Fifteenth Century Tools for Navigation

Captain Nuno Tristão employed many ancient tools for navigating his way down the coast of West Africa and home to Lisbon again. Here is a list of what he had at his disposal.

Telling Time

There were no clocks as we know them in Captain Tristão’s day. Marine chronometers and portable wind-up clocks were not developed until the mid-1700s. Since the Mesopotamians divided a day into twenty-four hours two thousand years earlier, navigators used the hourglass or sand-glass to determine when one hour stopped and a new one began.

The hourglass looked like two glass globes connected by a narrow opening. One of the globes was filled with sand. The ship’s officer in charge of minding the hourglass positioned the instrument with the sand-filled globe on top. In a full-sized hourglass, it took exactly one hour for the sand to seep through the narrow passageway to the globe at the bottom. To measure another hour, the navigator inverted the hourglass to allow the sand to seep back into the other globe. For the confined quarters of a ship, navigators carried mini sand-glasses that measured a half-hour or less.

Here is a factor that will confuse you. Hourglasses only measured regular hours. In Captain Tristão’s day, a farmer, who was more interested in the placement of the sun in the sky than in the mathematics of traveling a distance, simply divided every day into twelve hours no matter whether it was summer or winter. The farmer was measuring the day in irregular hours. In winter time, daytime hours were short and nighttime hours were long. In the summer time, daytime hours were long and nighttime hours were short. The astrolabe, an instrument we will introduce in the next article, had conversion tables on them to help navigators translate regular hours into irregular hours and visa versa.

Time zones, as we know them, were not developed until train schedules demanded them in the 1800s. Until then, noon in Gibraltar occurred at a different time than noon occurred in Lisbon, even though today, those places are in the same time zone, and noon occurs at the same moment.

Time of Day

To figure out which hour it was in the day, Captain Tristão used another ancient device called a sun dial. It measured the sun’s daily path across the sky in irregular hours – i.e. it divided every day into twelve hours. Hatch marks around the circumference of the base were numbered with the twelve hours using Roman Latin numerals. A narrow piece of metal or wood projected perpendicularly from the middle of the disk. That was called a shadow vane or gnomon.(1)

Like the ancient stone circles, the sun dial worked in a position level with the earth. Just as stone circles were oriented toward the north star, the navigator turned the sundial so that the Roman numeral XII [twelve] pointed toward the north. [We will talk about compasses in a second.] As the sun shone on the sundial during the day, the gnomon’s shadow moved around the dial. In the example above, the gnomon’s shadow lands on the Roman numeral ‘I‘ for one o’clock mark [o’clock is an abbreviation of one of the clock]. Again, this is a bit difficult to maneuver when rolling around at sea.

Noon, or twelve o’clock, was observed when the gnomon cast no shadow. The word noon comes from the Latin word nine and meant ninth-hour. The ninth-hour was originally three o’clock in the afternoon, an important time to Christians because that is the time that Jesus died on the cross – “at the ninth hour. ” As you have probably noticed, Medieval people followed many traditions based on Biblical stories.

By the 1400s, noon had become the time of day when the sun cast no shadow. Aboard ship, noon marked the end of one day and the beginning of the next. Translating the irregular hours into regular hours, when the navigator announced “make it noon,” an officer struck the ship’s bell eight times [signifying that the ninth hour was about to begin] while another officer started the hourglass. For the rest of the day, each time the sand ran out, the ship’s bell ‘sang again’ and the officer turned the glass. In order for the navigator to translate hours into degrees, he needed to know the amount of time that passed in regular hours.

When sailing directly east or west, the time of sunrise changes by one regular hour every 15 degrees longitude. For example: if you were floating at 150°W longitude and the sun rose at seven o’clock, then it was eight o’clock where another ship was floating at 165°W longitude.

Direction North, South, East, or West

The earth is like a great big battery. The north pole is the positive end of the battery and the south pole is the negative end. Someone back in time noticed that a certain rock had a natural attraction to the metal iron. We call that type of rock, or mineral, magnetite. As a tool, the magnetic rock is referred to as a lodestone. If iron is present, the positive end of the stone will turn its attention to the iron. If no iron is present, the positive end will turn its attention to the north pole.

To allow the stone freedom of movement, you can attach a lodestone to a piece of wood and float the wood in a pool of water. The positive end of the rock will make the wood turn to point toward the Earth’s magnetic north [which is not exactly the North Pole]. Every time you turn the wood and let go, the floating lodestone will turn the wood back to the first direction to which it pointed.

The oldest known extant lodestone – actually, only half of it still exists –  was uncovered in the Olmec ruins near Veracruz, Mexico. It is classified as M-160 [i.e. Michigan University’s artifact No.160]. Carbon dating of the site where the lodestone was found indicates the device was used by the Olmecs between 1400 and 1000 BCE. According to John B. Carlson, an astronomy instructor at the University of Maryland who tested the tool extensively, it is the earliest known way-finding device found in the Americas and predates the Chinese version of a compass by a millennium.(2)

Photo of M-160 by John B. Carlson, Science Magazine, 1975. Measure marks in centimeters, i.e. the remains of the stone is over four centimeters long.(3)

You can magnetize a piece of iron, such as a needle, by stroking a lodestone along it. Then if you place the needle on the piece of wood, it, too, will turn and point to the magnetic north pole. We call this a magnet. Eventually, mankind learned to balance the needle on a fulcrum, insert both the needle and the fulcrum in a small container, and mark the edges of the container with the 360 degrees of a circle to make a magnetic compass.

Some compasses, as shown by the illustration at the beginning of this section, had a folding shadow vane or gnomon that created a sun dial. That made it easy for the navigator to point his sun dial north.

The Chinese were credited for inventing the magnetic compass during the Han Dynasty between 220 to 280 CE. However, the discovery of the Olmec lodestone has put that date into question. The Chinese used the magnetic compass for the same purpose that the Olmecs used a lodestone: to orient their cities toward a north-south axis. The Chinese built grave monuments for important people with a north-south orientation. They did not use the magnetic compass for navigation on the open seas until around 1040 CE. By that time, the Arabs had already learned about the device and taken the technology to the Iberian Peninsula where the Western Europeans learned about it.

The North Star

The other way Captain Tristão found north was by finding the north star in the night sky. The Babylonians had figured out that one star in the northern sky stayed in place while the other stars swirled around it. They called that star the pole star, hence Western man, during the first millennium, named the star in that position with the Latin name Polaris. However, Polaris – the fiftieth brightest star in the sky – has only been the north star since 500 CE. During the 26,000-year Precessional Cycle, Earth slowly changes position in relation to the other stars. Different stars take their place as the north star. In 10,000 BCE the fifth brightest star, Vega, held the position. In 3942 BCE it was Thuban. In 1900 BCE it was Kochab, which is much brighter than Thuban. In 1793 BCE it was Theta Boötis. Polaris will reach the most northern point in our sky on March 24, 2100 CE. Then it will move away from the north pole. In 20,346 CE, Thuban will be the north star again!

To find Polaris, Captain Nuno Tristão first found the constellation we call the Big Dipper. [The English called it the Plough [Plow] or Charles’ Wain. Wain was Dutch-German for wagon. The Latin name is Ursa Major, which means Big Bear.] Captain Tristão followed the line of the outer edge of the Big Dipper to the star at the end of the handle of the Little Dipper. That was Polaris. [You will notice that Kochab, the north star while the Egyptians were building their pyramids, is in the same constellation with Polaris.]

At noon on the winter solstice, December 23, a navigator on a ship floating over the equator will find the sun directly overhead. At midnight, he will find Polaris at the horizon, a difference of 90° [a quarter of the earth’s circumference].

Latitudes and Longitudes

Cartographers have been developing a grid system since before Eratosthenes (276-195/4 BCE) drew the map shown above. He divided the world with parallels and meridians, but he did not relate the divisions by degrees as we do now.

To help measure and place landmasses in relation to oceans and each other on maps, cartographers developed a grid for the globe. The north-south lines are known as longitudes. The east-west lines are known as latitudes. In the diagram above, a longitude or latitude line marks every 15 degrees. As mentioned earlier, every 15 degrees translates to one hour. [360°/15° = 24 regular hours in a day].

Each latitude and longitude is numbered. The equator, which is a latitude, runs east and west at zero degree [0°] latitude. The north pole is at 90 degrees north latitude [90°N]. The south pole is at 90 degrees south latitude [90°S]. Longitudes are labeled east [E] and west [W] in relation to the Prime Meridian, which is at zero degree [0°] longitude.

The Prime Meridian is to longitudes what the equator is to latitudes – point zero. But whereas the equator is always in the same position relative to the earth’s axis, the Prime Meridian is arbitrary. A cartographer could choose any longitude he wished to be the Prime Meridian. But then, navigators needed more consistency than that.

Ptolemy, back in 150 CE, was the first cartographer who wanted a consistent longitude coordinate. He placed 0° Longitude at the farthest point west he knew of, which was the western edge of the the Fortunate Islands [either the Canary or the Cape Verde islands]. That was the left extreme of his map. The rest of the longitudes were numbered to the right.

The Portuguese thought Cape St. Vincent was the farthest point west, so they placed the Prime Meridian there. When they [re-]discovered the Cape Verde Islands farther west, they moved the Prime Meridian . Some Muslim Arabic maps lined the Prime Meridian up with Mecca in the center. Relatively recently, it was internationally agreed that the Prime Meridian would line up with Greenwich, England.(4) That is also the international date line. The line that continues from the Prime Meridian to the other side of the globe along 180° Latitude is called the Anti-meridian.

By the 1400s, mathematicians, astronomers, and cartographers were dividing latitudes and longitudes into smaller units called minutes. There are sixty minutes in each degree of latitude and longitude, just as there are sixty minutes in an hour. The minutes are noted with the foot symbol [']. For example, if you want to find Cape Cod, Massachusetts, you point your ship to the position where 41°40'N crosses with 70°12'W as shown in the image of the globe above.


Captain Nuno Tristão determined latitudes with an instrument developed back in Babylonian times called a cross-staff. The cross-staff measured the angle of the sun to the horizon at its highest point at noon or by the height of Polaris at night.

You can find a latitude by yourself. At noon, stand straight and tall. Point one of your hands toward the sun and the other toward the horizon. Measure the angle between your arms. With that angle, you can determine your latitude.

A mariner needs to be a bit more precise. An error of one minute of latitude translates to an error of 1.15 nautical miles at sea.

The earliest written records about the cross-staff come from Baghdad and India in the 6th century. A Persian mathematician and astronomer named al Fazari wrote about using one in the 8th century. Al Fazari may have learned about the instrument from the old astronomical texts from India that he was employed to translate. Like so many ancient instruments, the Arabs brought the technology with them to the Iberian Peninsula, which is where Henry the Navigator and his captains learned about them.

The device consisted of a wooden stick about one and a half feet long that passed through two or more sticks of gradated sizes called crosspieces or transoms. All the parts were marked in units to measure degrees. The navigator held one end of the long stick to his eye and pointed the other end between the sun and the horizon. Then he moved the transoms up and down the stick until the tops of all three lined up with the sun, and the bottoms of all three lined up with the horizon. This gave the navigator the angle measurement he was looking for.

Unfortunately, mariners burned the retinas of their eyes when they stared at the sun. Historians wonder if that is why pirate captains earned the reputation for being blind in one eye. It probably explains the ‘eye problems’ Christopher Columbus suffered from at the end of his career. A navigator could use Polaris in place of the sun, but for that, he needed a device called an astrolabe, which we discuss in the next article.

Measuring the Depth of the Sea Floor

Mariners needed to know how far below their ship lay the sea floor. If the water was too shallow, his boat ran aground. If it was too deep, his anchor line could not reach. To find out, he employed a device used by the ancient Phoenicians called a sounding lead.

Sounding leads do not make noise. The word sound came from the old English word sund that meant swimming, water, or sea. The tool was a cylindrical plummet [rod], made out of heavy lead attached to the end of a sounding line [long rope]. The plummets varied in size. A plummet found on the ruin of the 16th century Mary Rose [now in the Mary Rose Museum in Plymouth, England] is about an inch in diameter and a foot long.

The plummet was cast out of iron with a ring at one end and a cavity at the other. The sounding line attached to the ring. The navigator filled the cavity – about an inch or two deep – with bees’ wax or tallow [animal fat] before he threw the plummet in the water. [We will tell you the purpose of the wax or tallow in a minute.]

The measurement unit for depth is a fathom. A fathom is the distance between a man’s fingertips if he stands tall and extends his hands in each direction side to side. It is about six feet. We suggest you try it for yourself.

The old English word fæthm meant “something that embraces” [the length of an embrace]. The fathom measurement has since been standardized to six feet. Way back in the early days, a seaman pulled the line out of the water and measured how long it was by stretching the line between his arms, one length at a time [one fathom at a time].

To make counting easier and faster, navigators tied small leather tags, or knots of various shapes and sizes [most no bigger than an inch] along the sounding line at fathom measures. The ties were known as markers, or marks. Each marker looked and felt different from the others. A skilled boatswain [bosun] could tell which marker was which with his eyes closed. For example, the two-fathom marker might be a short tie with a round knot, and the seven-fathom marker might be a longer tie with a square knot. That allowed the bosun to differentiate the markers at night, or on a foggy morning when he could not see clearly. It also double checked his counting.

Markers were not tied at every fathom measure on the line. Traditionally, they were tied at two fathoms, three fathoms, five, seven, ten, thirteen, fifteen, seventeen, and twenty fathoms. [What would twenty fathoms be in feet?(5)]

To use the sounding lead, a bosun stood on the starboard side [the right side of the ship if facing forward] at the waist [center of the ship] facing the bow [front end]. He held the lead in one hand and the sounding line loosely coiled in the other hand. If the boat was stationary, the bosun dropped the lead in the water from where he stood. The extra line threaded through the bosun’s hands – not as easy as it sounds. The bosun needed to hold the coil loosely enough for the line to feed through his fingers, but tightly enough to prevent the line from running out and dropping into the water. It was not a good idea to drop the sounding lead in deep water. Storage space on ships was at a premium. There were few spare parts.

When the plummet touched the sea floor, the bosun looked at the line to see which knot was closest to his hand. Or, if it was dark, he felt for the nearest knot with his fingers. If the measurement was right ‘on the mark,” the bosun called out to the captain, master, or helmsman, “by the mark,” such as “by the mark ten.” But if the measurement fell between the markers, he called “by the deep” and guessed the number in between. For example, if the measure landed between markers ten and thirteen, he might call out “by the deep eleven.”

The process was a bit more complicated when the boat was moving along in the water. First the boson tied himself to the ship’s rigging so he did not fall overboard. Again, he held the coil in one hand and the line with the plummet attached in the other hand. He let out some of the line with the plummet attached, allowing himself to swing the plummet back and forth along the side of the ship.

Once the plummet had gathered speed and was swinging high enough, the bosun let go of the line, throwing the plummet far forward of the ship; sort of like sending out a fishing line, only underhand. Hopefully, the plummet sank to the sea bottom by the time the ship caught up to the line. When the line was perpendicular to the level of the sea, the boson grabbed tightly to the line and looked, or felt, to see which fathom marker was nearest his hand.

Determining the Condition of the Ocean Floor

The purpose of the wax or tallow inserted in one end of the sounding lead was to determine the condition of the sea floor. Was it sandy? Rocky? Or muddy? The condition of the sea floor determined how well your anchor would catch and which anchor you should use.

Before dropping or throwing the plummet, the boson made sure the wax in the end of the plummet was polished to a smooth finish.

After the bosun measured the depth of the sea floor, he hauled up the plummet and inspected the wax [or tallow] in the cavity at the end of the plummet. If the wax was still smooth, the bosun knew the sea floor was hard mud, shale, or smooth rocks. If there was sand impressed in the wax, the floor was sandy. If the wax was scratched, the sea floor was covered in sharp rocks.

Determining Speed

Navigators used a chip-reel, or log-reel, [also called chip-log, ship-log and just log] to measure how fast the ship traveled.

This ancient tool consisted of a chip or small log of wood tied to the end of a long line. Similar to a sounding line, knots tied at consistent intervals provided measurement marks. The measurement units were termed with the obvious name knots. One knot signified one nautical mile. In actuality, the knots were tied at intervals that were fractions of a mile; otherwise, the line would have been a mile long. The chip or log kept the line afloat when the line was thrown in the water – in contrast to the plummet of a sounding line that was meant to sink.

A chip-reel was used in conjunction with an hourglass. The navigator, or boson, stood at the stern of the ship as she moved along, facing away from her. In simultaneous actions, another officer turned the hourglass, while the navigator threw the log into the water toward where the ship had just been. The log remained in one place on the water as the ship moved away from it, and as the navigator let out the line. The speed was determined by how many knots passed through the navigator’s fingers during the fraction of an hour shown by the hourglass. Today, we still measure the speed of a ship in knots per hour, only we use more sophisticated tools for obtaining the information.


  1. The word gnomon was a Latin word that came from the Greek word for a carpenter’s square. In other words, it originally referred to the right angle it made with the base.
  2. According to China expert Joseph Needham, the Chinese geographer Shen Gua from the Song dynasty described, in 1088 CE, the technique for using a magnetized needle suspended from a length of a silk string to determine the direction of south. The next reference to a ‘magnet’ anywhere else in the world was written in 1188 CE.
  3. The lodestone was excavated by P. Krotser of the Yale University Excavation Project headed by well renowned archaeologist Michael D. Coe. John Carlson was an instructor in astronomy in the Astronomy Program, Department of Physics and Astronomy, University of Maryland, College Park 20742. Carlson, John B/ “Lodestone Compass: Chinese or Olmec Primacy?: Multidisciplinary analysis of an Olmec hematite artifact from San Lorenzo, Veracruz, Mexico.” Science Magazine, September 5, 1975. Found online at
  4. Later maps indicated the degrees longitude in their relationship to London. For example, it might say “45 degrees west longitude from London” or “22 degrees east longitude from London.” Placing the Prime Meridian at Greenwich gave navigators sailing from London, versus other ports, a slight mathematical advantage. It was easy to start counting from zero when they recorded the distances they traveled in degrees longitude.
  5. 20 fathoms x 6 feet = 120 feet

Now for the most amazing tool of all, the Astrolabe.