Read the excerpt from "Ship's Chronometer from HMS Beagle” and consider the accompanying image.If you wanted to sail, it was impossible to keep a precise record of time. And at sea, if you can’t tell the time, you don’t know how far east or west you are. It is relatively easy to calculate latitude – your distance north or south of the equator – by measuring the height of the Sun above the horizon at noon; but this won’t let you calculate longitude – your position east or west.