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Backstaff
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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 upper vane on a horizon vane. It was invented by the English navigator John Davis, who described it in his book Seaman's Secrets in 1594.[1]
Types of backstaffs
[edit]Backstaff is the name given to any instrument that measures the altitude of the sun by the projection of a shadow. It appears that the idea for measuring the sun's altitude using back observations originated with Thomas Harriot.[2] Many types of instruments evolved from the cross-staff that can be classified as backstaffs. Only the Davis quadrant remains dominant in the history of navigation instruments. Indeed, the Davis quadrant is essentially synonymous with backstaff. However, Davis was neither the first nor the last to design such an instrument and others are considered here as well.
Davis quadrant
[edit]
Captain John Davis invented a version of the backstaff in 1594. Davis was a navigator who was quite familiar with the instruments of the day such as the mariner's astrolabe, the quadrant and the cross-staff. He recognized the inherent drawbacks of each and endeavoured to create a new instrument that could reduce those problems and increase the ease and accuracy of obtaining solar elevations.
One early version of the quadrant staff is shown in Figure 1.[3] It had an arc affixed to a staff so that it could slide along the staff (the shape is not critical, though the curved shape was chosen). The arc (A) was placed so that it would cast its shadow on the horizon vane (B). The navigator would look along the staff and observe the horizon through a slit in the horizon vane. By sliding the arc so that the shadow aligned with the horizon, the angle of the sun could be read on the graduated staff. This was a simple quadrant, but it was not as accurate as one might like. The accuracy in the instrument is dependent on the length of the staff, but a long staff made the instrument more unwieldy. The maximum altitude that could be measured with this instrument was 45°.
The next version of his quadrant is shown in Figure 2.[3] The arc on the top of the instrument in the previous version was replaced with a shadow vane placed on a transom. This transom could be moved along a graduated scale to indicate the angle of the shadow above the staff. Below the staff, a 30° arc was added. The horizon, seen through the horizon vane on the left, is aligned with the shadow. The sighting vane on the arc is moved until it aligns with the view of the horizon. The angle measured is the sum of the angle indicated by the position of the transom and the angle measured on the scale on the arc.
The instrument that is now identified with Davis is shown in Figure 3.[4] This form evolved by the mid-17th century.[4] The quadrant arc has been split into two parts. The smaller radius arc, with a span of 60°, was mounted above the staff. The longer radius arc, with a span of 30° was mounted below. Both arcs have a common centre. At the common centre, a slotted horizon vane was mounted (B). A moveable shadow vane was placed on the upper arc so that its shadow was cast on the horizon vane. A moveable sight vane was mounted on the lower arc (C).
It is easier for a person to place a vane at a specific location than to read the arc at an arbitrary position. This is due to Vernier acuity, the ability of a person to align two line segments accurately. Thus an arc with a small radius, marked with relatively few graduations, can be used to place the shadow vane accurately at a specific angle. On the other hand, moving the sight vane to the location where the line to the horizon meets the shadow requires a large arc. This is because the position may be at a fraction of a degree and a large arc allows one to read smaller graduations with greater accuracy. The large arc of the instrument, in later years, was marked with transversals to allow the arc to be read to greater accuracy than the main graduations allow.[5]
Thus Davis was able to optimize the construction of the quadrant to have both a small and a large arc, allowing the effective accuracy of a single arc quadrant of large radius without making the entire instrument so large. This form of the instrument became synonymous with the backstaff. It was one of the most widely used forms of the backstaff. Continental European navigators called it the English Quadrant.
A later modification of the Davis quadrant was to use a Flamsteed glass in place of the shadow vane; this was suggested by John Flamsteed.[4] This placed a lens on the vane that projected an image of the sun on the horizon vane instead of a shadow. It was useful under conditions where the sky was hazy or lightly overcast; the dim image of the sun was shown more brightly on the horizon vane where a shadow could not be seen.[5]
-
Figure 1 – A simple precursor to the Davis Quadrant after an illustration in his book, Seaman's Secrets. The arc was limited to measuring angles to 45°. -
Figure 2 – The second Davis Quadrant after an illustration in his book, Seaman's Secrets. The arc above is replaced with an arc below and a shadow-casting transom above. This instrument can now measure up to 90°. -
Figure 3 – The Davis Quadrant as it evolved by the mid-17th century. The upper transom has been replaced with a 60° arc.
Usage
[edit]In order to use the instrument, the navigator would place the shadow vane at a location anticipating the altitude of the sun. Holding the instrument in front of him, with the sun at his back, he holds the instrument so that the shadow cast by the shadow vane falls on the horizon vane at the side of the slit. He then moves the sight vane so that he observes the horizon in a line from the sight vane through the horizon vane's slit while simultaneously maintaining the position of the shadow. This permits him to measure the angle between the horizon and the sun as the sum of the angle read from the two arcs.
Since the shadow's edge represents the limb of the sun, he must correct the value for the semidiameter of the sun.
Instruments that derived from the Davis quadrant
[edit]The Elton's quadrant derived from the Davis quadrant. It added an index arm with spirit levels to provide an artificial horizon.
Demi-cross
[edit]
The demi-cross was an instrument that was contemporary with the Davis quadrant. It was popular outside England.[4]
The vertical transom was like a half-transom on a cross-staff, hence the name demi-cross. It supported a shadow vane (A in Figure 4) that could be set to one of several heights (three according to May,[4] four according to de Hilster[6]). By setting the shadow vane height, the range of angles that could be measured was set. The transom could be slid along the staff and the angle read from one of the graduated scales on the staff.
The sight vane (C) and horizon vane (B) were aligned visually with the horizon. With the shadow vane's shadow cast on the horizon vane and aligned with the horizon, the angle was determined. In practice, the instrument was accurate but more unwieldy than the Davis quadrant.[6]
Plough
[edit]
The plough was the name given to an unusual instrument that existed for a short time.[4] It was part cross-staff and part backstaff. In Figure 5, A is the transom that casts its shadow on the horizon vane at B. It functions in the same manner as the staff in Figure 1. C is the sighting vane. The navigator uses the sighting vane and the horizon vane to align the instrument horizontally. The sighting vane can be moved left to right along the staff. D is a transom just as one finds on a cross-staff. This transom has two vanes on it that can be moved closer or farther from the staff to emulate different-length transoms. The transom can be moved on the staff and used to measure angles.
Almucantar staff
[edit]The Almucantar staff is a device specifically used for measuring the altitude of the sun at low altitudes.
Cross-staff
[edit]The cross-staff was normally a direct observation instrument. However, in later years it was modified for use with back observations.
Quadrant
[edit]There was a variation of the quadrant – the Back observation quadrant – that was used for measuring the sun's altitude by observing the shadow cast on a horizon vane.
Thomas Hood cross-staff
[edit]
Thomas Hood invented this cross-staff in 1590.[4] It could be used for surveying, astronomy or other geometric problems.
It consists of two components, a transom and a yard. The transom is the vertical component and is graduated from 0° at the top to 45° at the bottom. At the top of the transom, a vane is mounted to cast a shadow. The yard is horizontal and is graduated from 45° to 90°. The transom and yard are joined by a special fitting (the double socket in Figure 6) that permits independent adjustments of the transom vertically and the yard horizontally.
It was possible to construct the instrument with the yard at the top of the transom rather than at the bottom.[7]
Initially, the transom and yard are set so that the two are joined at their respective 45° settings. The instrument is held so that the yard is horizontal (the navigator can view the horizon along the yard to assist in this). The socket is loosened so that the transom is moved vertically until the shadow of the vane is cast at the yard's 90° setting. If the movement of just the transom can accomplish this, the altitude is given by the transom's graduations. If the sun is too high for this, the yard horizontal opening in the socket is loosened and the yard is moved to allow the shadow to land on the 90° mark. The yard then yields the altitude.
It was a fairly accurate instrument, as the graduations were well spaced compared to a conventional cross-staff. However, it was a bit unwieldy and difficult to handle in wind.
Benjamin Cole quadrant
[edit]
A late addition to the collection of backstaves in the navigation world, this device was invented by Benjamin Cole in 1748.[4]
The instrument consists of a staff with a pivoting quadrant on one end. The quadrant has a shadow vane, which can optionally take a lens like the Davis quadrant's Flamsteed glass, at the upper end of the graduated scale (A in Figure 7). This casts a shadow or projects an image of the sun on the horizon vane (B). The observer views the horizon through a hole in the sight vane (D) and a slit in the horizon vane to ensure the instrument is level. The quadrant component is rotated until the horizon and the sun's image or shadow are aligned. The altitude can then be read from the quadrant's scale. In order to refine the reading, a circular vernier is mounted on the staff (C).
The fact that such an instrument was introduced in the middle of the 18th century shows that the quadrant was still a viable instrument even in the presence of the octant.
English scientist George Adams created a very similar backstaff at the same time. Adam's version ensured that the distance between the Flamsteed glass and horizon vane was the same as the distance from the vane to the sight vane.[8]
Cross bow quadrant
[edit]
Edmund Gunter invented the cross bow quadrant, also called the mariner's bow, around 1623.[4] It gets its name from the similarity to the archer's crossbow.
This instrument is interesting in that the arc is 120° but is only graduated as a 90° arc.[4] As such, the angular spacing of a degree on the arc is slightly greater than one degree. Examples of the instrument can be found with a 0° to 90° graduation or with two mirrored 0° to 45° segments centred on the midpoint of the arc.[4]
The instrument has three vanes, a horizon vane (A in Figure 8) which has an opening in it to observe the horizon, a shadow vane (B) to cast a shadow on the horizon vane and a sighting vane (C) that the navigator uses to view the horizon and shadow at the horizon vane. This serves to ensure the instrument is level while simultaneously measuring the altitude of the sun. The altitude is the difference in the angular positions of the shadow and sighting vanes.
With some versions of this instrument, the sun's declination for each day of the year was marked on the arc. This permitted the navigator to set the shadow vane to the date and the instrument would read the altitude directly.
References
[edit]- Ephraim Chambers, Cyclopædia, The First Volume, 1728 explaining the use of a backstaff
- Maurice Daumas, Scientific Instruments of the Seventeenth and Eighteenth Centuries and Their Makers, Portman Books, London 1989 ISBN 978-0-7134-0727-3
- Gerard L'Estrange Turner, Antique Scientific Instruments, Blandford Press Ltd. 1980 ISBN 0-7137-1068-3
Notes
[edit]- ^ Willem Frederik Jacob Mörzer Bruyns; Richard Dunn (2009). Sextants at Greenwich: a catalogue of the mariner's quadrants, mariner's astrolabes, cross-staffs, backstaffs, octants, sextants, quintants, reflecting circles and artificial horizons in the National Maritime Museum, Greenwich. Oxford University Press. pp. 72–. ISBN 978-0-19-953254-4. Retrieved 16 September 2011.
- ^ Taylor, E.G.R. (1953). "The Doctrine of Nautical Triangles Compendius". Journal of the Institute of Navigation. 6: 134–135.
- ^ a b The Seaman's Secrets Archived 22 June 2008 at the Wayback Machine; text of Davis' publication with illustrations.
- ^ a b c d e f g h i j k May, William Edward, A History of Marine Navigation, G. T. Foulis & Co. Ltd., Henley-on-Thames, Oxfordshire, 1973, ISBN 0-85429-143-1
- ^ a b Bennett, Jim, "Catadioptrics and commerce in eighteenth-century London", in History of Science, vol xliv, 2006, pages 247–277.
- ^ a b Nicolàs de Hilster's web site Web page documenting an excellent reproduction of a demi-cross.
- ^ De Hilster, N. (2009). "Master Hood's cross-staff: a reconstruction" (PDF). Bulletin of the Scientific Instrument Society. 101. Archived from the original (PDF) on 21 July 2011.
- ^ Charles Hutton, "A Mathematical and Philosophical Dictionary" Archived 5 June 2011 at the Wayback Machine
External links
[edit]- "Backstaff" at answers.com – Good diagram of how a backstaff is held in use.
- Attribution
This article incorporates text from a publication now in the public domain: Chambers, Ephraim, ed. (1728). Cyclopædia, or an Universal Dictionary of Arts and Sciences (1st ed.). James and John Knapton, et al. {{cite encyclopedia}}: Missing or empty |title= (help)
Backstaff
View on GrokipediaHistorical Development
Invention by John Davis
John Davis (c. 1550–1605), an English navigator and explorer from Sandridge near Dartmouth, Devon, dedicated much of his career to the pursuit of the Northwest Passage, a hypothetical sea route connecting the Atlantic and Pacific Oceans through Arctic waters. Commissioned by English merchants and supported by figures like Queen Elizabeth I's advisor Sir Francis Walsingham, Davis led three major expeditions between 1585 and 1587 aboard vessels such as the Sunshine and Moonshine, penetrating deep into areas like Cumberland Sound and Baffin Bay but ultimately failing to locate the passage. These voyages, documented in contemporary accounts like those compiled by Richard Hakluyt, exposed the limitations of existing navigational tools in high-latitude conditions, where accurate solar observations were essential for determining latitude yet fraught with hazards.[4][5][6] During these Arctic expeditions, Davis conceived an innovative instrument to address the perils of direct solar observation, which often caused temporary blindness among navigators using tools like the cross-staff. By around 1587, he had developed the prototype of what became known as the backstaff or Davis quadrant, a device that allowed the user to measure the sun's altitude while facing away from it, relying instead on shadow projection. This invention stemmed directly from the practical demands of his voyages, where foggy and icy conditions compounded the risks of eye strain from staring at the low-angled sun near the horizon. Davis refined the concept over the following years, formalizing its design amid his broader contributions to seamanship knowledge.[7][8] Davis first publicly described the backstaff in his 1594 treatise The Seaman's Secrets, published in London by Thomas Dawson as a practical guide to navigation for mariners. The initial design featured a straight wooden staff with a fixed horizon vane at one end—a slotted plate for sighting the sea horizon—and two arcs—one of 30 degrees for the horizon vane and one of 60 degrees for the shadow vane—totaling 90 degrees at the opposite end, calibrated in degrees for angular measurement. A movable shadow vane, typically a small horizontal board or pin, slid along this arc to cast a shadow onto the horizon vane when aligned with the sun's position; the observer, with their back to the sun, adjusted the vane until the shadow precisely overlapped a mark on the horizon vane, yielding the solar altitude reading. This configuration eliminated the need for direct eye exposure to sunlight, thereby preventing the common issue of impaired vision that hindered latitude calculations during meridian passages. The instrument's simplicity, using readily available wood and minimal metal fittings, made it accessible for shipboard use in the Elizabethan era.[9][7][10]Adoption and Improvements in the 17th-18th Centuries
Following the publication of John Davis's The Seaman's Secret in 1594, the backstaff—also known as the Davis quadrant—gained rapid adoption among English navigators for measuring solar altitudes without direct exposure to the sun's glare. By the early 17th century, it had become a standard tool in maritime expeditions, including those of the English East India Company, where Davis himself served as chief pilot on the company's inaugural 1601 voyage to the East Indies. Dutch navigators also embraced the instrument during this period, incorporating it into voyages of the Dutch East India Company (VOC) for celestial observations in the Indian Ocean trade routes, as evidenced by its presence in inventories from the 1620s onward. The original design evolved by the early 17th century to include two distinct arcs (30° and 60°) on a central staff for improved usability, as seen in later depictions.[7][11][7] In the 17th century, refinements focused on enhancing readability and stability, building on ideas from instrument makers like Thomas Hood, who in the 1590s introduced shadow-vane mechanisms in cross-staffs that directly influenced Davis's design. Later makers, such as Thomas Tuttell of London (active c. 1700), added index arms for precise alignment and early vernier scales to improve angular resolution beyond the original fixed graduations, allowing readings to within 10-15 arcminutes under ideal conditions. These modifications made the backstaff more reliable for long-distance navigation, with examples from the period showing wooden frames reinforced with mortise-and-tenon joints for durability at sea.[7][12][7] By the 18th century, further enhancements emphasized precision and material quality, with instrument makers like John Elton and George Adams incorporating inset ivory scales for finer graduations, polished brass fittings for smoother adjustments, and larger overall sizes—often up to 24 inches in length—to reduce observational errors from hand tremors. These ivory-inlaid versions, common in English workshops, achieved accuracies of about 5-10 arcminutes, making them suitable for extended voyages.[12][12][2] Despite these advances, the backstaff's use declined sharply in the late 18th century as the reflecting octant, invented around 1730, offered superior accuracy (to 2-5 arcminutes) and versatility for both sun and star sightings without requiring the observer to face away from the horizon. The Dutch VOC officially discontinued it by 1731 due to its limitations in precision, and by the 1780s, most European fleets, including the British Royal Navy, had transitioned fully to octants, though backstaffs persisted in some merchant vessels until around 1800.[7][13][7]Design Principles
Shadow Projection Mechanism
The backstaff's shadow projection mechanism enables precise measurement of the sun's altitude by casting the shadow of a horizontal shadow vane onto a flat horizon vane while the observer faces away from the sun, thereby avoiding direct exposure to glare. The instrument's design positions the shadow vane on the pivoting index arm above the horizon vane, with sunlight passing over the observer's shoulder to project the shadow onto the horizon vane's slit, aligned with the sighted horizon. This indirect method relies on adjusting the index arm until the shadow's edge aligns with the horizon viewed through the slit to determine the angular elevation.[7][1] Geometrically, the mechanism employs a pair of perpendicular arcs—one typically of 30 degrees and the other of 60 degrees—mounted on a central staff with radius , calibrated to span altitude angles from 0° at the horizon to 90° at the zenith via their combined graduations, allowing the index arm's position to directly indicate the sun's elevation. The altitude angle is read directly from the degree markings on the arc at the point where the index arm intersects it. This geometric arrangement ensures the arc scale can be graduated in angular units for direct readout.[7][1] By eliminating direct solar observation, the shadow projection mechanism enhances usability and accuracy compared to sighting instruments, achieving measurements within 10-20 arcminutes even in hazy or bright conditions, as glare-induced errors are minimized. In contrast to direct line-of-sight tools like the cross-staff, which require viewing the sun through a transverse slit and suffer from visual distortion, the backstaff's reliance on shadow projection provides a more stable and eye-friendly alternative for marine navigation.[7]Construction and Components
The backstaff, also known as the Davis quadrant, features a pair of wooden arcs mounted on a central staff, typically forming a radius of 12 to 18 inches to facilitate precise angular measurements up to 90 degrees.[14] The instrument's frame consists of a main strut or staff, often 24 to 26 inches in length, along which key components slide or pivot, ensuring stability during use at sea.[7] Key components include the horizon vane, a sliding rectangular plate attached to the staff with a narrow slit for aligning the observer's eye with the horizon; the shadow vane, a perpendicular crosspiece fixed or adjustable on the index arm to cast a shadow onto the horizon vane; and the index arm itself, a pivoting radial arm equipped with peep sights or a small aperture for sighting the horizon while aligning the shadow.[15] These elements work together via the shadow projection principle, where the shadow vane's position indicates the solar altitude as read from the arc's graduation.[2] Early designs from the 1590s, such as John Davis's original, lacked additional struts and relied on simple mortise-and-tenon joints for assembly.[7] Construction materials evolved from entirely wooden builds in the late 16th century, using durable hardwoods like pear tree wood for the frame and arcs, to more refined versions in the 1700s incorporating ebony or lignum vitae for the frame, boxwood for the arcs and inlays, and ivory for scale plates to enhance readability and durability against marine conditions.[7][2] By the 18th century, metal elements such as brass rivets and pivots were commonly added for the index arm and vane fittings, improving precision and reducing wear, as seen in examples from makers like Joseph Halsey and Clark Elliott.[14][16] Backstaffs were designed for shipboard portability, typically weighing 1 to 2 pounds and measuring about 25 to 30 inches overall when assembled, allowing easy storage in a navigator's kit without compromising structural integrity.[2] Calibration marks on the arcs were engraved or inlaid in degrees from 0 to 90, with finer divisions of 1 degree on the larger arc and 10 or 5 arcminutes on the smaller, often including specialized noon sight scales for latitude determination corrected for the sun's semi-diameter.[14][7]Operation and Usage
Measuring Solar Altitude
The procedure for measuring solar altitude with the backstaff involves the navigator positioning themselves with their back to the sun to avoid direct glare, allowing for safer observation compared to earlier instruments like the cross-staff.[1][17] The instrument is held against the shoulder or to the eye, with the longer arm horizontal and the shorter arm perpendicular.[18] Through a combination of sighting vanes and shadow projection, the altitude—the angular height of the sun above the horizon—is determined directly from the instrument's scale.[7] To perform the measurement, the navigator first aligns the horizon vane's slit with the visible horizon by sighting through the peephole in the index vane and the slit simultaneously.[1] Next, the shadow vane (mounted on the smaller arc) is adjusted along its scale until the edge of the vane casts a sharp shadow that precisely aligns with the horizon line viewed through the sights.[18][7] This alignment ensures the shadow vane's position corresponds to the sun's altitude, which is then read from the graduated scales on the arcs—typically marked in degrees and minutes.[1] If the altitude exceeds 30 degrees, the larger 60-degree arc may be used for finer resolution.[7] Under hazy conditions, a small lens could be fitted to the shadow vane to project a clearer image of the sun's disk.[7] Observations are best conducted at local solar noon, when the sun reaches its maximum altitude, providing the meridian altitude necessary for latitude determination.[17][13] For longitude calculations, the solar altitude measurement must be paired with precise timekeeping to compare local solar time with a reference meridian, a practice enabled after the 1700s by marine chronometers that maintained accurate Greenwich time aboard ship.[17] Key sources of error in backstaff measurements include ship motion, which disrupts steady alignment, and parallax from imprecise vane positioning relative to the eye.[13][7] These were mitigated primarily through a steady hand and careful positioning, though no specialized mounts like gimbals were typically used with the backstaff.[13] The direct scale reading yields the solar altitude h. To obtain the zenith distance z—the angle from the sun to the zenith—subtract the altitude from 90 degrees: z = 90^\circ - h.[7] This value, combined with the sun's declination from astronomical tables, allows computation of latitude.[17] Historically, the backstaff achieved an accuracy of approximately 12 arc minutes (0.2 degrees) under ideal conditions, readable to 1 arc minute on well-graduated scales, making it sufficient for open-ocean navigation where positions accurate to tens of miles were adequate.[7] At sea, errors from motion often increased this to 0.5–1 degree, still a significant improvement over predecessors.[13]Observations of Moon and Stars
The backstaff could theoretically be adapted for measuring the altitude of the moon above the horizon via direct sighting when the moon was sufficiently bright, though such observations were rare and less common than solar ones due to the moon's variable illumination, lack of strong shadow projection, and frequent adverse weather conditions.[19][20] The technique involved adjusting the vane until the moon aligned with the horizon through the peep sights, but accuracy was reduced compared to solar use. Moon altitude measurements were primarily employed for latitude determination, similar to solar observations, and the backstaff was not suitable for the lunar distance method, which required instruments capable of measuring angular separations between celestial bodies.[21][22] Stellar observations using the backstaff were limited and supplemental, focusing on brighter bodies like Polaris for estimating latitude in the northern hemisphere during nighttime when direct sighting was feasible. However, the design's emphasis on shadow projection greatly reduced its effectiveness for faint objects, resulting in lower accuracy and making such sightings impractical for most stars; the procedure relied on aligning the star with the horizon through the peep sights, but was constrained by the need to face away from the celestial body.[21][19][1]Precursors
Cross-Staff
The cross-staff, also known as the Jacob's staff or fore-staff, was invented in the mid-14th century, with its earliest detailed description appearing in 1342 in the astronomical treatise De numeris harmonicis by the Jewish scholar Levi ben Gerson (1288–1344) of Provence, France.[23] Although initially developed for astronomical and surveying purposes, the instrument began to see adoption in European navigation in the early 16th century, with the earliest known instructions for its use at sea appearing in a Portuguese manuscript around 1514, particularly among Portuguese explorers.[23] Its design consisted of two perpendicular wooden staffs: a longer main staff, typically graduated with degree markings, and a shorter transverse crosspiece that slid along the main staff.[24] The crosspiece featured sights or vanes at each end, allowing the observer to align one sight with the horizon and the other with a celestial body, such as the sun or a star, to measure the angular altitude directly from the scale on the main staff.[23] In operation, the cross-staff enabled direct sighting of celestial objects to determine angles up to 90 degrees, making it suitable for latitude calculations by measuring the elevation of the sun at noon or Polaris at night.[24] For solar observations, users employed simple filters, such as smoked glass or colored vanes, to reduce glare and protect the eyes, though this method still required the observer to face the sun directly while holding the instrument horizontally against the cheek.[23] The procedure involved adjusting the crosspiece until the horizon and celestial body aligned in the sights, then reading the angle from the scale—no additional mathematical computations were needed for basic readings. The cross-staff's primary advantages lay in its simplicity and portability, requiring no complex mechanisms or prior calculations, which made it accessible to sailors without advanced training.[24] However, it had notable drawbacks, including significant eye strain from prolonged exposure to bright sunlight even with filters, awkward handling on a pitching ship that often demanded an assistant to steady the horizon view, and limitations to clear weather conditions where visibility was unobstructed.[23] These challenges, particularly the discomfort of direct solar viewing, highlighted the need for an instrument that allowed observations without facing the sun, influencing later innovations like John Davis's backstaff in 1594.Early Quadrants
Early quadrants emerged in 15th-century Europe as adaptations of Islamic astrolabes, which had been refined for astronomical purposes since the Islamic Golden Age and introduced to the continent via Spain around the 12th century.[25][26] By the 1460s, Portuguese navigators had developed the mariner's quadrant, a simplified instrument tailored for maritime use during the expanding Age of Discovery.[22] These instruments featured a curved arc spanning 45 to 90 degrees, typically constructed from wood or metal, with engraved degree markings along the edge.[27] A plumb bob suspended from the apex provided a vertical reference line, while adjustable sights or vanes allowed alignment with celestial bodies; handheld designs made them portable for use at sea.[28][29] They were often employed alongside cross-staffs as part of a basic navigational ensemble, combining arc-based and linear sighting methods for versatility in observations.[30] In operation, users aligned the horizon through one sight and the target star or sun through another, reading the altitude where the plumb bob intersected the scale to determine latitude.[27] This direct sighting method worked adequately for stars but proved challenging for solar measurements due to the need to stare into the sun, often mitigated with smoked glass yet still leading to dazzle and misalignment.[31] Overall accuracy hovered around 1-2 degrees, limited by ship motion, wind interference with the plumb bob, and the instrument's inherent instability on deck.[32][31] Portuguese explorers, such as those under Prince Henry the Navigator and later Vasco da Gama, relied on these quadrants during voyages along Africa's coast and into the Indian Ocean in the 1400s and 1500s, enabling more reliable latitude fixes amid the uncertainties of open-ocean travel.[33][34]Backstaff Variants
Davis Quadrant
The Davis Quadrant, also known as the original backstaff or English Quadrant, was invented by the English navigator John Davis in 1594 as a significant advancement in maritime instrumentation for celestial observations.[35] Described in Davis's treatise The Seaman's Secrets, it represented the first practical device allowing the observer to face away from the sun while measuring its altitude, thereby preventing the eye damage associated with direct solar viewing in earlier tools like the cross-staff.[36] This back-to-sun positioning was its key innovation, earning it the alternative name "backstaff" and enabling safer navigation during daylight hours.[37] The foundational design featured a graduated wooden staff serving as the main body, with a sliding arc or half-transom marked in degrees up to 45° for measuring lower solar altitudes, typically constructed from woods like pear tree for durability at sea.[7] A single shadow vane was fixed at a 45° angle from the arc's center to project the sun's shadow onto the horizon line, while the horizon vane included a narrow slit for precise alignment with the sea's horizon.[36] Later refinements extended the range to 90° with an additional arc, but the original 45° model sufficed for northern latitudes where Davis primarily operated.[35] Production of the Davis Quadrant was largely handmade by skilled shipwrights, utilizing mortise-and-tenon joints for assembly, reflecting the era's shipbuilding craftsmanship rather than specialized instrument-making.[37] No surviving examples from 1594 exist, but period replicas and early instruments, often in ebony or lignum vitae with boxwood scales, are preserved in collections such as the National Maritime Museum in Greenwich and the Mariners' Museum in Newport News.[1] These artifacts, dating from the early 17th century onward, illustrate the quadrant's robust, portable form suitable for shipboard use. In practice, the instrument was employed primarily at sea to determine latitude by observing the sun's meridian altitude, with the observer holding the staff vertically or horizontally and adjusting the arc until the shadow vane's projection aligned perfectly with the horizon through the slit.[35] It integrated seamlessly with the magnetic compass for dead reckoning, allowing navigators to plot courses by combining solar latitude fixes with estimated speeds and directions.[37] The Davis Quadrant's historical significance lies in its role in facilitating safer transatlantic and exploratory voyages during the late 16th and 17th centuries, as it minimized the risks of sun-induced blindness that plagued earlier sighting methods and thereby boosted confidence in open-ocean navigation.[37] Popularized through Davis's voyages in search of the Northwest Passage, it became a staple tool for English mariners, contributing to the expansion of global trade routes until its gradual replacement by more precise instruments in the 18th century.[1]Demi-Cross and Plough Instruments
The demi-cross, a simplified hybrid of the cross-staff and early backstaff designs, emerged in the 17th century as a portable instrument for measuring celestial altitudes, particularly suited for quick horizon observations without direct solar exposure. Developed in the Netherlands as an evolution of John Davis's 45-degree backstaff, it was first documented in Willem Jansz. Blaeu's 1621 pilot book Het Licht der Zeevaert and appeared in subsequent nautical texts up to 1693.[38] Unlike the full cross-staff's forward-facing setup, the demi-cross featured a back-facing arc for shadow projection, with a vertical transom resembling half of a cross-staff's crossbar—hence its name, derived from the French demi meaning half.[39] In design, the demi-cross consisted of a rectangular wooden staff (typically 867–1156 mm long) engraved with up to four scales divided in 15-minute intervals, starting from 90 degrees at the base. A sliding cross (433–610 mm) carried a shadow vane, horizon vane, and sight vane; the shadow vane projected the sun's shadow onto the white-painted horizon vane for alignment, while the sight vane allowed peeking at the horizon. Constructed from ebony for the staff and pear or boxwood for the vanes, with brass springs for tension, it measured altitudes up to 90 degrees but was limited to a reduced arc of 45–60 degrees in practice for stability.[38] Fixed vanes and cheaper pine alternatives in later versions enhanced portability over the more elaborate Davis quadrant, though wooden warping posed handling challenges.[39] Primarily applied in coastal navigation, the demi-cross enabled latitude determination via solar or stellar altitudes, with field tests demonstrating an average error of -2.6 arc minutes—sufficient for merchant voyages but less precise than refined quadrants (1–2 degree potential error in suboptimal conditions). No original artifacts survive, but depictions in Blaeu's 1625 Tafelen van de Declinatie title page illustrate its form, marking an evolutionary step toward mass-produced wooden instruments by the late 17th century.[38] The plough, or plow, represented another downsized backstaff variant, named for its distinctive plow-shaped vane and evolving directly from Davis's 90-degree backstaff design. Documented in 17th- and 18th-century texts such as John Seller's works and Jonas Moore's A New Systeme of Mathematicks (1681), it featured a compact wooden frame (approximately 10 inches long) with a single scale and fixed vanes for simplified shadow-based measurements.[39] Primarily constructed from inexpensive pine or similar softwoods, the plough prioritized affordability and portability over precision, achieving readings with a typical 1–2 degree error suitable for non-professional use. An example, attributed to Seller, is preserved in Amsterdam's Het Scheepvaartmuseum (inventory no. S.4793).[39] Gaining popularity among colonial American merchants in the 1700s for coastal trade routes, the plough's reduced arc (45–60 degrees) and basic components made it ideal for quick, on-deck horizon checks during voyages, reflecting a trend toward practical, low-cost tools amid growing transatlantic commerce. Artifacts from the 1720s, including American-made versions, highlight its role in early mass production efforts, bridging traditional European designs with New World adaptations.[16]Almucantar Staff
The almucantar staff is a specialized variant of the backstaff, designed to measure small angular altitudes of celestial bodies close to the horizon, such as the sun near sunrise or sunset, or stars at low elevations. This instrument facilitated observations along the almucantar, the celestial circle parallel to the horizon, enabling determinations of amplitude—the angular distance from the east-west line to the center of the rising or setting sun—for calculating magnetic compass variation.[40][41] Developed in the early 17th century by English instrument makers building on John Davis's backstaff of 1594, the almucantar staff featured a more compact form suited to precise low-angle readings, with an extended wooden arm and sliding cursor (transom) for alignment. Typically crafted from pear or boxwood, it incorporated a graduated arc spanning 15 to 30 degrees rather than a full quadrant, allowing for fine adjustments in shadow positioning without direct viewing of bright objects. For night observations, adaptations included translucent or open screens to permit faint light from stars or the moon to cast usable shadows on the horizontal vane.[42] In usage, the observer aligned the shadow of the celestial body on the horizon vane using the sliding cursor along the staff, reading the angle from the scale to achieve accuracies around 5 arcminutes, sufficient for navigational computations including longitude via related stellar positions. Such measurements were particularly valuable for moon-star angular separations when bodies shared nearly the same almucantar, aiding early methods of longitude determination at sea.[41][42] Due to its specialized design and the challenges of stable low-altitude observations, the almucantar staff was less common than broader backstaff variants like the Davis quadrant, though it appeared in 17th-century astronomical navigation manuals, such as those from the 1650s describing advanced celestial techniques. Surviving examples are rare, reflecting limited production and the instrument's eventual supersession by more versatile tools.[40]Thomas Hood and Benjamin Cole Variants
Thomas Hood, the first appointed lecturer in mathematics in England and a pioneering figure in the promotion of mathematical instruments, developed an early variant of the cross-staff in the late 16th century that integrated shadow-casting capabilities, marking it as a foundational precursor to later backstaff designs.[43] His instrument consisted of two perpendicular wooden rulers—a transom and a yard—each engraved with degree scales from 1° to 90° and subdivided into 1,000 equal linear parts for measurement versatility. A brass vane with a pinhole, mounted on a screw-adjustable socket, cast a shadow onto the scale to determine the altitude of the sun or stars without requiring the observer to face the light source directly, thus reducing eye strain compared to earlier sighting methods.[44] This design supported both celestial navigation at sea and terrestrial surveying tasks, such as measuring heights and distances. Hood detailed its construction and usage in his 1590 treatise The Use of the Two Mathematicall Instruments, which advocated for the practical application of such tools in English maritime endeavors and influenced subsequent instrument makers through its emphasis on shadow-based observation.[44] Building on these foundations, instrument maker Benjamin Cole introduced refinements to backstaff designs in the mid-18th century, creating a hybrid variant that merged cross-staff linearity with quadrant arcs for improved solar altitude measurements. His 1748 "New Quadrant," a late evolution of the Davis quadrant, featured a pivoting arc at the end of a staff, shadow and horizon vanes, and peep sights to align the observer's view with the horizon while capturing the sun's shadow, enabling more accurate readings even in hazy conditions.[45] Cole's models often employed a brass frame for durability, measuring approximately 18 inches in length, with scales engraved in degrees and equipped with vernier adjustments for precision down to one-tenth of a degree, surpassing the coarser graduations of earlier wooden backstaffs.[45] These innovations retained the core shadow-based principle but enhanced usability for dynamic environments, including artillery ranging where quick angular computations were essential. Cole documented his design in The Description and Use of a New Quadrant (1749), promoting its adoption among naval officers.[39] Hood's and Cole's variants differed primarily in their refinement levels and applications: Hood's emphasized accessible construction for broad educational use, achieving accuracies around 5 arcminutes in field tests, while Cole's incorporated metal components and sighting aids for professional military contexts, such as Royal Navy voyages in the 1760s where they supported latitude determinations until superseded by reflecting instruments.[44][39] Both advanced the integration of cross-staff mechanics into backstaff forms, prioritizing shadow projection over direct sighting to mitigate solar glare, and their treatises played key roles in disseminating these techniques among English navigators and surveyors.[45]Cross-Bow Quadrant
The cross-bow quadrant represents a specialized variant of the backstaff, distinguished by its compact, bow-shaped arc that prioritizes portability for celestial navigation. Developed in the early 17th century, it employs Thomas Hood's shadow-casting method, where a horizon vane projects a shadow onto a graduated scale to measure altitudes without direct solar exposure.[46] This design evolution from straight-staff instruments allowed for easier handling during extended voyages or terrain-challenging journeys.[46] The instrument's origins trace to English navigators, with the earliest description appearing in George Waymouth's 1604 treatise The Jewell of Artes, portraying it as a backstaff adaptation. Edward Wright enhanced the model in the 1610 edition of Certaine Errors in Navigation by adding a nocturnal scale to one arc end for timekeeping. By 1623, Edmund Gunter formalized its construction and application in The Description and Use of the Sector, the Cross-Staffe, and the Bow, emphasizing its crossbow-like curvature for stability and reduced weight. Mid-17th-century examples suggest possible Dutch influences, as seen in instruments preserved in Amsterdam collections, aligning with its use in overland exploration where maneuverability was essential.[46] Key features include a wooden arc—typically curved with a tensioned chord spanning the ends to support adjustable vanes—and provisions for shadow projection along the bow's curve, enabling precise sightings. Its lightweight build, often under one pound using woods like boxwood or mahogany, facilitated folding mechanisms in some variants for storage in leather cases. These attributes made it ideal for Arctic expeditions and land surveys, where bulky tools were impractical. Historical assessments indicate a practical accuracy of around 5–10 arcminutes, comparable to other wooden backstaffs of the era.[46][47] Surviving 1680s specimens, such as those in Sweden's Skokloster Castle (inventory no. 10590) and Amsterdam's Het Scheepvaartmuseum (inventory no. S.4793(415)), exemplify this compact evolution, showcasing refined arc scaling and vane adjustments for enhanced mobility in exploratory contexts.[46] These artifacts highlight the cross-bow quadrant's role in bridging early backstaff designs toward more versatile field instruments.[46]Legacy
Replacement by Octant and Sextant
The reflecting octant, invented by English mathematician John Hadley in 1731 and presented to the Royal Society, revolutionized celestial navigation by employing mirrors to enable simultaneous observation of the horizon and a celestial body without requiring the user to avert their gaze from the sun.[48] This innovation addressed key limitations of earlier instruments, allowing measurements during daylight or in varying weather conditions where direct sighting was challenging.[49] The octant's introduction initiated a transitional period in the mid-18th century, with the backstaff gradually phased out from major navies by the 1780s as reflecting instruments proved more reliable for routine latitude determinations.[13] The subsequent development of the sextant in the late 1750s, notably by instrument maker John Bird, extended the reflecting principle to a 60-degree arc, enhancing portability and precision to approximately 1 arcminute, which was essential for advanced techniques like lunar distance measurements.[50] Several factors contributed to the backstaff's obsolescence: its inherent sensitivity to ship motion, which caused the shadow vane to jitter and degraded readings in rough seas, contrasted sharply with the octant's stabilized view through fixed mirrors that mitigated such instability.[51] Moreover, the backstaff's requirement for the observer to face away from the celestial body limited its versatility, whereas the octant and sextant permitted forward-facing observations, reducing errors from awkward positioning.[52] Reflecting instruments gained endorsement through trials that favored their accuracy in lunar observations critical to longitude determination, solidifying the shift away from shadow-based tools like the backstaff. Despite this, the backstaff persisted in niche applications, such as on whaling ships into the early 1800s, where crews valued its straightforward construction amid harsh conditions.[53] The superior performance of reflecting instruments is evident in their comparative accuracies, as summarized below:| Instrument | Typical Practical Accuracy |
|---|---|
| Backstaff | ~12 arcminutes |
| Octant/Sextant | ~1 arcminute |