Astronomical Observations and Navigation
|Figure 1. The kind of telescope (left) and astronomical quadrant (right) Cook and Green used on the Endeavour voyage. In the centre is an image of the Transit of Venus in 2012.|
|Figure 2. Chart of the Island of Otaheite (Tahiti) with Point Venus located beneath the red star. This is where they documented the Transit of Venus (Hawkesworth, 1773, Vol. 2 and online at the Internet Archive).|
|Figure 3. Fort Venus on the Island of Otaheite (Tahiti or King George's Island), a drawing by Sydney Parkinson, the artist on board the Endeavour and in the employ of Sir Joseph Banks. They constructed the fort to protect themselves and their observatory from attack by the natives.|
|Figure 4. Four pages from Hawkesworth (1773, Vol. 2) describing the events around the Transit of Venus and the missing Astronomical Quadrant, stolen by the natives.|
Prior to Harrison's invention, Galileo Galilei in 1612 had worked out another method to determine longitude, which Cook also employed. With sufficiently accurate knowledge of the orbits of the moons of Jupiter one could use their positions as a universal clock and this would make possible the determination of longitude.
|Figure 5. Galileo's Jovilabe used to calculate the periods of Jupiter's moons. Found at brunelleschi.imss.fi.it.|
“This night Mr. Green and I observ'd an Emersion of Jupiter's first Satellite, which hapnd at 2 hours 58 minutes 53 seconds in the A.M.; the same Emersion hapnd at Greenwich, according to Calculation, on the 30th at 5 hours 17 minutes 43 seconds A.M. The difference is 14 hours 18 minutes 50 seconds, equal to 214 degrees 42 minutes 30 seconds of Longitude, which this place is West of Greenwich, and its Latitude 15 degrees 26 minutes South”.
|Figure 6. Excerpt from Cook's handwritten Journal. Notice the annotation in darker pen giving Latitude and Longitude. It may have been made during their time repairing the ship on the Endeavour River.|