Postby Master Gunner » 12 Jul 2012, 05:47
Hmm. Regarding the deserted island, if you had access to Twitter, or any other form of communications, determining your longitude (east-west location) would be rather easy. Just tweet when the sun crosses the horizon, and someone can compare that to when the sun the sun rises in Greenwich, England, and that will significantly narrow down the search field. The tricky part, in an interesting reversal, is finding your latitude. In the past, sailors would be familiar with the constellations and would use them to determine their north-south position (as at any given longitude, the stars are going to stay at the same angle above the horizon), but it wasn't until relatively recently (late 18th century) that we could keep time accurately enough for ships to determine east-west location. In contrast, the average person today is more likely to have an accurate timepiece than detailed knowledge of the constellations.
Nonetheless, if you could tweet a picture, that would be an easy solution. A shot of the horizon and some stars above it would be enough for a rescue team to narrow down your location. Otherwise, it would be down to trying to recognize one of the major constellations (Orion, Big Dipper, Polaris in the North, Southern Cross in the south), and trying to estimate its angle above the horizon. A sextant app would be very helpful here.
So, with the time difference from the Prime Meridian, and approximate altitude of a star in the sky, you would be able to direct a rescue team to your approximate location. Still probably going to be a large search area, but with some luck, rescue is not beyond reach through twitter alone.