The transit of Venus
The weather in Exeter yesterday was best described as “ocr“, so I missed the transit this time. Fortunately, the skies were clear back in 2004, and I remember the experience of peering through a pair of binoculars equipped with a sun-filter and seeing the small black dot of Venus silhouetted against the sun.
The transit of Venus is a periodic event, and the big year was in 1769. I understand that the astronomers valued the transit as a way to get a handle on the size of the universe; but the real virtue was that it provided an excuse for the British Government to send an expedition down into the South Pacific. That expedition was commanded by James Cook, and it started the career of the greatest explorer of them all.
Of course, as a naval officer (Lieutenant, at the time), Cook kept a journal. If your eyesight is up to it, you can read his account of June 3rd 1769 in the original handwriting; but I admit that I looked up Project Gutenberg’s transcription:
Saturday, 3rd. This day proved as favourable to our purpose as we could wish. Not a Cloud was to be seen the whole day, 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 Sun’s Disk. We very distinctly saw an Atmosphere or Dusky shade round the body of the planet, which very much disturbed the times of the Contact, particularly the two internal ones. Dr. Solander observed as well as Mr. Green and myself, and we differ’d from one another in Observing the times of the Contact much more than could be expected. Mr. Green’s Telescope and mine where of the same Magnifying power, but that of the Doctor was greater than ours. It was nearly calm the whole day, and the Thermometer Exposed to the Sun about the Middle of the day rose to a degree of heat we have not before met with.
The ideal weather observer does not expose his thermometer to the sun (shade temperatures please), so perhaps it’s no great loss that Cook’s journal does not contain regular weather observations. For those, we must turn to the Master’s log of HM Bark Endeavour (the closest equivalent to the familiar modern-day logs). For the day of the transit, this records “Little wind and variable with fine pleasant clear weather”. I reckon that’s “Lt. Airs, Var., 1, b” in our notation. Sadly, none of the logs contain regular thermometer or barometer observations – Cook did better on his subsequent voyages – but we do get wind speed and direction reports for every day.