For the longest time, the solution to calculating longitude eluded mariners because no one could construct a clock that was both accurate enough and durable enough. Much like our skill at flinging sophisticated pieces of technology into orbit has advanced, so too has our ability to construct watches. That sounds easy enough: just take a watch with you that’s set to London time, and at noon at your current location, check what time the watch reports. The only way to calculate longitude is to measure the difference in time between two points: the reference point (e.g., 0° longitude) and your current location. Longitude, or rather the “starting point” for lines of longitude, is entirely arbitrary. But how far away were you from the nearest charted island? And were you to the east of it, or the west? So you could figure out how far away from the equator or the poles. Latitude corresponds both to the sun’s altitude in the sky (at noon) and to the altitude of certain stars (if you are in the Northern Hemisphere, Polaris is a good choice) at night. Latitude was relatively easy-well, easy enough once you dealt with the pitching deck, the storms, and the scurvy. I’m not sure how to describe how lost you could be, out there on the ocean, no longer in sight of shore. In the days-centuries-prior to GPS, you could get lost. GPS might not be great at giving directions, but that doesn’t mean you’re lost. Thanks to GPS, we can forget that calculating longitude without the help of a network of satellites is difficult and requires great mathematical and engineering expertise. I don’t use it that much personally, because I don’t tend to go anywhere, but I’m sure all this technology I love to use makes use of GPS.

Longitude by davasobel