Rainbow's Arc Positioning

A camera sextant, for the fun of it. Noon, a clean horizon, and an hour on watch. Rainbow’s Arc takes a sun sight from a photograph: the sun and the sea horizon in one frame, worked against a built-in almanac into a line of position, and two lines into a fix. If you own a sextant, its drill scores your sights against where you really are. Every boat carries GPS these days, usually several. This app is not a contingency plan for the day they all fail at once — though it would serve. It is a fun alternative to try, and it holds your position three ways — a camera sextant, a drill for your real sextant, and a deck log of dead reckoning — that reinforce each other: the log carries you between sights, the sights correct the log, and the gap between them is the current talking. The nautical almanac (sun, four planets, twenty-one named stars, computed on the phone through 2050), the math, and…