Dead reckoning is how you know where you are without looking. Take your last known position, your heading, your speed and the time gone by, and mark the chart. The speed came from throwing a wooden chip over the stern and counting the knots in the line that ran out while a 28-second glass emptied, which is why ships still go in knots. But the chip floats in the same water as the ship, so if the whole sea is sliding sideways, the line reports nothing at all. Each small error goes into the next position as fact, and the next, and the sum can’t catch itself, because the only thing that can correct it is something outside it: a star, a headland, a lead dropped to the bottom. Until then the navigator keeps ruling neat lines, and a cross drawn from arithmetic looks exactly like one drawn from a star.