Dead Reckoning

This ancient form of navigation known as dead reckoning begins with a known position, or fix, which is then advanced, mathematically or directly on the chart, by means of recorded heading, speed, and time.

The navigator finds a position by measuring the course and distance they have sailed from some known point. Starting from a port, the navigator measures the distance from that point on a chart. Each day’s end position would be the starting point for the next day’s course and distance-measurement.

The navigator plots his 9am position, indicated by the triangle, and, using his course and speed, estimates his position at 9:30am and 10am.

Without an accurate way to tell time, dead reckoning was notoriously inaccurate in the open ocean, making Columbus’s feats all the more remarkable.