Friday, August 22, 2014

Special Collections Celebrates National Science Week: Age of Discovery and Astronomical Observations and Navigation


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.
The measurement of longitude is important both to cartography and ocean navigation. Latitude is relatively easy to determine using a quadrant or astrolabe (above right), because latitude is always at right angles to the earth rotation. Longitude is more difficult to determine because it is a moving target as the earth rotates over a period of a day, and thus to be measured relative to a given standard, commonly Greenwich Mean Time. Before John Harrison invented his sea clock with a large balance wheel to cope with a ship's movement, longitude could not be measured on board a ship. See Dava Sobel's book, Longitude: the true story of a lone genius who solved the greatest scientific problem of his time. Captain James Cook used one of Harrison clocks on his second and third world voyages, but not on the Endeavour voyage.
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).
The Endeavour and its crew were commissioned by the Royal Society of London primarily to record the Transit of Venus in Tahiti in order to calculate longitude at that point (See Point Venus on the map above). Transits of Venus occur in a pattern that repeats every 243 years with pairs of transits eight years apart separated by gaps of 121.5 years and 105.5 years. The last six transits of Venus were recorded in 1639, 1761, 1769, 1874, 1882, 2004  and the most recent one on June 6, 2012, watched by Australians on television!   
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.
 James Cook, Charles Green and Daniel Solander were successful in their observations (Cook & Green, 1771), but there was an incident that might have made things turn out quite differently for these men. The day after they had built their fort, to “secure us against the natives”, the Astronomical Quadrant used to measure it went missing. A great search ensued and a handsome reward was offered. Without the Quadrant the principal purpose of the Endeavour voyage was in peril! Read on from Hawkesworth (1773, Vol. 2) below:
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.
During the time that the Endeavour was being repaired on the Endeavour River, Cook took several astronomical observations. From the transcription of Captain Cook’s Journal, Project Gutenberg, Australia:

“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.

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