Navigation Tools

Lead Line

Sounding Lead, 7 lbs., marked in 25 fathom increments (156 feet), The Mariners’ Museum

Sounding Lead, 7 lbs., marked in 25 fathom increments (156 feet), The Mariners’ Museum

The Lead Line, a device for measuring the depth of the water as well as obtaining a sample of the ocean floor, is one of the oldest of all navigating tools. The word “lead” is pronounced the same way as in “lead pencil”.

It began with the oldest known boat trading peoples, the Egyptians. We have images of their river trading craft going back to about 3400 BCE. Then, as now, it was inconvenient to run aground….it could ruin your entire day just as quickly as a collision. The earliest device to measure depth was a stick. At first it was unmarked with any depth scales. In time, it would have been. As trading expanded beyond the rivers of Egypt and onto the coast and into the Mediterranean Sea, a stick was no longer adequate. A rock could be tied to the end of a line and dropped over the side. The depth of the water could be measured as you retrieved the line and stretched the line between your arms. By the Fifth Century, BCE, the Greeks were using a lead line which is mentioned by the Greek historian Herodotus.

By 1600 in England, the lead line was being marked at certain depths to make the reading easier: 2, 3, 5, 7, 10, 15, 20, and 25 fathoms. A fathom, from the Old Norse word, fathmr, for “outstretched arms” was standardized at six-feet: an average distance between a man’s outstretched arms as he held the lead line. The standard lead linewas 20 fathoms long–120 feet–and the lead weight 7 pounds. That number may seem curious now, but in England of the 1600’s, weights were routinely measured in a 14 pound increment called a “stone”. A half stone, or a “clove” was seven pounds. The lead itself was cylinder-like, but slightly fatter at the bottom than the top and a loop was cast into the metal so that a line could be attached.

Besides the depth, the lead could also tell the mariner about the type of ocean bottom he was sailing over. The bottom of the lead weight was hollowed out so that a glob of tallow, or animal fat, could be inserted. A glob is a non-scientific, but highly descriptive term for tallow about the size of a golf ball. Since Golf hadn’t been invented yet, the “glob” would do. When the fat glob hit the bottom, some of the material stuck into the fat. Just as on land the surface varies from rock, to sand, to dirt, to pebbles, so too does the ocean bottom. Knowing the material on the bottom along with the depth was an additional means to determine where you were on the featureless ocean as the bottom changes drastically as you travel.

As the leadsman retrieved the line, he’d call out the depth. If it were exactly as measured on the line at 2, 3, 5, 7, etc. fathoms, he called out a “mark”: “By the mark seven”. If he estimated it to be one-quarter less, he’d say “A quarter less seven”. If it were more,” and a quarter seven”, or “and a half seven”. If he estimated a reading in whole fathoms, but not marked, he called it a “deep”; “by the deep four”. Estimates were only made in quarter, half fathoms and whole fathoms.

Time Keeping

Chronometer, Reproduction to Look like a 1785 Chronometer made by John Arnold and Sons, London, England, The Mariners’ Museum(1933.0162)

Time keeping on a ship has some special problems: the great variation of temperature on a ship at sea, the motion, and the humidity can all affect the accuracy of the time keeper.

At first, sailors could track time just by estimating the position of the sun. From sunrise to noon was about six hours, and another six hours from noon to sunset. There were actually several times during the year when the day was 12 hours long and this system worked fine. But most days are either longer or shorter. Of course, for several thousands of years, no one cared that much about the exact time and this estimating was acceptable. The sand-glass or hour-glass was developed about 800-900 AD and allowed for a reasonably accurate measure of the passage of time.

When modern sailors tried to “keep” time they found the sand-glass met their needs quite well. They used at least two or three variations. To keep track of their speed they used a device called a “Chip Log” which required a sand-glass of 30 seconds duration. To keep track of how long they worked, they used a sand-glass of 30 minutes duration. Now the duty time aboard ship, called a watch, is traditionally four hours, and this can be traced back to Egyptian times thousands of years ago, so you might expect a sand-glass lasting four hours. But this would be too heavy and the sand in it more likely to clog. Instead, sailors used a sand-glass of 30 minutes length. By the way, the content of the “sand” glass usually was not sand, but a mixture of ground-up sea shells, stone, egg shells, marble or other materials which would be less like to stick together than sand. A “glass” of an hour’s duration might also be found aboard ship.

To make sure the ship’s boy was paying attention, for it was usually his job to turn the sand-glass every 30 minutes, and to let everyone know the time, each time the glass was turned, the ship’s boy would ring the ship’s bell. It would be rung once for each half hour of a four hour watch. So after the second hour, four bells would be rung; after the third hour, six bells. To enable people’s ears to distinguish how many bells there were, they were rung in pairs, two rings at a time. When eight bells were rung, four hours had passed and the next watch hearing the eight bells would come up to relieve the crew currently working. The count would begin again with one bell being rung after half an hour into the new watch, and so on till eight bells would be rung again. With 24 hours in the day, there were six watch.

Noon was a significant event to the sailor right up until modern times and electronic navigation. Before electronic navigation, the sailor was able to find his position on the voyage by measuring the altitude of the sun above the horizon at noon. This was so important, that, at sea, the day began at noon, not at midnight. In the morning it might be Thursday, but at noon, it became Friday. This was true right up through 1924.

Accurate time keeping was highly desirable, especially for navigating across the ocean, but it was not possible until the invention of a very accurate watch in 1759 by John Harrison of England. While it was accurate, it was also expensive and could cost as much as an entire year’s salary for the captain of a ship. As more people produced these accurate clocks, called “Chronometers”, the price came down. Today, a $10 electric watch is as accurate as the older chronometer of the late 1700’s. costing over 1,000 times as much.


Back-Staff, Peter Ifland Collection, The Mariners Museum, (1998)

The Cross-Staff, in one very early version, goes back as early as 400 BCE. That’s a very long time ago. The Chaldeans in the Mid-East used it, but sailors did not use it until the early 1500s; the first recorded date was 1514. As with other early navigation instruments, the first use for the Cross-Staff was in astrology, in measuring the altitude of stars to help forecast the future. Sailors only became interested in it as a navigational tool when their voyages took them to places unfamiliar to them, such as Africa, India and theNew World.

The first European to learn of a device similar to the Cross-Staff, the Ka-Mal, was Vasco da Gama, who learned of the Ka-Mal from the Arabs when he visited India in 1498. The Cross-Staff, as used by sailors, was developed from the Ka-Mal.

The observer of the sun or Polaris would place the end of the long part of the Cross-Staffbelow his eye and observe the sun/Polaris across the upper part of the cross-bar while also observing the horizon at the bottom of the cross-bar. This can be done by moving the crossbar closer or farther from the observer’s eye along the long central bar. The crossbar was called the transom and the long central part was the staff. When both the heavenly body (the sun or Polaris) and the horizon were lined up with the transom, the observer could read the angular altitude (degrees) on a scale on the staff.

This angular altitude could then be mathematically converted to the latitude of the observer.

The transom (cross-bar) came in different lengths depending on the altitude of the body to be measured. The smallest transom was for altitudes of about 15 °, the next size was for altitudes of about 30° and the last size for altitudes of about 60°. The best range for the use of the Cross-Staff was for bodies between 20 ° and 60 °. Smaller and larger angles could be read, but they were not as accurate. The staff (long part of the instrument) had a different scale for each transom marked on the staff. Therefore, there were usually three scales inscribed on the staff.

When a person was holding the Cross-Staff, it looked like that person was shooting a bow and arrow; the transom looked like the bow and the staff like an arrow. This gave rise to the term “shooting the sun” whenever someone measured the angle of the sun above the horizon, even when the instrument did not look like a Cross-Staff.


One of the oldest of all the altitude measuring devices, the Astrolabe is an angle-measuring tool that’s name comes from the Greek, “to take a star.” It was possibly invented by the Greek astronomer and mathematician, Hipparchus (190-120 BCE). However, in its earliest uses, it was for astronomy and astrology. Only when the need to measure angular heights of Polaris became important did we see these instruments adopted for sea-going use. As an astronomer’s tool, the Astrolabe was introduced to the Europeans by Arab astronomers in the 10th century, CE. But the first documented use of it used at sea is in 1481 on a voyage down the African coast by Portuguese explorers. It is likely, though, that it was in use by sailors for many years before that.

VK801J35UsingAstrolabe.jpgSo how does it work? To correctly measure the angle of the sun or a star, the Astrolabe must hang down so that it is perpendicular to the ocean. If it’s tilted right or left, or front to back, the angle will not be accurate. To keep it straight, the user holds it with a finger through a ring and lets the Astrolabe dangle. Next, take a look at the diagram and see that there are two plates on a rotating arm. They each have a pinhole perfectly lined up so that when the sun shines through the top one, and hits the second pinhole, the angle is accurate. You then read from the scale along the circumference.

For Polaris, you can sight over the edge of the two plates. One advantage of the Astrolabe is that you do not need a clear horizon to use the instrument; you do need a clear horizon when measuring the height of the sun or Polaris with other navigational instruments.

Astrolabe, A New Collection of Voyages, Discoveries and Travels: Containing Whatever is Worthy of Notice in Europe, Asia, Africa, and America, 1767, From The Library at The Mariners’ Museum, G160.K75 rare.

It is not a particularly accurate tool at sea because of the difficulty in keeping it steady in a rolling ship and high winds. Usually, however, the Portuguese explorers would take their Astrolabe ashore and set it up to avoid this problem; this is what they did when they were mapping the coast of Africa in their early exploration. Using it at sea could result in errors as much as five degrees, or 300 miles. However, ashore, it would be much more accurate, certainly less than one-half degree, or 30 miles. The sea-going version might be 6″ in diameter, whereas the one they took ashore (and which would be awkward to use at sea) might be two feet in diameter, making it more accurate and easier to read.

Polynesian Navigation

Marshall Islands Stick Chart, Marshall Islands, Kwajalein. The bamboo slats represent routes to specific places marked by the shell, such as islands. Reproduction, The Mariners’ Museum

As Captain James Cook was conducting his voyages of exploration and discovery, Polynesian navigators had already successfully explored and settled the islands from New Zealand to Hawaii. Remarkably, the Polynesians had developed a sophisticated and reliable means of wayfinding based not on science and mathematics, but rather on their innate knowledge of the seas and sky.

By using the sun, stars, sea swell patterns, cloud formations, and seamarks such as bird flight habits, Polynesian navigators were able to steer their canoes over distances that amazed European navigators – including the two thousand miles between Tahiti and the Hawaiian Islands.

The Polynesian star compass was the key to finding direction at sea. The four cardinal points (north, south, east, and west) were located according to the rising and setting sun. During night voyaging, stars formed reference points. Polynesian navigators memorized the star compass as well as known islands whose locations corresponded to points of the compass. In training, a navigator would name an island as the center point, then go around the compass points naming the islands that lay in each direction.

Beyond navigating by the sun and stars, the Polynesians used their extensive knowledge of the sea to successfully guide them through their voyages. By careful observation of sea swell patterns, wind direction, cloud formations, and patterns of bird flight and flotsam, traditional Pacific navigators pieced together the course they chose to follow.

Sea Swells: Sea swells are waves that have moved beyond the wind or storms that generated them. Swells tend to be more regular and persistent in their flow than waves. By observing the swells and understanding the winds that created them, Polynesian navigators could steer their canoes according to the swell patterns. Interestingly, swells are more easily felt than seen.

Winds: Winds were also used to determine direction. However, wind changes could occur during the course of a day’s voyage. To better observe these changes, Polynesian navigators fixed lightweight wind pennants made of feathers and bark to the masts of their canoes.

Cloud Formations: As clouds moved over sea and land, the Polynesians noted that clouds tend to be drawn to land in distinctive “V” formations. This cloud pattern is created by the reflection of heat radiated from the island. Many navigators also noted slight color changes in clouds over land, and were able to distinguish the landform from the color; a slight green indicated lagoon islands, bright clouds indicated sand, and dark clouds marked forested areas.

Flight of birds: Flight patterns of specific species provided a reliable means of determining the direction of land. The fairy and noddy terns were especially important, as both species nest on land, and neither swims. Both terns fly to sea in the morning and return to land at dusk. By observing the habits of these birds, Polynesian navigators could not only determine the direction of land, but also its approximate distance. Fairy terns have a flight range of about one hundred twenty miles, while noddy terns have a range of about forty miles.

Flotsam: Floating debris such as palm fronds, coconuts, and other vegetation also signaled nearby land.

Experimental “wayfinding” has been traditionally performed in Micronesia, and is regaining credence as an art in Polynesia.

Back Staff

A major problem in using the Cross-Staff was in having to look at the sun. This led to blindness or at least damaged eyesight for navigators. To solve the problem, John Davis (sometimes spelled “Davies”) invented an instrument in 1594 (published description–1595) that used the shadow of the sun instead of the direct view of the sun to obtain the altitude. It also eliminated the need to look in two directions at the same time. Now a navigator could look at the horizon and line up the shadow of the sun with the horizon at the same point on the instrument.

The observer adjusted the shadow vane, the upper, left scale in the diagram, so that the sun would cast its shadow on the horizon vane, the lower, left-hand object. Sighting from the back (right-hand side in the diagram), the observer would adjust the eyepiece at the back of the Back-Staff so that the horizon and sun’s shadow were aligned. He would then read the scale of the shadow vane, add it to the scale of the eyepiece and thereby obtain the altitude of the sun. As with other height-measuring methods, this number was used to obtain the latitude of the observer.

Davis invented the instrument in 1594, but both he and others made a number of modifications of his original instrument. Of course, it could not be used to measure the altitude of Polaris. Why? No shadow. The Back-Staff was very popular and soon most mariners used it in place of the astrolabe or Cross-Staff. While it was more accurate than either of those instruments, it would be replaced in the mid-1700s by the Octant or Sextant, which would be even easier to use and more accurate.


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