Before the days of GPS and SatNavs, how did early seafarers crossing the open ocean manage? The answer was to use the positions of stars. By measuring the angles to different stars you can work out your line of latitude - how far north or south you are. And some ancient mariners had clever ways of doing it, as Captain James Taylor, president of the Royal Institute of Navigation, explained to Chris Smith...
James - If you go back to the South Seas centuries ago, the South Sea islanders used latitude sailing, which was effectively using a coconut shell part filled with water, with apertures drilled in it so that a reflected heavenly body would be seen by the user at the appropriate angle. So the mariner would keep that heavenly body at that particular angle and that led to latitude sailing.
So you would go along a line of latitude until you got to where you wanted to be and then go straight north or south away from or towards the Sun. And it was that, the limitation of latitude sailing which led to the absolute need to establish a means of determining longitude.
Chris - Why can we work out latitude using a coconut shell but we can't work out longitude then?
James - Because the Earth rotates around its north/south axis and, therefore, you're using heavenly bodies and they're tracking across the sky basically to follow. But if you need a true position, and much of this was generated by various accidents where people did not know the longitude, the legendary loss of the treasures in Admiral Cloudesley Shovells' wreck off the Scillys, for example, is that you need to determine longitude and true position, and that was much more economical. Instead of going through a series of right angles in order to complete your journey, you could go directly.
Chris - So, basically, because the Earth is turning you need to know what time of day it is, very precisely to know where those stars in the sky that you are following should be?
James - Indeed. And the easiest indicator of your own longitude is the Sun. When the Sun is at it's highest point in the sky, effectively you're on that line of latitude. So once Harrison had invented his chronometer, an accurate form of timekeeping. And you knew the longitude of your point of departure, and the time difference between your point of departure and your current position when the Sun is directly overhead, you had a longitude relative to your point of departure. And custom and practice dictated that the standard point of departure was Greenwich. Greenwich became the zero degrees east/west longitude and you could now measure time when the Sun was at it highest point relative to that. So we now had latitude and longitude.