Long before GPS or detailed nautical charts, medieval sailors crossed open seas using carefully measured distances, directions, tides, and even the phases of the moon. Lorris Chevalier explores the practical techniques that helped mariners navigate the waters of northern Europe during the Middle Ages.
By Lorris Chevalier
In the northern seas of medieval Europe, navigating the open water required a practical mastery of distance, direction, and time. Between the thirteenth and fifteenth centuries, mariners developed a working system of measurement that allowed them to steer across largely featureless waters. Their knowledge was not abstract or scientific in a modern sense, but was deeply practical: it was designed to keep ships on course in an environment that constantly erased visible reference points.
Unlike land routes, which are fixed and persistent, sea routes exist only in the moment of travel. Every journey across the ocean had to be reconstructed anew, without permanent markers. Once out of sight of land, sailors entered a space without stable landmarks, where orientation depended on memory, observation, and measurement. In this context, measurement became a substitute for geography itself.
Measuring Distance at Sea
Frontispice du Grand routier de Pierre Garcie (1520 edition)
To navigate offshore, sailors relied on two main strategies: following visible coastal features when possible, or using abstract systems of distance and direction when at sea. The latter required precise knowledge of bearings and lengths, even if expressed in non-standardised units such as leagues or estimated nautical distances.
Medieval sailing manuals known as routiers recorded these relationships in a highly formulaic style. Pierre Garcie Ferrande’s Grand Routtier de la mer repeatedly describes routes in terms of direction and distance, as in the following examples:
Sensuyt de lisle d’Ieux. Le bout damont de lisle d’Ieux et la barge d’Olonne gisent noroest et suest, et ya entre deux : neuf lieues.
Lisle d’Ieux et Sainct Sebastian gisent nort et su e prens ung quart de noroest et ung quart de suest et ya de lun a lautre : lxvi lieues.
Which mean:
Next follows the Isle of Yeu. The northern tip of the Isle of Yeu and the harbour of Olonne lie northwest and southeast of one another, and the distance between them is nine leagues.
The Isle of Yeu and Saint Sebastian lie north and south, with a quarter point toward the northwest and a quarter point toward the southeast, and the distance between them is sixty-six leagues.
These instructions do not describe landscapes in a pictorial sense, but rather construct a mental network of routes defined entirely through bearing and distance. To navigate safely, sailors had to internalise such descriptions and reproduce them from memory while at sea.
When sailing beyond sight of land, measurement of depth also became crucial. The lead line, marked at regular intervals, allowed sailors to sound the seabed and determine their position indirectly. Changes in depth and sediment could signal proximity to coasts or underwater features.
Maritime routes described by Pierre Garcie Ferrande – image by CRHIP / Wikimedia Commons
An English sailing instruction from the fifteenth century illustrates this technique clearly:
And yf ye haue an c fadim deep or ellis iiijxx and x, than ye schal go northe vntil ye sownde ayen in lxxij fadim in fayre gray sonde, and that is the rigge þat lieth betwene Clere and Cille.
This can be translated as:
And if you have a depth of 100 fathoms (or approximately 80 plus 10), then you must sail north until you sound again at 72 fathoms with a light grey sand, and that is the ridge that lies between Cape Clear and the Scilly Isles.
Here, the sailor is guided not by visible land but by a calculated shift in depth and seabed texture. The sea floor itself becomes a form of map.
Measuring Time at Sea
BNF Français 9685 fol. 94r
Time was equally essential for navigation. Long voyages, whether for trade, fishing, or warfare, required ways of measuring duration that extended beyond simple daylight observation. Mariners therefore developed multiple systems combining celestial observation, tidal cycles, and mechanical instruments.
One of the more sophisticated examples is the use of lunar phases to calculate extended periods of time, as recorded by Pierre Garcie Ferrande:
Qui veult bien compter la lune par les heures lon doit prendre par chascun quart de la lune vingt et deux heures et demye. Item pour deux quars, xlv heures. Item pour troys quars, lxvii heures et demye. Item pour quattre quars, iiii. xx. x heures.
Which can be translated as:
Whoever wishes to reckon the moon by hours must take for each quarter of the moon twenty-two and a half hours. Likewise, for two quarters, forty-five hours. For three quarters, sixty-seven and a half hours. For four quarters, one hundred and ten hours.
This system shows how lunar cycles were converted into calculable units of time, extending the sailor’s ability to estimate duration far beyond the limits of the day-night rhythm.
Tides also served as temporal markers. Expressions such as “a tide and a half” or “a quarter tide” corresponded to measurable periods and were especially useful in coastal waters where timing was critical for safe entry into harbours.
For shorter intervals, sailors increasingly relied on sandglasses, known in England as “running glasses” or sea-clocks. These devices allowed time to be divided into standard units that could then be linked to distance and speed.
Memory, Direction, and Mental Mapping
15th-century ship – British Library MS Cotton Titus A. XXVI, fol.41
Despite the increasing use of instruments, medieval navigation remained fundamentally dependent on memory. Sailing directions were not meant to be casually read at sea but memorised in advance. Pilots needed to recall sequences of bearings and distances, often structured in repetitive formulae.
A typical example from Pierre Garcie Ferrande demonstrates this reliance on directional relationships:
Lisle d’Ieux et le boucaut de Bayonne gisent nort noroest et su suest et ya de lun a lautre : lxxiiii lieues.
Which means:
The Isle of Yeu and the mouth of the Bayonne Channel lie north-northwest and south-southeast of one another, and the distance between them is seventy-four leagues.
Such instructions formed a cognitive framework through which sailors mentally reconstructed routes. Combined with the magnetic compass, which spread through Europe from the twelfth century onward, these memorised directions allowed navigators to maintain orientation even far from land. In this sense, memory acted as a navigational technology in itself—a “mental chart” constructed from learned measurements.
Medieval maritime navigation in northern Europe was built upon a delicate interplay between observation, measurement, and memory. Sailors transformed the sea from an apparently featureless expanse into a space structured by depth, direction, time, and duration.
Whether through lunar calculations, tidal rhythms, sounding lines, or memorised sailing directions, they created a practical system that made movement across the ocean possible. These techniques did not constitute a theoretical conquest of the sea, but rather a pragmatic adaptation to its uncertainties.
Dr Lorris Chevalier, who has a Ph.D. in medieval literature, is a historical advisor for movies, including The Last Duel and Napoleon. Click here to view his website.
Long before GPS or detailed nautical charts, medieval sailors crossed open seas using carefully measured distances, directions, tides, and even the phases of the moon. Lorris Chevalier explores the practical techniques that helped mariners navigate the waters of northern Europe during the Middle Ages.
By Lorris Chevalier
In the northern seas of medieval Europe, navigating the open water required a practical mastery of distance, direction, and time. Between the thirteenth and fifteenth centuries, mariners developed a working system of measurement that allowed them to steer across largely featureless waters. Their knowledge was not abstract or scientific in a modern sense, but was deeply practical: it was designed to keep ships on course in an environment that constantly erased visible reference points.
Unlike land routes, which are fixed and persistent, sea routes exist only in the moment of travel. Every journey across the ocean had to be reconstructed anew, without permanent markers. Once out of sight of land, sailors entered a space without stable landmarks, where orientation depended on memory, observation, and measurement. In this context, measurement became a substitute for geography itself.
Measuring Distance at Sea
Frontispice du Grand routier de Pierre Garcie (1520 edition)
To navigate offshore, sailors relied on two main strategies: following visible coastal features when possible, or using abstract systems of distance and direction when at sea. The latter required precise knowledge of bearings and lengths, even if expressed in non-standardised units such as leagues or estimated nautical distances.
Medieval sailing manuals known as routiers recorded these relationships in a highly formulaic style. Pierre Garcie Ferrande’s Grand Routtier de la mer repeatedly describes routes in terms of direction and distance, as in the following examples:
Sensuyt de lisle d’Ieux. Le bout damont de lisle d’Ieux et la barge d’Olonne gisent noroest et suest, et ya entre deux : neuf lieues.
Lisle d’Ieux et Sainct Sebastian gisent nort et su e prens ung quart de noroest et ung quart de suest et ya de lun a lautre : lxvi lieues.
Which mean:
Next follows the Isle of Yeu. The northern tip of the Isle of Yeu and the harbour of Olonne lie northwest and southeast of one another, and the distance between them is nine leagues.
The Isle of Yeu and Saint Sebastian lie north and south, with a quarter point toward the northwest and a quarter point toward the southeast, and the distance between them is sixty-six leagues.
These instructions do not describe landscapes in a pictorial sense, but rather construct a mental network of routes defined entirely through bearing and distance. To navigate safely, sailors had to internalise such descriptions and reproduce them from memory while at sea.
When sailing beyond sight of land, measurement of depth also became crucial. The lead line, marked at regular intervals, allowed sailors to sound the seabed and determine their position indirectly. Changes in depth and sediment could signal proximity to coasts or underwater features.
An English sailing instruction from the fifteenth century illustrates this technique clearly:
And yf ye haue an c fadim deep or ellis iiijxx and x, than ye schal go northe vntil ye sownde ayen in lxxij fadim in fayre gray sonde, and that is the rigge þat lieth betwene Clere and Cille.
This can be translated as:
And if you have a depth of 100 fathoms (or approximately 80 plus 10), then you must sail north until you sound again at 72 fathoms with a light grey sand, and that is the ridge that lies between Cape Clear and the Scilly Isles.
Here, the sailor is guided not by visible land but by a calculated shift in depth and seabed texture. The sea floor itself becomes a form of map.
Measuring Time at Sea
Time was equally essential for navigation. Long voyages, whether for trade, fishing, or warfare, required ways of measuring duration that extended beyond simple daylight observation. Mariners therefore developed multiple systems combining celestial observation, tidal cycles, and mechanical instruments.
One of the more sophisticated examples is the use of lunar phases to calculate extended periods of time, as recorded by Pierre Garcie Ferrande:
Qui veult bien compter la lune par les heures lon doit prendre par chascun quart de la lune vingt et deux heures et demye. Item pour deux quars, xlv heures. Item pour troys quars, lxvii heures et demye. Item pour quattre quars, iiii. xx. x heures.
Which can be translated as:
Whoever wishes to reckon the moon by hours must take for each quarter of the moon twenty-two and a half hours. Likewise, for two quarters, forty-five hours. For three quarters, sixty-seven and a half hours. For four quarters, one hundred and ten hours.
This system shows how lunar cycles were converted into calculable units of time, extending the sailor’s ability to estimate duration far beyond the limits of the day-night rhythm.
Tides also served as temporal markers. Expressions such as “a tide and a half” or “a quarter tide” corresponded to measurable periods and were especially useful in coastal waters where timing was critical for safe entry into harbours.
For shorter intervals, sailors increasingly relied on sandglasses, known in England as “running glasses” or sea-clocks. These devices allowed time to be divided into standard units that could then be linked to distance and speed.
Memory, Direction, and Mental Mapping
Despite the increasing use of instruments, medieval navigation remained fundamentally dependent on memory. Sailing directions were not meant to be casually read at sea but memorised in advance. Pilots needed to recall sequences of bearings and distances, often structured in repetitive formulae.
A typical example from Pierre Garcie Ferrande demonstrates this reliance on directional relationships:
Lisle d’Ieux et le boucaut de Bayonne gisent nort noroest et su suest et ya de lun a lautre : lxxiiii lieues.
Which means:
The Isle of Yeu and the mouth of the Bayonne Channel lie north-northwest and south-southeast of one another, and the distance between them is seventy-four leagues.
Such instructions formed a cognitive framework through which sailors mentally reconstructed routes. Combined with the magnetic compass, which spread through Europe from the twelfth century onward, these memorised directions allowed navigators to maintain orientation even far from land. In this sense, memory acted as a navigational technology in itself—a “mental chart” constructed from learned measurements.
Medieval maritime navigation in northern Europe was built upon a delicate interplay between observation, measurement, and memory. Sailors transformed the sea from an apparently featureless expanse into a space structured by depth, direction, time, and duration.
Whether through lunar calculations, tidal rhythms, sounding lines, or memorised sailing directions, they created a practical system that made movement across the ocean possible. These techniques did not constitute a theoretical conquest of the sea, but rather a pragmatic adaptation to its uncertainties.
Dr Lorris Chevalier, who has a Ph.D. in medieval literature, is a historical advisor for movies, including The Last Duel and Napoleon. Click here to view his website.
Click here to read more from Lorris Chevalier
Further Readings:
Laget, Frédérique, “Mesurer l’espace et le temps en mer à la fin du Moyen Âge,” in Mesure et histoire médiévale, édité par Société des historiens médiévistes de l’Enseignement supérieur public, Éditions de la Sorbonne, 2013.
Top Image: UBK 2° Ms. poet. et roman. 1 fol. 21r
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