Sun in our position (the altitude), one can calculate their latitude (as I will show later). At night, a good way to find the latitude is to measure the altitude of Polaris, whose declination is always approximately 90°. The problem was how to measure the altitude. Many inventions tried to deal with this problem: astrolabes, quadrants, octants, but none of those was accurate enough to rely on completely. However, in 1731 two men from different parts of the world John Hadley in England and Thomas Godfrey in Philadelphia created a revolutionary device: a sextant. 14 In 1914-1916 Sir Ernest Shackleton OA made one of the world most famous marine journeys when he was planning to reach the South Pole on his ship ‘Endurance’. Unfortunately, she got trapped in ice, making the rest of the journey impossible. The crew decided to escape on 3 little boats, so that they reached Elephant Island. This is when Shackleton with 5 of his crew members sailed 1300 miles on his boat ‘James Caird’ (placed in our school’s new lab building) to get help. Of course neither of the crew members had an access to Google Maps or other satellite technology (GPS wasn’t here until 1960s). This is why Frank Worsely [6] (the skipper) was obliged to use the old good sextant and the Nautical Almanac (a book containing all the astronomical data published every year) to find their way on the sea. This was not an easy task as the horizon and the sky must be clear so that the measurement was accurate enough. A mistake of just one minute could be as bad as 4 kilometres on the sea! Another extremely useful piece of equipment is the chronometer, invented by a British mathematician John Harrison in 1737, (or a precise clock) that provided the sailors with the Coordinated Universal Time (UTC – the solar time on the Prime Meridian). This should remain unchanged even on the Elephant Island, where the Time Zone is GMT -3. Shackleton’s journey 15,16
Using a sextant 17,18
Sextant, presented on the Figure 2 below, consists of a telescope, 2 mirrors, frame, a couple of shade glasses and an index bar. The horizon mirror in the
14 P. Iflant, “The History of Sextant,” [Online]. Available: http://www.mat.uc.pt/~helios/Mestre/Novemb00/H61iflan.htm. [Accessed 15 April 2015]. 15 D. TV, “Shackleton: Death Or Glory: Navigation With Sextant,” [Online]. Available: https://www.youtube.com/watch?v=9zzYxdWGda8. [Accessed 16 April 2015]. 16 “Ernest Shackleton,” [Online]. Available: http://www.bbc.co.uk/history/historic_figures/shackleton_ernest.shtml. [Accessed 15 April 2015]. 17 “How to use a sextant,” [Online]. Available: http://www.robinsdocksideshop.com/how_to_use_a_sextant.htm. [Accessed 14 April 2015]. 18 “ Your position with a sextant,” [Online]. Available: http://www.idea2ic.com/Manuals/YOUR%20POSITION%20WITH%20A%20SEXTANT%20.pdf. [Accessed 16 April 2015].
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