The Mariner's Astrolabe

Voyagers have always had to work out their position at sea out of sight of land. Contrary to popular belief, people have long known that Earth is a sphere. Over 2200 years BPE (before the present era) the Greeks realised that you could work out where you were on the globe by calculating the angle of the sun above the horizon at midday. Thus, over many generations, with the progressive refinement of navigation instruments, the first astrolabes were produced. They were simple, though heavy, disks, with 360 degrees marked around the perimeter, and a pointer pivoted in the middle. The navigator held it up to eye level and marked the angle of the sun by moving the pointer to align with the solar disk. With the angle so calculated, a navigator could determine the position of his vessel on the surface of the Earth along lines of latitude but, there was always danger of eye damage from looking at the sun. This astrolabe came from the Dutch East Indiaman Gilt Dragon, wrecked near Ledge Point in 1656. Of the five known East Indiamen wrecked on the WA coast, four Dutch and one English Gilt Dragon was the furthest off course. Clearly, even the best instruments had their limitations when navigating unknown waters.