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The term Jacob's staff is used to refer to several things, also known as cross-staff, a ballastella, a fore-staff, a ballestilla, or a balestilha. In its most basic form, a Jacob's staff is a stick or pole with length markings; most staffs are much more complicated than that, and usually contain a number of measurement and stabilization features. The two most frequent uses are: * in
astronomy Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology of the Universe, evolution. Objects of interest ...
and
navigation Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navigation, ...
for a simple device to measure
angle In Euclidean geometry, an angle is the figure formed by two Ray (geometry), rays, called the ''Side (plane geometry), sides'' of the angle, sharing a common endpoint, called the ''vertex (geometry), vertex'' of the angle. Angles formed by two ...
s, later replaced by the more precise
sextant A sextant is a doubly reflecting navigation instrument that measures the angular distance between two visible objects. The primary use of a sextant is to measure the angle between an astronomical object and the horizon for the purposes of celes ...
s; * in
surveying Surveying or land surveying is the technique, profession, art, and science of determining the terrestrial two-dimensional or three-dimensional positions of points and the distances and angles between them. A land surveying professional is ca ...
(and scientific fields that use surveying techniques, such as
geology Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ear ...
and
ecology Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overlaps wi ...
) for a vertical rod that penetrates or sits on the ground and supports a compass or other instrument. The simplest use of a Jacob's staff is to make qualitative judgements of the height and angle of an object relative to the user of the staff.


In astronomy and navigation

In
navigation Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navigation, ...
the instrument is also called a cross-staff and was used to determine angles, for instance the angle between the horizon and
Polaris Polaris is a star in the northern circumpolar constellation of Ursa Minor. It is designated α Ursae Minoris ( Latinized to ''Alpha Ursae Minoris'') and is commonly called the North Star or Pole Star. With an apparent magnitude that ...
or the
sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
to determine a vessel's
latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pol ...
, or the angle between the top and bottom of an object to determine the distance to said object if its height is known, or the height of the object if its distance is known, or the horizontal angle between two visible locations to determine one's point on a map. 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

The origin of the name of the instrument is not certain. Some refer to the Biblical patriarch
Jacob Jacob (; ; ar, يَعْقُوب, Yaʿqūb; gr, Ἰακώβ, Iakṓb), later given the name Israel, is regarded as a patriarch of the Israelites and is an important figure in Abrahamic religions, such as Judaism, Christianity, and Islam. J ...
, specifically in the
Book of Genesis The Book of Genesis (from Greek ; Hebrew: בְּרֵאשִׁית ''Bəreʾšīt'', "In hebeginning") is the first book of the Hebrew Bible and the Christian Old Testament. Its Hebrew name is the same as its first word, ( "In the beginning") ...
().Turner, Gerard L'E. ''Antique Scientific Instruments'', Blandford Press Ltd. 1980 It may also take its name after its resemblance to Orion, referred to by the name of Jacob on some medieval star charts.Harriet Wynter and Anthony Turner, ''Scientific Instruments'', Studio Vista, 1975, Orion
This article indicates the three belt stars are sometimes called Jacob's Ladder or Jacob's Stick
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 is a term for physical manifestations resembling a common cross or Christian cross. The label can be extended to architectural shapes, biology, art, and design. Cruciform architectural plan Christian churches are commonly described ...
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 French-Jewish mathematician
Levi ben Gerson Levi ben Gershon (1288 – 20 April 1344), better known by his Graecized name as Gersonides, or by his Latinized name Magister Leo Hebraeus, or in Hebrew by the abbreviation of first letters as ''RaLBaG'', was a medieval French Jewish philosoph ...
of
Provence Provence (, , , , ; oc, Provença or ''Prouvènço'' , ) is a geographical region and historical province of southeastern France, which extends from the left bank of the lower Rhône to the west to the Italian border to the east; it is bor ...
, in his "Book of the Wars of the Lord" (translated in Latin as well as Hebrew). He used a Hebrew name for the staff that translates to "Revealer of Profundities", while the term "Jacob's staff" was used by his Christian contemporaries. Its invention was likely due to fellow French-Jewish astronomer
Jacob ben Makir Jacob ben Machir ibn Tibbon ( he, יעקב בן מכיר ׳ן תיבון), of the Ibn Tibbon family, also known as Prophatius, was a Jewish astronomer; born, probably at Marseilles, about 1236; died at Montpellier about 1304. He was a grandson o ...
, who also lived in Provence in the same period. Attribution to 15th century Austrian astronomer Georg Purbach is less likely, because Purbach was not born until 1423. (Such attributions may refer to a different instrument with the same name.) Its origins mayMay, William Edward, ''A History of Marine Navigation'', G. T. Foulis & Co. Ltd., Henley-on-Thames, Oxfordshire, 1973, be traced to the
Chaldea Chaldea () was a small country that existed between the late 10th or early 9th and mid-6th centuries BCE, after which the country and its people were absorbed and assimilated into the indigenous population of Babylonia. Semitic-speaking, it was ...
ns around 400 BC. Although it has become quite accepted that ben Gerson first described Jacob's staff, the British Sinologist
Joseph Needham Noel Joseph Terence Montgomery Needham (; 9 December 1900 – 24 March 1995) was a British biochemist, historian of science and sinologist known for his scientific research and writing on the history of Chinese science and technology, in ...
theorizes that the
Song Dynasty The Song dynasty (; ; 960–1279) was an imperial dynasty of China that began in 960 and lasted until 1279. The dynasty was founded by Emperor Taizu of Song following his usurpation of the throne of the Later Zhou. The Song conquered the rest ...
Chinese scientist
Shen Kuo Shen Kuo (; 1031–1095) or Shen Gua, courtesy name Cunzhong (存中) and pseudonym Mengqi (now usually given as Mengxi) Weng (夢溪翁),Yao (2003), 544. was a Chinese polymathic scientist and statesman of the Song dynasty (960–1279). Shen wa ...
(1031–1095), in his ''
Dream Pool Essays ''The Dream Pool Essays'' (or ''Dream Torrent Essays'') was an extensive book written by the Chinese polymath and statesman Shen Kuo (1031–1095), published in 1088 during the Song dynasty (960–1279) of China. Shen compiled this encycloped ...
'' of 1088, described a Jacob's staff.Needham, Joseph. (1986). ''Science and Civilization in China: Volume 3, Mathematics and the Sciences of the Heavens and the Earth''. Taipei: Caves Books Ltd. Pages 573–575. Shen was an
antiquarian An antiquarian or antiquary () is an fan (person), 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 artifact (archaeology), artifac ...
interested in ancient objects; after he unearthed an ancient crossbow-like device from a home's garden in
Jiangsu Jiangsu (; ; pinyin: Jiāngsū, Postal romanization, alternatively romanized as Kiangsu or Chiangsu) is an Eastern China, eastern coastal Provinces of the People's Republic of China, province of the China, People's Republic of China. It is o ...
, 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 medieval European
Renaissance The Renaissance ( , ) , from , with the same meanings. is a period in European history marking the transition from the Middle Ages to modernity and covering the 15th and 16th centuries, characterized by an effort to revive and surpass ideas ...
, the Dutch mathematician and surveyor
Adriaan Metius Adriaan Adriaanszoon, called Metius, (9 December 1571 – 6 September 1635), was a Dutch geometer and astronomer born in Alkmaar. The name "Metius" comes from the Dutch word ''meten'' ("measuring"), and therefore means something like "measurer" o ...
developed his own Jacob's staff; Dutch mathematician
Gemma Frisius Gemma Frisius (; born Jemme Reinerszoon; December 9, 1508 – May 25, 1555) was a Frisian physician, mathematician, cartographer, philosopher, and instrument maker. He created important globes, improved the mathematical instruments of his d ...
made improvements to this instrument. In the 15th century, the German mathematician Johannes Müller (called
Regiomontanus Johannes Müller von Königsberg (6 June 1436 – 6 July 1476), better known as Regiomontanus (), was a mathematician, astrologer and astronomer of the German Renaissance, active in Vienna, Buda and Nuremberg. His contributions were instrumental ...
) made the instrument popular in geodesic and astronomical measurements.


Construction

In the original form of the cross-staff, the pole or main staff was marked with
graduations Graduation is the awarding of a diploma to a student by an educational institution. It may also refer to the ceremony that is associated with it. The date of the graduation ceremony is often called graduation day. The graduation ceremony is al ...
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 is an alloy of copper (Cu) and zinc (Zn), in proportions which can be varied to achieve different mechanical, electrical, and chemical properties. It is a substitutional alloy: atoms of the two constituents may replace each other with ...
fittings on the ends, with holes in the brass for observation. (In
marine archaeology Maritime archaeology (also known as marine archaeology) is a discipline within archaeology as a whole that specifically studies human interaction with the sea, lakes and rivers through the study of associated physical remains, be they vessels, s ...
, these fittings are often the only components of a cross-staff that survive.)Swanick, Lois Ann. ''An Analysis Of Navigational Instruments In The Age Of Exploration: 15th Century To Mid-17th Century'', MA Thesis, Texas A&M University, December, 2005 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 just one with pegs to indicate the ends. These pegs were 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' version had a sliding vane on the transom as an end point.


Usage

The user places one end of the main staff against their cheek, just below the eye. By sighting the horizon at the end of the lower part of the transom (or through the hole in the brass fitting) 'B'' then adjusting the cross arm on the main arm until the sun is at the other end of the transom 'C'' the altitude can be determined by reading the position of the cross arm 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 reported to be used at sea, until the
Age of Discoveries The Age of Discovery (or the Age of Exploration), also known as the early modern period, was a period largely overlapping with the Age of Sail, approximately from the 15th century to the 17th century in European history, during which seafarin ...
. Its use was reported by
João de Lisboa João de Lisboa (c.1470 – 1525) was a Portuguese explorer. He is known to have sailed together with Tristão da Cunha, and to have explored Río de La Plata and possibly the San Matias Gulf, around 1511-12. The Brazilian historian Francisco Ado ...
in his ''Treatise on the Nautical Needle'' of 1514.
Johannes Werner Johann(es) Werner ( la, Ioannes Vernerus; February 14, 1468 – May 1522) was a German mathematician. He was born in Nuremberg, Germany, where he became a parish priest. His primary work was in astronomy, mathematics, and geography, although he ...
suggested the cross-staff be used at sea in 1514 and improved instruments were introduced for use in navigation.
John Dee John Dee (13 July 1527 – 1608 or 1609) was an English mathematician, astronomer, astrologer, teacher, occultist, and alchemist. He was the court astronomer for, and advisor to, Elizabeth I, and spent much of his time on alchemy, divinatio ...
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 Oxford Companion to Ships and the Sea'', Peter Kemp ed., 1976 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 uncomfortable exercise and made it difficult to obtain an accurate
altitude Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...
for the sun. Mariners took to mounting smoked-glass 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 The backstaff is a navigational instrument that was used to measure the altitude of a celestial body, in particular the Sun or Moon. When observing the Sun, users kept the Sun to their back (hence the name) and observed the shadow cast by the u ...
or quadrant, neither of which required the user to stare directly into the sun, and later by the octant and the
sextant A sextant is a doubly reflecting navigation instrument that measures the angular distance between two visible objects. The primary use of a sextant is to measure the angle between an astronomical object and the horizon for the purposes of celes ...
. 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.Daumas, Maurice, ''Scientific Instruments of the Seventeenth and Eighteenth Centuries and Their Makers'', Portman Books, London 1989 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. Nicolàs de Hilster's web site
Tests performed on various instruments are described. In addition, de Hilster describes the handling characteristics found by the testers on the Nav List mailing list.
However, it has been proven by several authors that in terms of accuracy, the cross-staff was superior to the backstaff.Bruyns, Willem Mörzer, ''The Cross-staff, History and development of a navigational instrument'', Nederlandsch Historisch Sheepvaart Museum, Amsterdam, and Walburg Instituut, Zutphen, Netherlands, 1994 Backstaves were no longer allowed on board
Dutch East India Company The United East India Company ( nl, Verenigde Oostindische Compagnie, the VOC) was a chartered company established on the 20th March 1602 by the States General of the Netherlands amalgamating existing companies into the first joint-stock ...
vessels as per 1731, with octants not permitted until 1748.


In surveying

In
surveying Surveying or land surveying is the technique, profession, art, and science of determining the terrestrial two-dimensional or three-dimensional positions of points and the distances and angles between them. A land surveying professional is ca ...
, the term ''jacob staff'' refers to a
monopod A monopod, also called a unipod, is a single staff or pole used to help support cameras, binoculars, rifles or other precision instruments in the field. Camera and imaging use The monopod allows a still camera to be held steadier, allowing t ...
, 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 A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself with ...
, 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.Turner, Gerard L'E., ''Nineteenth Century Scientific Instruments'', Sotheby Publications, 1983, #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 silvered mirrors at 45° to each other. This permitted the surveyor to see along both axes of the instrument at once.Rankine, William J. M., ''A Manual of Civil Engineering'', Charles Griffin & Company (1926), p.21 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 *
Graphometer The graphometer, semicircle or semicircumferentor 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 ...
*
Circumferentor A circumferentor, or surveyor's compass, is an instrument used in surveying to measure horizontal angles. It was superseded by the theodolite in the early 19th century. A circumferentor consists of a circular brass box containing a magnetic n ...
* Holland circle * Miner's dial * Optical square * Surveyor's sextant * Surveyor's target *
Abney level An Abney level and clinometer is an instrument used in surveying which consists of a fixed sighting tube, a movable spirit level that is connected to a pointing arm, and a protractor scale. An internal mirror allows the user to see the bubble i ...
Some devices, such as the modern optical targets for laser-based surveying, are still in common use on a Jacob's staff.


In geology

In geology, the Jacob's staff is mainly used to measure stratigraphic thicknesses in the field, especially when
bedding Bedding, also known as bedclothes or bed linen, is the materials laid above the mattress of a bed for hygiene, warmth, protection of the mattress, and decorative effect. Bedding is the removable and washable portion of a human sleeping environm ...
is not visible or unclear (e.g., covered outcrop) and when due to the configuration of an
outcrop An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth. Features Outcrops do not cover the majority of the Earth's land surface because in most places the bedrock or superficial ...
, the apparent and real thicknesses of beds diverge therefore making the use of a tape measure difficult. There is a certain level of error to be expected when using this tool, due to the lack of an exact reference mean for measuring stratigraphic thickness. High-precision designs include a laser able to slide vertically along the staff and to rotate on a plane parallel to bedding.Patacci, M. (2016), A high-precision Jacob's staff with improved spatial accuracy and laser sighting capabilit
Open RepositoryDOI Link
/ref>


See also

*
Backstaff The backstaff is a navigational instrument that was used to measure the altitude of a celestial body, in particular the Sun or Moon. When observing the Sun, users kept the Sun to their back (hence the name) and observed the shadow cast by the u ...
* Cross of St James *
Pilgrim's staff The pilgrim's staff is a walking stick used by Christian pilgrims during their pilgrimages, like the Way of St. James to the shrine of Santiago de Compostela in Spain or the Via Francigena to Rome. The pilgrim's staff has a strong association wit ...
*
Tacheometry Tacheometry (; from Greek for "quick measure") is a system of rapid surveying, by which the horizontal and vertical positions of points on the earth's surface relative to one another are determined without using a chain or tape, or a separate l ...
*As a symbol in Scouting:
5th World Scout Jamboree The 5th World Scout Jamboree (Dutch: ''5e Wereldjamboree'') was the World Scout Jamboree where 81-year-old Robert Baden-Powell gave his farewell. Organizational details The Jamboree in Vogelenzang, Bloemendaal in the Netherlands was opened on J ...


References


Further reading


Levi ben Gerson and the Cross Staff Revisited, Bernard R Goldstein
{{Authority control 14th-century introductions Angle measuring instruments History of astronomy Historical scientific instruments Navigational equipment Song dynasty Surveying instruments