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Glossary of terms

The study which assumes, and professes to interpret, the influence of the heavenly bodies on human affairs.
The measurement of the positions, motions, and distances of the celestial bodies.
The science of the celestial bodies, their motions, positions, distances, magnitudes, structures, etc.
The study of measures and weights.
To determine the form, boundaries, position, extent, etc., of, as a part of the earth's surface, by linear and angular measurements and the application of the principles of geometry and trigonometry.
The process, occupation, or art of making surveys of land, etc.; the act of one who surveys.
Surveying has many links with navigation, as they both use the same underlying principles of determining the present location of a terrestrial object, whether it be an unmoving mountain, or a moving ship on the sea.
That point in the visible celestial hemisphere which is vertical to the spectator; the point of the heavens directly overhead;—opposed to nadir (the point of the celestial sphere directly under the place where we stand).


Zenith Sector

An astronomical instrument, the limb of which embraces a small portion only of a circle, used for measuring differences of declination too great for the compass of a micrometer. When it is used for measuring zenith distances of stars, it is called a zenith sector.

www.nmm.ac.uk National Maritime Museum, Greenwich, LONDON. Home of the Prime Meridian of the World Longitude 0° 0' 0", Latitude 51° 28' 38"

Bradley's Zenith Sector

This telescope was constructed by George Graham in 1727 for Bradley's personal use in studying the parallax of the star Gamma Draconis. With it, he discovered two major phenomena: the aberration of light and the nutation (wobbling) of the Earth's axis. When he was appointed third Astronomer Royal in 1747, Bradley only agreed to bring this instrument to Greenwich after the government paid him the princely sum of £45 for it.  It was used at Greenwich until 1837.


According to the Flamsteed Astronomy Society http://www.flamsteed.info/fasbradley.htm 280 years ago, in 1724, James Bradley embarked on a programme of observation that would transform astronomy. In the south-west corner of the ROG Meridian Building, mounted opposite Halley’s Mural Quadrant, is an unassuming “piece of drain-pipe” which doesn’t get a second glance from most of the visitors to the ROG, but... James Bradley’s Zenith Sector is arguably, with Airy’s Transit Circle, one of the two most influential instruments on display there. A zenith sector or telescope points almost vertically upwards to the zenith or point directly overhead.

Bradley got the right result for the wrong reason. To prove the Earth is in orbit around the Sun, he’d set-out to measure the apparent change of position of the star Gamma Draconis, expected due to the different viewpoints from the extreme positions of the Earth at each end of its orbit six months apart. Measurement of this annual ‘parallax’ would finally prove that the Earth was indeed orbiting the Sun. However, instead of measuring the apparent change of the star’s position due to parallax, Bradley stumbled onto the first direct evidence for the Earth’s motion through space round the Sun, an effect he called the aberration of light.

In 1725 Bradley, working with Samuel Molyneux, began observations of Gamma Draconis. Based on the direction of the star, Bradley expected its apparent position to shift due to parallax with the maximum displacements happening in December and June. The pair carried out a programme of 80 observations stretching into 1727 using Molyneux’s 24-foot zenith telescope in his Kew mansion. They found that the apparent position was indeed changing up to 20 arc-seconds each way on a 365-day cycle, but to their surprise the maximum displacements came not in December and June, but in March and September. Gamma Draconis, also known as Eltamin (which derives from Al Ras al Tinnen, “the Dragon’s Head”), was running three months late.

Molyneux was called away into the Admiralty and Bradley worked on alone. He couldn’t understand the meaning of the measurements and he needed to see if other stars were subject to the same effect. He had George Graham build him a new and better zenith telescope. Graham was London’s best craftsman and he’d made clocks and instruments for Halley at Greenwich. The 24-foot zenith sector at Kew was his work. Three years later, Graham was to give support and encouragement to John Harrison when Harrison came to London seeking the Longitude prize. At 12-feet, Bradley’s new zenith sector (the one now on display) was shorter than the Kew instrument but had a wider adjustment to allow Bradley to check more star positions. In August 1727 he installed it in his late uncle’s house at Wanstead and began work. Every star position he measured showed the same mysterious effect.

James Bradley — 3rd Astronomer Royal

Bradley scratched his head until Autumn 1728 (it was probably the wig that did it) when he had his ‘eureka’ moment. The story has it that he was taking a pleasure cruise in a sailing boat on the Thames. After a while he took to watching the burgee, the small flag at the masthead that was showing the wind direction. He noticed that every time the boat changed course, the wind direction shown by the flag also appeared to change. This was too much of a coincidence so he asked the crew about it. The sailors told Bradley that it wasn’t really a change in wind direction, but the flag’s behaviour was also affected by the boat’s course. A change in course would cause the flag to shift too. Bradley realised he was seeing the same effect through the zenith sector. The incoming starlight was equivalent to the wind, and the Earth’s motion was equivalent to the boat’s course. What he was measuring was affected both by the incoming light’s direction and by the Earth’s motion around the Sun. It was the sum of the vectors. He was having to tilt the telescope fractionally in the direction of the Earth’s motion to centre the star’s apparent position.

Bradley had directly detected the effect of the Earth’s motion around the Sun. Aristarchus, Copernicus and Galileo were vindicated: “eppur si muove” — and yet it does move; but... it was clear that all previous star catalogues, including Flamsteed’s, would need correction, and... he hadn’t managed to measure the annual parallax. The true dimensions of the universe remained unknown.



According to the South African Astronomical Observatory S.A.A.O (http://www.saao.ac.za/assa/html/his-obj-tel_-_transit_instrume.html) the Bradley Sector was loaned to the Royal Observatory at the Cape of Good Hope (Cape Observatory), South Africa, from 1837 to 1850 for the re-measuring of the Arc of the Meridian. When De La Caille measured the Arc in 1751-53? his readings at Table Mountain and Piketberg were apparently distorted by the gravity exerted by the mountains, which pulled askew the plumb lines he used for vertical alignment. With the vertical reference incorrect, his calculations showed that the Earth was pear shaped, not orange shaped as was expected. The Zenith Sector allowed Maclear to determine the correct vertical angle in the presence of gravitational forces that would confound instruments like plumb lines.


According to http://profsurv.com/ps_scripts/article.idc?id=147 the zenith sector was a relatively new instrument, designed for astronomical observation and used in surveying, it was among the most sophisticated instrumentation of the eighteenth century. The first example of the zenith sector had been made in 1727 by the eminent London clockmaker and instrument maker George Graham (1673-1751) for the amateur astronomer Samuel Molyneux (1687-1728). Molyneux, accompanied by James Bradley (1693-1762), Savilian Professor of Astronomy at Oxford University, used the sector to observe the annual parallax of the star Draconis and led to the discovery of apparent stellar motions due to the effects of aberration of light and of nutation. In 1729, by means of another sector made by Graham, Bradley was able to calculate the speed of light in space. Graham also produced the sector used in 1736 by the party of French astronomers led by mathematician Pierre Louis Moreau de Maupertuis (1697-1759) to measure the length of the meridian at Torneo in Lapland, which provided the first reliable value of the elliptical nature of the earth’s surface.

The zenith sector is a fixed vertical telescopic instrument designed for measuring the zenith distances that come within its arc, and serves also for discovering the aberration of stars and the nutation of the earth’s axis. It is used for determining the parallels of latitude by repeated observations of a number of fixed stars near the zenith as they cross the meridian at differing hours. Because stars near the zenith are free from refraction, they are observed in preference to others for determining latitude. Six or seven stars are observed at varying times, with the observations repeated on a number of nights. The long focal length of the instrument’s objective made the slightest deviation in a star’s zenith distance readily perceptible as it culminated. The instrument consists of a brass tube between 5-1/2 and 6 feet in length which is suspended by means of trunnions of a framework erected within an observatory or in a field observatory tent in such a manner that its upper end projects through an opening. The tube could be moved like a pendulum on a horizontal axis near the object lens.

Zenith sectors were produced also by two other contemporary English makers of mathematical instruments, John Bird (1709-1776) and Jonathan Sisson (1690-1740). Prices for the instruments varied according to the radius, from £60 to £160. Sisson produced a zenith sector for Cecilius Calvert, who was acting on behalf of his nephew Frederick Lord Baltimore, and Thomas Penn commissioned one to be made for him by John Bird. These instruments were used to define the border between Pennsylvania (grant given to William Penn by King George II in 1682 that included the territory lying west of the Delaware River from 12 miles north of the town of New Castle as far as the 42º parallel) and Cecil Calvert (second Lord Baltimore, who received a grant of the entire peninsula east of the Bay of Chesapeake and all the land “not yet husbanded or planted” as far as 40º north 50 years earlier by King Charles I in 1632). This constituted a two degree latitude overlap, and 80 years of disputes. Finally, at the urging of the King’s Council, the Proprietors of both provinces, the aristocratic Penn family and the Barons of Baltimore, both residing in London, jointly took positive action. They hired the experienced mathematician/astronomers Charles Mason and Jeremiah Mason to use instruments in the field in 1763 to define the boundary, now known as the Mason-Dixon line.

After these had been inspected first by themselves and then by the commissioners for both provinces, the astronomers declared their preference for the Bird sector over the one made by Sisson. The Bird sector made for Penn was the first example of the instrument to incorporate a modification in the suspension of the plumb bob that had previously been suggested by the astronomer Nevil Maskelyne. The Bird sector was used constantly during the course of the survey, “to measure the angle between the zenith and a star as it crossed the meridian. The instrument was mounted to rotate about both vertical and horizontal axes. A telescope attached to a sector of a vertical circle with a graduated limb was trained upon the star as it came to the meridian. A fine plumb-line hanging past the center of the sector and its limb served as a reference for the reading of angles in the plane of the meridian.” 

The survey required five years to complete, after which Mason and Dixon departed for England and the instruments were returned to their respective owners. The Bird sector was borrowed by David Rittenhouse in the late summer of 1769 and used in establishing the New York-New Jersey boundary to determine the latitude at either end of the line. It was used again by Rittenhouse in October 1774 while he was engaged with Samuel Holland in establishing the beginning of the 43rd degree of latitude on behalf of the province of Pennsylvania. Rittenhouse later built a copy for his own use. See Rittenhouse Zenith Sector below .


A further account of the Mason Dixon Line at the January 2002 meeting of the Delmarva Stargazers at http://www.delmarvastargazers.org/newsletter/feb02/page1.html was presented by Bob Mentzer. In 1750 the British court ruled that the boundary between Maryland and southern Pennsylvania should be a line 15 miles south of the southern boundary of Philadelphia.

Since the beginning of their east-west line would be west of Philadelphia, they had to begin their measurements to the east of the east-west line. The exact point was in the middle of Mr.Alexander Bryan’s plantation house, and an observation post was built in his front yard. From there the survey moved west following the line of latitude 39 degrees, 43 minutes and 17.6 seconds North.

Bob then reminded us that the shortest distance between 2 points on earth is a great circle whereas latitude lines on earth are not the shortest distance. A great circle is the line formed on earth, by the intersection of a plane passing thru the earth’s center. This means that a great circle line superimposed on a latitude line will actually cross the longer curved latitude line twice. The survey therefore consisted of making star measurements with the zenith sector every 12 miles while making constant corrections to convert the great circle survey lines to latitude lines. Surveying in the wild forests was difficult and slow and not without all kinds of pit falls. One of the most dangerous was the Indians, particularly the Shawnees, near the western end of the line. To avert this danger, the group hired 14 Iroquois as escorts. The Iroquois prestige and influence among the Indian nations kept the party safe from the more hostile tribes to the west. However at a point 36 miles short of the 233 mile goal, the Iroquois escort told them to go no farther and on October 9, 1767, the survey was finished. The horizontal black bar on the map below is the final Mason Dixon line.

The vertical line is the border between Delaware and Maryland and is called the Transpeninsular Line. Bob Mentzer concluded his presentation discussing how symbolic the Mason Dixon line was to become, especially in terms of Slavery and Freedom and even in the American Civil War still a hundred years away


http://www.mhs.ox.ac.uk/students/96to97/znsketch.htm "Back of the Envelope" sketch

Sketch by Jonathan Sills

The picture on the right above from http://www.mhs.ox.ac.uk/students/96to97/zenith.htm is a Zenith Sector by John Bird, 1773, 12ft, aperature 3.5in, Oxford, Museum of the History of Science. This zenith sector was part of the original equipment of the Radcliffe Observatory, Oxford.

The zenith sector pointed straight and directly overhead. A telescope rotated on a pivot and allowed astronomers to measure the zenith distances (the angle between the star and the highest point in the sky) of celestial bodies. This also necessitated aligning the instrument in the meridian (a line through the poles).

Since the graduated scale was so low to the ground, the astronomer usually had to lie on his back or a special reclined seat in order to effectively make observations with the zenith sector.


Rittenhouse Zenith Sector 1786-1800 (http://historywired.si.edu/object.cfm?ID=35)

Scientific precision in 18th-century America The zenith sector--a telescope designed to determine latitude by observing stars as they pass overhead--was the most precise geodetic instrument of the 1700s. And this zenith sector made by Philadelphia, PA inventor/surveyor/scientist David Rittenhouse (1732-1796) was the most precise scientific instrument made in America. It served as tangible evidence that Americans could achieve the same level of technological sophistication and scientific accuracy as their European counterparts. The American zenith sector debuted in the summer of 1786, when Andrew Ellicott surveyed the boundary between the State of New York and the Commonwealth of Pennsylvania. Ellicott noted that the latitude of points along the line "may be depended on, within two inches and perhaps less."

The model for the American version was the London-made instrument used for Mason and Dixon's survey of the boundary between the colonies of Maryland and Pennsylvania.















A Virtual Exhibition by Jonathan Sills

Museum of the History of Science, Oxford University


During the eighteenth century, new official observatories began popping up all over Europe. In the past, observatories were dependent on the vision and energy of ambitious astronomers like Tycho Brahe for their establishment. The new "official" observatories, sponsored by states, universities, or scientific societies, were run as organizations.

The professionals working in these European observatories spoke different languages, pursued different objectives, and had different political leanings. Yet during the eighteenth century they shared unprecedented agreement on the proper way to practice astronomy.


Why was this consensus achieved? Despite their diverse backgrounds and goals, they all used the same tools. And the design of these instruments primarily emerged from the vision of one man:


George Graham.


To run an observatory in the eighteenth century you needed at least two basic instruments:

The mural quadrant was a quarter circle fixed to a sturdy wall.

This circle was divided into increments and allowed an astronomer to measure the altitude of stars or other celestial bodies.

This one hangs over the staircase on our museum's first floor.




A transit instrument was the term used to describe any tool that allowed astronomers to time the passage of a celestial body over the meridian of a place or through the field of a telescope.

(the meridian is a line through the north and south poles)

This one from our collection was made in the late eighteenth century.



George Graham, an English mechanic, was the first instrument maker to specialize in these large measuring instruments which observatories needed.

His devices were celebrated for their accuracy and workmanship, particularly after Royal Astronomers Edmond Halley (of Halley's Comet fame) and James Bradley used them to make significant discoveries.

Their glowing endorsements for Graham piqued the interest of other astronomers, who in turn championed the English technology in other countries.

A French Fellow of the Royal Society of London brought a Graham mural quadrant to France, and then lent it to the Berlin Academy for a collaborative project.

Their published papers cited the "superior" English technology and in turn influenced the purchase decisions of Italian, Russian, and Hungarian astronomers.


Pretty soon, all of Europe was using tools based on Graham's model, constructed by Graham or other English instrument makers under his tutelage (like John Bird, who constructed the mural quadrant above).

Graham had developed a lucrative export trade.

Riding this success, Graham offered a grab bag of other tools to boost astronomers' productivity, some of which were more successful than others:


The zenith sector (pictured left) measured angular distances directly above an observer's head, and could be used to measure aberration in the movement of stars.

This one from our collection was made by John Bird, one of Graham's proteges mentioned earlier.

The astronomical regulator was a very accurate clock used to time transit observations. It featured Graham's own proprietary mechanism to ensure the pendulum wasn't affected by changes in temperature.

John Shelton's regulator (pictured right) is one of two in the museum's collection.

Finally, the equatorial sector was a flashy and expensive instrument for taking measurements based directly on the equator rather than the poles.

This instrument was so large we only have a piece of it on display.


Not everybody used Graham's instruments. Local suppliers and resident instrument makers still provided competition, and people could certainly use other clocks besides "astronomical regulators" to time transits. But even these instruments had to operate within the structure of Graham's system, simply because it was so prevalent in official observatories. Other successful English contemporaries naturally took their leads from his designs.

Graham's instruments did more than just enforce technical standards. They helped create a working community and fostered communication between observatories by virtue of a common platform.