The Age of Exploration: Chapter 5, Exercise 1 — Possible Solution
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DEAD RECKONING
Calculating a current position from a previously known position (a fix),
using estimates of speed, heading and elapsed time: distance equals speed
times time.

WHY ERRORS MATTER MORE ON A LONG VOYAGE
Errors are cumulative, and the method does not account for wind drift,
currents or directional deviation, so each uncorrected error carries into
the next stage. Accuracy improves when a new fix is obtained part way through.