Before satellite navigation, sailors used a combination of observation and calculation to keep track of a vessel. A compass indicated direction, while charts described coastlines, hazards, and known routes. Mariners estimated distance traveled using speed and time, though changing currents, wind, and sea conditions could make those estimates uncertain. A ship's position was rarely known with perfect precision, especially far from land or in poor weather.
Latitude could be estimated by measuring the height of the sun at noon or observing particular stars, using instruments available in the period. Determining longitude accurately was a harder problem because it required comparing local time with the time at a known reference location. Over time, improved astronomical methods and reliable marine clocks made longitude estimates more dependable. These methods developed over centuries, so the tools available to one crew could differ from those of another.
Practical navigation also relied on experience. Sailors learned how currents affected familiar passages, watched for seabirds or changes in water color, and recognized coastal landmarks when they came into view. Such signs were useful but not foolproof. Charts could be incomplete, observations could be obscured, and mistakes could put a ship on a reef or far from its intended route. The best navigators combined instruments with cautious judgment and repeated checks, treating a plotted position as an estimate rather than an unquestionable fact.