When a ship is approaching a rocky coast and the
life of the ship and its crew depends on a fast and accurate answer. It's the Navigator's job to provide the answer.
So what do navigators need to find their position on the earth's surface by observing the stars?
They need an Almanac prepared by the astronomers to forecast precisely where the heavenly bodies, the sun, moon planets and selected navigational stars,
are going to be, hour by hour, years into the future, relative to the observatory that prepared the almanac, Greenwich, England in modern times.
They need a chronometer or some other means of telling the time back at the observatory that was the reference point for the data in the almanac,
It is the cartographer's job to provide accurate charts so that navigators can establish their position in latitude and longitude or in reference
to landmasses or the hazards of rocks and shoals.
The navigators need a quick and easy mathematical method for reducing the data from their celestial observations to a position on the chart
Finally, navigators need an angle-measuring instrument, a sextant, to measure the angle of the celestial body above a horizontal line of reference.
How do navigators use the stars, including our sun, the moon, and planets to find their way? Well, for at least two millennia, navigators
have known how to determine their latitude, their position north or south of the equator. At the North Pole, which is 90 degrees latitude,
Polaris (the North Star) is directly overhead at an altitude of 90 degrees. At the equator, which is zero degrees latitude, Polaris is on
the horizon with zero degrees altitude. Between the equator and the North Pole, the angle of Polaris above the horizon is a direct measure
of terrestrial latitude. If we were to go outside tonight and look in the northern sky, we would find Polaris at about 40 degrees
13 minutes altitude - the latitude of Coimbra. In ancient times, the navigator who was planning to sail out of sight of land would
simply measure the altitude of Polaris as he left homeport, in today's terms measuring the latitude of home port. To return after
a long voyage, he needed only to sail north or south, as appropriate, to bring Polaris to the altitude of home port,
then turn left or right as as appropriate and "sail down the latitude," keeping Polaris at a constant angle. The Arabs knew all about
this technique. In early days, they used one or two fingers width, a thumb and little finger on an outstretched arm or an arrow held
at arms length to sight the horizon at the lower end and Polaris at the upper.
To return to homeport, he would sail north or south as needed to bring Polaris to the altitude he had observed when he left home,
then sail down the latitude. Over time, Arab navigators started tying knots in the string at intervals of one issabah. The word
issabah is Arabic for finger, and it denotes one degree 36 minutes, which was considered to be the width of a finger. They even
developed a journal of different ports that recorded which knot on the kamal corresponded to the altitude of Polaris for each
port they frequently visited.
Throughout antiquity, the Greeks and Arabs steadily advanced the science of astronomy and the art of astrology. About a thousand
years ago, in the 10th century, Arabs introduced Europe to two important astronomical instruments, the quadrant and the
In the word "astrolabe", astro "means" star
and "labe" roughly translates as to 'take' or 'to find.'
The astronomer's beautiful, intricate and expensive astrolabe was the grandfather of the much simpler, easy to use mariner's
quadrant and astrolabe. The mariner's quadrant, a quarter of a circle made of wood or brass, came into widespread use for
navigation around 1450, though its use can be traced back at least to the 1200s.
Mariner's brass quadrant.
The quadrant was a popular instrument with
Portuguese explorers. Columbus would have marked the observed altitude of Polaris on his quadrant at selected ports of call just
as the Arab seaman would tie a knot in the string of his kamal.
Alternatively, the navigator could record the altura, or altitude, of Polaris quantitatively in degrees at Lisbon and at other
ports to which he might wish to return. It wasn't long before lists of the alturas of many ports were published to guide the
seafarer up and down the coasts of Europe and Africa.
During the 1400's, Portuguese explorers were traveling south along the coast of Africa searching for a route to the orient. As a
seafarer nears the equator heading south, Polaris disappears below the horizon. So, in southern seas, mariners had to have a
different way of finding their latitude. Under orders from the Portuguese Prince Henry, The Navigator, by 1480, Portuguese
astronomers had figured out how to determine latitude using the position of the sun as it moved north and south of the equator
with the seasons, what we now call its "declination." In simple terms, the navigator could determine his altura, his latitude,
by using his quadrant to take the altitude of the sun as it came to it's greatest altitude at local apparent noon, and then
making a simple correction for the position of the sun north or south of the equator according to the date.
The mariner's quadrant was a major conceptual step forward in seagoing celestial navigation. Like the knots-in-a string method of the Arab kamal, the quadrant provided a quantitative measure, in degrees, of the altitude of Polaris or the sun, and related this number to a geographic position, the latitude,n the earth's surface. But for all its utility, the quadrant had two major limitations: On a windy, rolling deck, it was hard to keep it exactly vertical in the plane of a heavenly body. And it was simply impossible to keep the wind from blowing the plumb bob off line.
A beautiful mariners' astrolabe.
All the complex scales were eliminated, leaving
only a simple circular scale marked off in degrees. A rotatable alidade carried sighting pinnules. Holding the instrument at eye
level, the user could sight the star through the pinnules and read the star's altitude from the point where the alidade crosses
Astrolabe in use.
The Persian mathematician Avicenna wrote about a cross-staff in the eleventh century. The concept probably arrived in Europe when Levi ben Gerson, working in the Spanish school at Catalan in 1342, wrote about an instrument called a balestilla that he described as a being made from a "square stick" with a sliding transom.
The Cross-Staff in use.
After 1650, most "modern" cross-staffs have four transoms of varying lengths. Each transom corresponds to the scale on one of the four sides of the staff. These scales mark off 90, 60, 30, and 10 degrees, respectively. In practice, the navigator used only one transom at a time. The major problem with the cross-staff was that the observer had to look in two directions at once - along the bottom of the transom to the horizon and along the top of the transom to the sun or the star. A neat trick on a rolling deck!
The observer determined the altitude of the sun by observing its shadow while simultaneously sighting the horizon. Relatively inexpensive and sturdy, with a proven track record, Davis quadrants remained popular for more than 150 years, even after much more sophisticated instruments using double-reflection optics were invented. One of the major advantages of the Davis back-staff over the cross-staff was that the navigator had to look in only one direction to take the sight - through the slit in the horizon vane to the horizon while simultaneously aligning the shadow of the shadow vane with the slit in the horizon vane. The major problem with back-sight instruments was that it was difficult if not impossible to sight the moon, the planets or the stars. Thus, toward the end of the 1600's and into the 1700's, the more inventive instrument makers were shifting their focus to optical systems based on mirrors and prisms that could be used to observe the nighttime celestial bodies. The critical development was made independently and almost simultaneously by John Hadley in England and by Thomas Godfrey, a Philadelphia glazier, about 1731. The fundamental idea is to use of two mirrors to make a doubly reflecting instrument - the forerunner of the modern sextant.
Diagram of Sextant.
Hadley's first doubly reflecting octants were made from solid sheets of brass. They were heavy and had a lot of wind resistance. Lighter wooden instruments that could be made larger, with scales easier to divide accurately and with less wind resistance quickly replaced them.
Early Hadley octant.
We have seen how navigators could find their latitude for many centuries but ships, crews and valuable cargo were lost in shipwrecks because it was impossible to determine longitude. Throughout the seventeenth century and well into the eighteenth century, there was an ongoing press to develop techniques for determining longitude. The missing element was a way to measure time accurately. The clock makers were busy inventing ingenious mechanical devices while the astronomers were promoting a celestial method called "lunar distances". Think of the moon as the hand of a clock moving across a clock face represented by the other celestial bodies. Early in the 18th century, the astronomers had developed a method for predicting the angular distance between the moon and the sun, the planets or selected stars. Using this technique, the navigator at sea could measure the angle between the moon and a celestial body, calculate the time at which the moon and the celestial body would be precisely at that angular distance and then compare the ship's chronometer to the time back at the national observatory. Knowing the correct time, the navigator could now determine longitude. When the sun passes through the meridian here at Coimbra, the local solar time is 1200 noon and at that instant it is 1233 PM Greenwich Mean Time. Remembering that 15 degrees of longitude is equivalent to one hour of time gives us the longitude of 8 degrees, 15 minutes West of Greenwich. The lunar distance method of telling time was still being used into the early 1900's when it was replaced by time by radio telegraph. An octant measures angles up to 90 degrees and is ideally suited for observations of celestial bodies above the horizon. But greater angle range is needed for lunar distance observations. It was a simple matter to enlarge Hadley's octant, an eighth of a circle, to the sextant, a sixth of a circle, that could measure up to 120 degrees.
An early sextant by John Bird.
A brass sextant by Dollond.
Examples of sextant frame designs.
Probably the finest 18th century instrument maker was the Englishman Jesse Ramsden. His specialty was accurate scale division. Here's a small brass sextant that Ramsden made shortly before his death in 1800. Ramsden's major achievement was to invent a highly accurate "dividing engine" - the apparatus used to divide the scale into degrees and fractions of degrees. His design was considered so ingenious that the British Board of Longitude awarded Ramsden a prize of 615 pounds - in 18th century terms, a small fortune. His "dividing engine" now resides in the Smithsonian Institution in Washington. The development of more precise scale division was a milestone in instrument development. Certainly, it permitted more accurate observations but it also permitted smaller, lighter, more easily handled instruments.
The Modern sextant 1988 .
By kind permission of Peter Ifland, Ph. D. in Biochemistry (U. of Texas) Commander in the US Naval Reserve and A.S.Alves University of Coimbra.
Our collection of Fine Solid Brass hand crafted Sextants are probably the finest reproductions of the traditional Nautical sextants, which have been used in celestial navigation since 1757.Our Sextants are based an a design bought in by Captain Cambell, but the original Octant from which the modern sextant came was made by John Hadley about 1731.The sextants are workable but not meant to be used for serious navigation. They make ideal nautical gifts for those who love the sea, or are collectors of historic navigational instruments.