astronomy Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest includ ...

, sextants are devices depicting a sixth of a circle, used primarily for measuring the position of stars. There are two types of astronomical sextants, mural instruments and frame-based instruments. They are of significant historical importance, but have been replaced over time by transit telescopes, other

astrometry Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other Astronomical object, celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, th ...

techniques, and

satellites A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation ( GPS), broadcasting, scientif ...

such as

Hipparcos ''Hipparcos'' was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions and distances of ...

Mural sextants

The first known mural sextant was constructed in

Ray, Iran Shahre Ray, Shahr-e Ray, Shahre Rey, or Shahr-e Rey (, ) or simply Ray or Rey (), is the capital of Ray County, Iran, Rey County in Tehran Province, Iran. Formerly a distinct city, it has now been absorbed into the metropolitan area of Greater T ...

, by Abu-Mahmud al-Khujandi in 994. To measure the

obliquity of the ecliptic In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, which is the line perpendicular to its orbital plane; equivalently, it is the angle between its equatorial plane and orbital ...

, al-Khujandī invented a device that he called ''al-Fakhri sextant'' (al-suds al Fakhrī), a reference to his patron, Buwayhid ruler, Fakhr al Dawla (976–997). This instrument was a sixty-degree arc on a wall aligned along a meridian (north–south) line. Al Khujandi's instrument was larger than previous instruments; it had a radius of about twenty meters. Tekeli, Sevim (1958), 'Nasiruddin, Takiyuddin ve Tycho Brahe'nin Rasat Aletlerinin mukayesesi'. ''Ankara Universitesi Dil ve Tarih-Cografya Fakültesi Dergesi'', XVI, p. 4. The main improvement incorporated in al-Fakhri sextants over earlier instruments was bringing the precision of reading to seconds while older instruments could only be read in degrees and minutes. This was confirmed by al-Birūni, al-Marrākushī and al-Kāshī. Al-Khujandī used his device to measure the sun's angle above the horizon at the

summer Summer or summertime is the hottest and brightest of the four temperate seasons, occurring after spring and before autumn. At or centred on the summer solstice, daylight hours are the longest and darkness hours are the shortest, with day ...

and

winter solstice The winter solstice, or hibernal solstice, occurs when either of Earth's geographical pole, poles reaches its maximum axial tilt, tilt away from the Sun. This happens twice yearly, once in each hemisphere (Northern Hemisphere, Northern and So ...

s; these two measurements allow computation of the latitude of the sextant's location and the obliquity of the ecliptic.

Ulugh Beg Mīrzā Muhammad Tarāghāy bin Shāhrukh (; ), better known as Ulugh Beg (; 22 March 1394 – 27 October 1449), was a Timurid sultan, as well as an astronomer and mathematician. Ulugh Beg was notable for his work in astronomy-related ma ...

constructed a Fakhri Sextant that had a radius of 40.4 meters, the largest instrument of its type in the 15th century. Housed in the Ulugh Beg Observatory, the sextant had a finely constructed arc with a staircase on either side to provide access for the assistants who performed the measurements.

Framed sextants

A sextant based on a large metal frame had an advantage over a mural instrument in that it could be used at any orientation. This allows the measure of angular distances between astronomical bodies. These instruments differ substantially from a navigator's sextant in that the latter is a reflecting instrument. The navigator's sextant uses mirrors to bring the image of the sun, moon or a star to the horizon and measure the altitude of the object. Due to the use of the mirrors, the angle measured is twice the length of the instrument's arc. Hence, the navigator's sextant measures 120° on an arc with an included angle of 60°. By comparison, the astronomical sextants are large and measure angles directly — a 60° arc will measure at most 60°.

Construction

These large sextants are made primarily of wood, brass or a combination of both materials. The frame is heavy enough to be stiff and provide reliable measures without flexural changes in the instrument compromising the quality of the observation. The frame is mounted on a support structure that holds it in position while in use. In some cases, the position of the sextant can be adjusted to allow measurements to be made with any instrument orientation. Owing to the size and weight of the instrument, attention was paid to balancing it so that it could be moved with ease. Observations were typically made with an

alidade An alidade () (archaic forms include alhidade, alhidad, alidad) or a turning board is a device that allows one to sight a distant object and use the line of sight to perform a task. This task can be, for example, to Triangulation (surveying), tr ...

, though newer versions could use a

telescope A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...

. In some cases, a system of counter-weights and pulleys were used to allow the observer to manipulate the instrument in spite of its size.

Usage

These instruments were used in much the same way as smaller instruments, with effort possibly scaled due to the size. Some of the instruments might have needed more than one person to operate. If the sextant is permanently fixed in position, only the position of the alidade or similar index need be determined. In that case, the observer moved the alidade until the object of interest is centered in the sights and then reads the

graduation A graduation is the awarding of a diploma by an educational institution. It may also refer to the ceremony that is associated with it, which can also be called Commencement speech, commencement, Congregation (university), congregation, Convocat ...

s marked on the arc. For instruments that could be moved, the process was more complex. It was necessary to sight the object with two lines. The edge of the instrument would typically be supplied with sights and the instrument was aligned with one of the two objects of interest. The alidade was then aligned with the second object as well. Once each object was centred in one set of sights, the reading could be taken. This could be a challenge for a moving star observed with a very large instrument as a single person might not be able to confirm both sights with ease; an assistant was a great benefit. The illustration of the Hevelius instrument to the right shows how two persons would use such a sextant: his wife Elisabetha is aligning the instrument while Johannes sets the alidade.

Well-known framed sextants

* Taqi al-Din used a sextant for the determination of the

equinox A solar equinox is a moment in time when the Sun appears directly above the equator, rather than to its north or south. On the day of the equinox, the Sun appears to rise directly east and set directly west. This occurs twice each year, arou ...

es. *

Tycho Brahe Tycho Brahe ( ; ; born Tyge Ottesen Brahe, ; 14 December 154624 October 1601), generally called Tycho for short, was a Danish astronomer of the Renaissance, known for his comprehensive and unprecedentedly accurate astronomical observations. He ...

used a sextant for many of his stellar position measurements. *

Johannes Hevelius Johannes Hevelius Some sources refer to Hevelius as Polish: * * * * * * * Some sources refer to Hevelius as German: * * * * *of the Royal Society * (in German also known as ''Hevel''; ; – 28 January 1687) was a councillor and mayor of Danz ...

used a sextant with a particularly ingenious

to provide stellar position measurements of great accuracy. *

John Flamsteed John Flamsteed (19 August 1646 – 31 December 1719) was an English astronomer and the first Astronomer Royal. His main achievements were the preparation of a 3,000-star catalogue, ''Catalogus Britannicus'', and a star atlas called '' Atlas ...

, the first

Astronomer Royal Astronomer Royal is a senior post in the Royal Households of the United Kingdom. There are two officers, the senior being the astronomer royal dating from 22 June 1675; the junior is the astronomer royal for Scotland dating from 1834. The Astro ...

, used a sextant at the

Royal Greenwich Observatory The Royal Observatory, Greenwich (ROG; known as the Old Royal Observatory from 1957 to 1998, when the working Royal Greenwich Observatory, RGO, temporarily moved south from Greenwich to Herstmonceux) is an observatory situated on a hill in G ...

References