   before the transit. "At this time we had but very few men upon the Sick list .   
   the Ships compney had in general been very healthy owing in a great measure to   
   the Sour krout," wrote Cook.    
      
   http://science.nasa.gov/media/medialibrary/2012/06/02/ptvenus_med2.jpg   
      
   The view from Point Venus, Tahiti, where Cook and his men observed the transit   
   of Venus. Oil on canvas, William Hodges, 1744-1797. Credit: National Library   
   of Australia. Tahiti was as alien to Cook's men as Mars might seem to us   
   today. At least the island was comfortable and well provisioned for human   
   life; the islanders were friendly and eager to deal with Cook's men. Banks   
   deemed it "the truest picture of an arcadia (idyllic and peaceful) . that the   
   imagination can form." Yet the flora, fauna, customs and habits of Tahiti were   
   shockingly different from those of England; Endeavor's crew was absorbed,   
   amazed.    
      
   Perhaps that is why Cook and Banks had so little to say about the transit when   
   it finally happened on June 3, 1769. Venus' little black disk, which could   
   only be seen gliding across the blinding sun through special telescopes   
   brought from England, had a powerful rival: Tahiti itself.    
      
   Banks' log entry on the day of the transit consists of 622 words; fewer than   
   100 of them concern Venus. Mostly he chronicled a breakfast-meeting with   
   Tarr¢a, the King of the Island, and Tarr¢a's sister Nuna, and later in the   
   day, a visit from "three handsome women." Of Venus, he says, "I went to my   
   Companions at the observatory carrying with me Tarr¢a, Nuna and some of their   
   chief attendants; to them we shewd the planet upon the sun and made them   
   understand that we came on purpose to see it. After this they went back and   
   myself with them." Period. If the King or Banks himself was impressed, Banks   
   never said so.    
      
   Cook was a little more expansive: "This day prov'd as favourable to our   
   purpose as we could wish, not a Clowd was to be seen . and the Air was   
   perfectly clear, so that we had every advantage we could desire in Observing   
   the whole of the passage of the Planet Venus over the Suns disk: 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."    
      
   http://star.arm.ac.uk/history/TRANSIT2.GIF   
      
   Drawings of the 1769 transit of Venus by James Cook. [more]    
      
   Cook also observed the "black drop effect." When Venus is near the limb of the   
   sun--the critical moment for transit timing--the black of space beyond the   
   sun's limb seems to reach in and touch the planet. This makes it very   
   difficult to say precisely when a transit begins or ends. The effect was not   
   fully understood until 1999 when a team of astronomers led by Glenn Schneider   
   of the University of Arizona studied a similar black drop during a transit of   
   Mercury.  They proved1 the distortion is caused by a combination of solar limb   
   darkening and the point-spread function of the telescope. Cook's observations   
   were clearly affected.  Indeed, his measurements disagreed with those of   
   ship's astronomer Charles Green, who observed the transit beside Cook, by as   
   much as 42 seconds.    
      
   This was a problem for observers elsewhere, too. When all was said and done,   
   observations of Venus' 1769 transit from 76 points around the globe, including   
   Cook's, were not precise enough to set the scale of the solar system.   
   Astronomers didn't manage that until the 19th century when they used   
   photography to record the next pair of transits.    
      
   Cook wouldn't dwell on these matters; there was a lot more exploring to do.   
   Secret orders from the Navy instructed him to leave the island when the   
   transit was done and "search between Tahiti and New Zealand for a Continent or   
   Land of great extent."    
      
   For much of the next year Endeavor and her crew scoured the South Pacific,   
   searching for a continent that some 18th century scientists claimed was   
   necessary to balance the great land masses of the northern hemisphere. At one   
   point they were out of sight of land for almost two months. But the terra   
   australis incognita, the unknown "south land," didn't exist, just as Cook   
   thought all along. Along the way Cook met the fierce Maori of New Zealand and   
   the Aborigines of Australia (encounters both races would lament in later   
   years), explored thousands of miles of Kiwi and Aussie coastline, and had a   
   near-disastrous collision with the Great Barrier Reef.    
      
   http://science.nasa.gov/media/medialibrary/2012/06/02/endriver_strip.jpg   
      
   The Endeavour is beached in Australia following a collision with the Great   
   Barrier Reef. An engraving from John Hawkesworth's An Account of the voyages..   
   Credit: National Library of Australia.    
   Later, during a 10-week stopover in Jakarta for repairs, seven seamen died of   
   malaria. The port city was densely populated by people and diseases. Cook left   
   as quickly as possible, but the damage was done. Ultimately 38 of the   
   Endeavour's original company (and 8 who joined later) perished, including   
   astronomer Charles Green. "The ship's 40% casualty rate wasn't considered   
   extraordinary for the day," writes Horwitz. "In fact, Cook would later be   
   hailed for the exceptional concern he showed for the health of his crew."    
      
   On July 11, 1771, Cook returned to England at Deal. The survivers had   
   circumnavigated the globe, catalogued thousands of species of plants, insects   
   and animals, encountered new (to them) races of people, and hunted for giant   
   continents. It was an epic adventure.    
      
   In the end, the transit was just a tiny slice of Cook's adventure,   
   overshadowed by Tahiti and sabotaged by black drops. But because of the voyage   
   Venus and Cook are linked. In fact, it might be said that the best reason to   
   watch a transit of Venus is history.    
      
   Decide for yourself. On June 5-6, 2012, Venus is due to cross the face of the   
   Sun again. The event will be web cast, broadcast, and targeted by innumerable   
   sidewalk telescopes. In other words, you can't miss it. Look into the inky   
   black disk. It can carry you back to a different place and time: Tahiti, 1769,   
   when much of Earth was still a mystery and the eye at the telescope belonged   
   to a great explorer.    
      
   Can you feel the sand between your toes?    
      
      
   Author:Dr. Tony Phillips| Production editor: Dr. Tony Phillips | Credit:   
   Science@NASA   
      
   More Information    
   2012 Transit of Venus -- maps, timetables, and details from NASA    
      
   Live Webcast and Observing Tips -- from the Goddard Space Flight Center    
      
   Live Webcast -- from the Coca-Cola Science Center    
      
   Transit of Venus -- from Sky and Telescope    
      
   Transit of Venus photo gallery -- from spaceweather.com    
      
   2012 Transit of Venus -- ScienceCast video    
      
   Transit of Venus Images From the ISS - from the International Space Station   
   (available during and after the Transit)    
      
   Footnote: (1) TRACE observations of the 15 November 1999 transit of Mercury   
   and the Black Drop effect: considerations for the 2004 transit of Venus by   
   Glenn Schneider, Jay M. Pasachoff, and Leon Golub, Icarus 168 (2004) 249-256    
      
      
   Regards,   
      
   Roger    
   --- timEd/386 1.10.y2k+   
    * Origin: NCS BBS - Houma, LA - (1:3828/7)