Jacob's staff
Encyclopedia
- For the plant known as the ocotillo, sometimes called the Jacob's staff, see ocotilloOcotilloFouquieria splendens Engelm. is a desert plant of the southwestern United States and northern Mexico. Common names include ocotillo, desert coral, coachwhip, Jacob's staff, and vine cactus, although it is not a true cactus...
.
The Jacob's staff, also called a cross-staff, a ballastella, a fore-staff, or a balestilha is used to refer to several things. This can lead to considerable confusion unless one clarifies the purpose for the object so named. The most frequent uses of the terms are for astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
and navigation
Navigation
Navigation is the process of monitoring and controlling the movement of a craft or vehicle from one place to another. It is also the term of art used for the specialized knowledge used by navigators to perform navigation tasks...
and for surveying
Surveying
See Also: Public Land Survey SystemSurveying or land surveying is the technique, profession, and science of accurately determining the terrestrial or three-dimensional position of points and the distances and angles between them...
.
Astronomy and Navigation
In navigationNavigation
Navigation is the process of monitoring and controlling the movement of a craft or vehicle from one place to another. It is also the term of art used for the specialized knowledge used by navigators to perform navigation tasks...
the instrument is also called a cross-staff and was used to determine the vessel's latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
by measuring the altitude of Polaris
Polaris
Polaris |Alpha]] Ursae Minoris, commonly North Star or Pole Star, also Lodestar) is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole, making it the current northern pole star....
or the sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
.
The Jacob's staff, when used for astronomical observations, was also referred to as a radius astronomicus. With the demise of the cross-staff, in the modern era the name "Jacob's staff" is applied primarily to the device used to provide support for surveyor's instruments.
History
Jacob
Jacob "heel" or "leg-puller"), also later known as Israel , as described in the Hebrew Bible, the Talmud, the New Testament and the Qur'an was the third patriarch of the Hebrew people with whom God made a covenant, and ancestor of the tribes of Israel, which were named after his descendants.In the...
, specifically . It may also take its name after its resemblance to Orion
Orion (constellation)
Orion, often referred to as The Hunter, is a prominent constellation located on the celestial equator and visible throughout the world. It is one of the most conspicuous, and most recognizable constellations in the night sky...
, referred to by the name of Jacob
Jacob
Jacob "heel" or "leg-puller"), also later known as Israel , as described in the Hebrew Bible, the Talmud, the New Testament and the Qur'an was the third patriarch of the Hebrew people with whom God made a covenant, and ancestor of the tribes of Israel, which were named after his descendants.In the...
on some medieval star chart
Star chart
A star chart is a map of the night sky. Astronomers divide these into grids to use them more easily. They are used to identify and locate astronomical objects such as stars, constellations and galaxies. They have been used for human navigation since time immemorial...
s. Another possible source is the Pilgrim's staff, the symbol of St James (Jacobus in Latin). The name cross staff simply comes from its cruciform
Cruciform
Cruciform means having the shape of a cross or Christian cross.- Cruciform architectural plan :This is a common description of Christian churches. In Early Christian, Byzantine and other Eastern Orthodox forms of church architecture this is more likely to mean a tetraconch plan, a Greek cross,...
shape.
The original Jacob's staff was developed as a single pole device in the 14th century that was used in making astronomical measurements. It was first described by the Jewish mathematician Levi ben Gerson of Provence
Provence
Provence ; Provençal: Provença in classical norm or Prouvènço in Mistralian norm) is a region of south eastern France on the Mediterranean adjacent to Italy. It is part of the administrative région of Provence-Alpes-Côte d'Azur...
. However, its invention was likely due to Jacob ben Makir
Jacob ben Machir ibn Tibbon
Jacob ben Machir ibn Tibbon, of the Ibn Tibbon family, also known as Prophatius.Provençal, Jewish astronomer; born, probably at Marseilles, about 1236; died at Montpellier about 1304. He was a grandson of Samuel ben Judah ibn Tibbon. His Provençal name was Don Profiat Tibbon; the Latin writers...
who also lived in Provence in the same period. Attributions to 15th century astronomer Georg Purbach
Georg Purbach
Georg von Peuerbach was an Austrian astronomer, mathematician and instrument maker.-Biography:...
are less likely correct, since Purbach was not born until 1423. Such attributions may refer to a different instrument with the same name. May states that its origins can be traced to the Chaldeans around 400 BC.
Although it has become quite accepted that Levi ben Gerson first described Jacob's staff, the Sinologist Joseph Needham
Joseph Needham
Noel Joseph Terence Montgomery Needham, CH, FRS, FBA , also known as Li Yuese , was a British scientist, historian and sinologist known for his scientific research and writing on the history of Chinese science. He was elected a fellow of the Royal Society in 1941, and as a fellow of the British...
theorizes that the Song Dynasty
Song Dynasty
The Song Dynasty was a ruling dynasty in China between 960 and 1279; it succeeded the Five Dynasties and Ten Kingdoms Period, and was followed by the Yuan Dynasty. It was the first government in world history to issue banknotes or paper money, and the first Chinese government to establish a...
Chinese
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
scientist Shen Kuo
Shen Kuo
Shen Kuo or Shen Gua , style name Cunzhong and pseudonym Mengqi Weng , was a polymathic Chinese scientist and statesman of the Song Dynasty...
(1031–1095), in his Dream Pool Essays
Dream Pool Essays
The Dream Pool Essays was an extensive book written by the polymath Chinese scientist and statesman Shen Kuo by 1088 AD, during the Song Dynasty of China...
of 1088, described a Jacob's staff. Shen was an antiquarian
Antiquarian
An antiquarian or antiquary is an aficionado or student of antiquities or things of the past. More specifically, the term is used for those who study history with particular attention to ancient objects of art or science, archaeological and historic sites, or historic archives and manuscripts...
interested in ancient objects; after he unearthed an ancient crossbow-like device from a home's garden in Jiangsu
Jiangsu
' is a province of the People's Republic of China, located along the east coast of the country. The name comes from jiang, short for the city of Jiangning , and su, for the city of Suzhou. The abbreviation for this province is "苏" , the second character of its name...
, he realized it had a sight with a graduated scale that could be used to measure the heights of distant mountains, likening it to how mathematicians measure heights by using right-angle triangles. He wrote that when one viewed the whole breadth of a mountain with it, the distance on the instrument was long; when viewing a small part of the mountainside, the distance was short; this, he wrote, was due to the cross piece that had to be pushed further away from the eye, while the graduation started from the further end. Needham does not mention any practical application of this observation.
During the Renaissance
Renaissance
The Renaissance was a cultural movement that spanned roughly the 14th to the 17th century, beginning in Italy in the Late Middle Ages and later spreading to the rest of Europe. The term is also used more loosely to refer to the historical era, but since the changes of the Renaissance were not...
, the Dutch
Netherlands
The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...
mathematician and surveyor Metius
Metius
Adriaan Adriaanszoon, called Metius, , was a Dutch geometer and astronomer. He was born in Alkmaar. The name Metius comes from the Dutch word meten , and therefore means something like "measurer" or "surveyor."-Father and brother:Metius was born at Alkmaar, North Holland...
is known to have developed his own Jacob's staff. Gemma Frisius
Gemma Frisius
Gemma Frisius , was a physician, mathematician, cartographer, philosopher, and instrument maker...
is also known to have made improvements to this instrument. Johannes Müller, called Regiomontanus
Regiomontanus
Johannes Müller von Königsberg , today best known by his Latin toponym Regiomontanus, was a German mathematician, astronomer, astrologer, translator and instrument maker....
, made the Jacob's staff in the 15th century to a popular instrument in geodesic and astronomical measurements.
Construction
In the original form of the cross-staff, the pole or main staff was marked with graduationsGraduation (instrument)
-Linear graduation:Linear graduation of a scale occurs on a straight instrument. The graduation can identify linear measures, such as inches or millimetres on a rule. They can also be non-linear such as logarithmic or other transcendental scales....
for length. The cross-piece (BC in the drawing to the right), also called the transom or transversal, slides up and down on the main staff. On older instruments, the ends of the transom were cut straight across. Newer instruments had brass
Brass
Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.In comparison, bronze is principally an alloy of copper and tin...
fittings on the ends with holes in the brass for observation. In marine archaeology, these fittings are often the only components of a cross-staff that survive.
It was common to provide several transoms, each with a different range of angles it would measure. Three transoms were common. In later instruments, separate transoms were switched in favour of a single transom with pegs to indicate the ends. These pegs mounted in one of several pairs of holes symmetrically located on either side of the transom. This provided the same capability with fewer parts. The transom on Frisius
Gemma Frisius
Gemma Frisius , was a physician, mathematician, cartographer, philosopher, and instrument maker...
' version had a sliding vane on the transom as an end point.
Usage
The navigator places one end of the main staff against his cheek just below his eye. He sights the horizon at the end of the lower part of the transom (or through the hole in the brass fitting) (B), adjusting the cross arm on the main arm until he or she can sight the sun at the other end of the transom (C). The altitude can then be determined by reading the position of the transom on the scale on the main staff. This value was converted to an angular measurement by looking up the value in a table.Cross-staff for navigation
The original version was not used at sea. Johannes WernerJohannes Werner
Johann Werner was a German parish priest in Nuremberg and a mathematician...
suggested the cross-staff be used at sea in 1514 and improved instruments were introduced for use in navigation. John Dee
John Dee (mathematician)
John Dee was an English mathematician, astronomer, astrologer, occultist, navigator, imperialist and consultant to Queen Elizabeth I. He devoted much of his life to the study of alchemy, divination and Hermetic philosophy....
introduced it to England in the 1550s. In the improved versions, the rod was graduated directly in degrees. This variant of the instrument is not correctly termed a Jacob's staff but is a cross-staff.
The cross-staff was difficult to use. In order to get consistent results, the observer had to position the end of the pole precisely against his cheek. He had to observe the horizon and a star in two different directions while not moving the instrument when he shifted his gaze from one to the other. In addition, observations of the sun required the navigator to look directly at the sun. This could be a painful exercise and made it difficult to obtain an accurate altitude for the sun. Fortunately, mariners took to mounting smoked-glass
Smoked glass
The term smoked glass refers to two different types of glass. It can be either of the following: Smoked glass is a flat sheet of glass held in the smoke of a candle flame such that one surface of the sheet of glass is covered in a layer of smoke residue...
to the ends of the transoms to reduce the glare of the sun.
As a navigational tool, this instrument was eventually replaced, first by the backstaff
Backstaff
The backstaff or back-quadrant is a navigational instrument that was used to measure the altitude of a celestial body, in particular the sun or moon...
or quadrant
Quadrant (instrument)
A quadrant is an instrument that is used to measure angles up to 90°. It was originally proposed by Ptolemy as a better kind of astrolabe. Several different variations of the instrument were later produced by medieval Muslim astronomers.-Types of quadrants:...
, neither of which required the user to stare directly into the sun, and later by the octant
Octant (instrument)
The octant, also called reflecting quadrant, is a measuring instrument used primarily in navigation. It is a type of reflecting instrument.-Etymology:...
and the sextant
Sextant
A sextant is an instrument used to measure the angle between any two visible objects. Its primary use is to determine the angle between a celestial object and the horizon which is known as the altitude. Making this measurement is known as sighting the object, shooting the object, or taking a sight...
. Perhaps influenced by the backstaff, some navigators modified the cross-staff to operate more like the former. Vanes were added to the ends of the longest cross-piece and another to the end of the main staff. The instrument was reversed so that the shadow of the upper vane on the cross piece fell on the vane at the end of the staff. The navigator held the instrument so that he would view the horizon lined up with the lower vane and the vane at the end of the staff. By aligning the horizon with the shadow of the sun on the vane at the end of the staff, the elevation of the sun could be determined. This actually increased the accuracy of the instrument, as the navigator no longer had to position the end of the staff precisely on his cheek.
Another variant of the cross-staff was a spiegelboog, invented in 1660 by the Dutchman, Joost van Breen.
Ultimately, the cross-staff could not compete with the backstaff in many countries. In terms of handling, the backstaff was found to be more easy to use. However, it has been proven by several authors that in terms of accuracy, the cross-staff was superior to the backstaff. Backstaves were no longer allowed on board Dutch East India Company
Dutch East India Company
The Dutch East India Company was a chartered company established in 1602, when the States-General of the Netherlands granted it a 21-year monopoly to carry out colonial activities in Asia...
vessels as per 1731, with octants not permitted until 1748..
Surveying
In surveyingSurveying
See Also: Public Land Survey SystemSurveying or land surveying is the technique, profession, and science of accurately determining the terrestrial or three-dimensional position of points and the distances and angles between them...
the Jacob's staff, contemporaneously referred to as a jacob staff, is a single straight rod or staff made of nonferrous material, pointed and metal-clad at the bottom for penetrating the ground. It also has a screw base and occasionally a ball joint on the mount, and is used for supporting a compass
Compass
A compass is a navigational instrument that shows directions in a frame of reference that is stationary relative to the surface of the earth. The frame of reference defines the four cardinal directions – north, south, east, and west. Intermediate directions are also defined...
, transit, or other instrument.
The term cross-staff may also have a different meaning in the history of surveying. While the astronomical cross-staff was used in surveying for measuring angles, two other devices referred to as a cross-staff were also employed.
- Cross-head, cross-sight, surveyor's cross or cross - a drum or box shaped device mounted on a pole. It had two sets of mutually perpendicular sights. This device was used by surveyors to measure offsets. Sophisticated versions had a compass and spirit levels on the top. The French versions were frequently eight-sided rather than round.
- Optical square - an improved version of the cross-head, the optical square used two mirrors at 45° to each other. This permitted the surveyor to see along both axes of the instrument at once.
Use of the Jacob's Staff as a support
In the past, many surveyor's instruments were used on a Jacob's staff. These include:- Cross-head, cross-sight, surveyor's cross or cross
- GraphometerGraphometerThe graphometer or semicircle is a surveying instrument used for angle measurements. It consists of a semicircular limb divided into 180 degrees and sometimes subdivided into minutes. The limb is subtended by the diameter with two sights at its ends. In the middle of the diameter a "box and needle"...
- CircumferentorCircumferentorA circumferentor, or surveyor's compass, is an instrument used in surveying to measure horizontal angles, now superseded by the theodolite. See also alidade....
- Holland circle
- Miner's dial
- Optical square
- Surveyor's Sextant
- Surveyor's target
Some devices, such as the modern optical targets for laser-based surveying, are still in common use on a Jacob's staff.
See also
- BackstaffBackstaffThe backstaff or back-quadrant is a navigational instrument that was used to measure the altitude of a celestial body, in particular the sun or moon...
- Cross of St JamesCross of St JamesThe Cross of St. James is similar to a Cross flory fitchy and is formed by a cross flory, where the lower part is fashioned as a sword blade - making this a cross of a warrior...
- Pilgrim's staffPilgrim's staffThe pilgrim's staff is a walking stick used by pilgrims on the Way of St. James to the shrine of Santiago de Compostela in Spain. Generally, the stick has a hook on it so that something may be hung from it. The walking stick sometimes has a cross piece on it...
- TacheometryTacheometryTacheometry , is a system of rapid surveying, by which the positions, both horizontal and vertical, of points on the earth surface relatively to one another are determined without using a chain or tape or a separate levelling instrument....
- As a symbol in Scouting: 5th World Scout Jamboree5th World Scout JamboreeThe 5th World Scout Jamboree was the World Scout Jamboree where 81-year old Baden-Powell gave his farewell.- Organizational details :The Jamboree in Vogelenzang, Bloemendaal in the Netherlands was opened on 1937-07-31 by Queen Wilhelmina of the Netherlands, with 28,750 Scouts from 54 countries...