Sextants for astronomical observations were used primarily for measuring the positions of stars. They are little used today, having been replaced over time by transit telescopes, astrometry techniques, and satellites such as Hipparcos. STAR is an acronym for: Organizations Society of Ticket Agents and Retailers], the self-regulatory body for the entertainment ticket industry in the UK. Society for Telescopy, Astronomy, and Radio, a non-profit New Jersey astronomy club. ... A Transit mount is a system where a telescope can move in altitude (North-South), but not in azimuth (East-West). ... Illustration of the use of optical wavelength interferometry to determine precise positions of stars. ... Hipparcos (for High Precision Parallax Collecting Satellite) was an astrometry mission of the European Space Agency (ESA) dedicated to the measurement of stellar parallax and the proper motions of stars. ...

There are two types of astronomical sextants, mural instruments and frame-based instruments. Tycho Brahes mural quadrant A mural instrument is an angle measuring device mounted on or built into a wall. ...

Mural Sextants

Ulugh Beg's mural sextant, constructed in Samarkand, Uzbekistan during the 15^th century.

Mural sextants are a special case of a mural instrument. Many were made that were quadrants rather than sextants. They were a kind of speciality of Medival Muslims to whom the credit of building the first mural sextants is attributed. Image File history File links Metadata Size of this preview: 800 × 599 pixelsFull resolution (2288 × 1712 pixel, file size: 711 KB, MIME type: image/jpeg) File historyClick on a date/time to view the file as it appeared at that time. ... Image File history File links Metadata Size of this preview: 800 × 599 pixelsFull resolution (2288 × 1712 pixel, file size: 711 KB, MIME type: image/jpeg) File historyClick on a date/time to view the file as it appeared at that time. ... Tycho Brahes mural quadrant A mural instrument is an angle measuring device mounted on or built into a wall. ... A quadrant is an instrument that is used to measure angles up to 90°. // There are several types of quadrants: Mural quadrants used for measuring the altitudes of astronomical objects. ...

The first known mural sextant was constructed in Ray, Iran, by Abu-Mahmud al-Khujandi in 994.^[1]. To measure the obliquity of the ecliptic , 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). The main improvement incorporated in al-Fakhri sextant over earlier instruments was bringing the precision of reading to seconds while older instruments could only be read in degrees and minutes. This fact was confirmed by Al Birūni, al Marrākushī and al Kāshī. 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 such instruments, it had a radius of about twenty meters.^[2] Ray, is one of the oldest cities of Iran. ... Abu Mahmud Hamid ibn al-Khidr Al-Khujandi was a Persian (Tajik) astronomer and mathematician who lived in the late 10th century and helped build an observatory near in what is now Ray, Iran near Tehran. ... Events Otto III reaches his majority and begins to rule Germany in his own right. ...

• Ulugh Beg constructed the Fakhri Sextant that had a radius of 40.4 meters, the largest instrument of its type in 16th century. Seen in the image on the left, the arc was finely constructed with a staircase on either side to provide access for the assistants who performed the measurements.

Ulugh Beg, here depicted on a Soviet stamp, was one of Islams greatest astronomers during the Middle Ages. ...

Framed sextants

Tycho Brahe's sextant, used for measuring the angular distances between stars.

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. Image File history File links Size of this preview: 381 × 599 pixelsFull resolution (400 × 629 pixel, file size: 78 KB, MIME type: image/jpeg) Tycho Brahe drew the construction of his two person sextant (SEXTANS ASTRONOMICUS TRIGONICUS PRO DISTANTHIIS RIMANDIS) used for measuring the angular distances between celestial objects. ... Image File history File links Size of this preview: 381 × 599 pixelsFull resolution (400 × 629 pixel, file size: 78 KB, MIME type: image/jpeg) Tycho Brahe drew the construction of his two person sextant (SEXTANS ASTRONOMICUS TRIGONICUS PRO DISTANTHIIS RIMANDIS) used for measuring the angular distances between celestial objects. ... Monument of Tycho Brahe and Johannes Kepler in Prague Tycho Brahe, born Tyge Ottesen Brahe (December 14, 1546 – October 24, 1601), was a Danish nobleman from the region of Scania (in modern-day Sweden), best known today as an early astronomer, though in his lifetime he was also well known...

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°. A sextant is a measuring instrument generally used to measure the angle of elevation of a celestial object above the horizon. ...

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, though newer versions could use a telescope. 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. A simple alidade for use with a ceiling projector The Alhidade or alidade is the part of a theodolite that rotates around the vertical axis, and that bears the horizontal axis around which the telescope (or visor, in early telescope-less instruments) turns up or down. ... This article does not cite any references or sources. ...

Usage

Johannes and Elisabetha Hevelius observing with the sextant.

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. Image File history File links Metadata Size of this preview: 373 × 599 pixelsFull resolution (1495 × 2400 pixel, file size: 1. ... Image File history File links Metadata Size of this preview: 373 × 599 pixelsFull resolution (1495 × 2400 pixel, file size: 1. ...

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 centred in the sights and then reads the graduations 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. In the image, Elisabetha is aligning the instrument while Johannes sets the alidade.

Well-known framed sextants

• Tycho Brahe used a sextant for many of his stellar position measurements.
• Johannes Hevelius used a sextant with a particularly ingenious alidade to provide stellar position measurements of great accuracy.
• John Flamsteed, the first Astronomer Royal used a sextant at the Royal Greenwich Observatory

Monument of Tycho Brahe and Johannes Kepler in Prague Tycho Brahe, born Tyge Ottesen Brahe (December 14, 1546 – October 24, 1601), was a Danish nobleman from the region of Scania (in modern-day Sweden), best known today as an early astronomer, though in his lifetime he was also well known... Johannes Hevelius Johannes Hevelius (Latin), also called Johann Hewelke, Johannes Höwelcke or Johannes Hewel (in German), or Jan Heweliusz (in Polish), (born January 28, 1611 – died January 28, 1687), was a councillor and mayor in Danzig (Gdańsk). ... A simple alidade for use with a ceiling projector The Alhidade or alidade is the part of a theodolite that rotates around the vertical axis, and that bears the horizontal axis around which the telescope (or visor, in early telescope-less instruments) turns up or down. ... John Flamsteed - Wikipedia, the free encyclopedia /**/ @import /skins-1. ... Astronomer Royal is a senior post in the Royal Household of the Sovereign of the United Kingdom. ... Royal Observatory, Greenwich The original site of the Royal Greenwich Observatory (RGO), which was built as a workplace for the Astronomer Royal, was on a hill in Greenwich Park in Greenwich, London, overlooking the River Thames. ...

References

1. ^ O'Connor, John J; Edmund F. Robertson "Abu Mahmud Hamid ibn al-Khidr Al-Khujandi". MacTutor History of Mathematics archive.  
2. ^ Tekeli Sevim (1960),'Nasiruddin, Takiyuddin ve Tycho Brahe'nin Rasat Aletlerinin mukayesesi'. Ankara Universitesi, Dil ve Tarih-Cografya, p.4.