Sextant (astronomical)

Last updated

Sextants for astronomical observations were devices depicting a sixth of a circle, used primarily for measuring the positions of stars. They are of significant historical importance, but have been replaced over time by transit telescopes, astrometry techniques, and satellites such as Hipparcos.

Contents

There are two types of astronomical sextants, mural instruments and frame-based instruments.

Mural sextants

Remains of Ulugh Beg's mural sextant, Samarkand, Uzbekistan, 15th century. Ulugh Beg observatory.JPG
Remains of Ulugh Beg's mural sextant, Samarkand, Uzbekistan, 15th century.

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). This instrument was a sixty-degree arc on a wall aligned along a meridian arc (north–south line). Al Khujandi's instrument was larger than previous instruments; it had a radius of about twenty meters. [2] 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 and winter solstices; these two measurements allow computation of the latitude of the sextant's location and the obliquity of the ecliptic. [1]

Ulugh Beg 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

Tycho Brahe's sextant, used for measuring the angular distances between stars. Tycho instrument sextant 16.jpg
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.

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, 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.

Usage

Johannes and Elisabetha Hevelius observing with the sextant. Johannes Hevelius und Elisabetha Hevelius.jpg
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.

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 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: his wife Elisabetha is aligning the instrument while Johannes sets the alidade.

Well-known framed sextants

Related Research Articles

Astrometry Branch of astronomy involving positioning and movements of celestial bodies

Astrometry is the branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. The information obtained by astrometric measurements provides information on the kinematics and physical origin of the Solar System and our galaxy, the Milky Way.

Sextant Tool for angle measurement

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 celestial navigation.

Ulugh Beg Timurid ruler as well as an astronomer, mathematician and sultan (1394–1449)

Mīrzā Muhammad Tāraghay bin Shāhrukh, better known as Ulugh Beg, was a Timurid sultan, as well as an astronomer and mathematician.

Uraniborg Historic observatory

Uraniborg was a Danish astronomical observatory and alchemy laboratory established and operated by Tycho Brahe. It was built c. 1576 – c. 1580 on Hven, an island in the Øresund between Zealand and Scania, Sweden, which was part of Denmark at the time. It was expanded with the underground facility Stjerneborg on an adjacent site.

Alidade

An alidade 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 draw a line on a plane table in the direction of the object or to measure the angle to the object from some reference point. Angles measured can be horizontal, vertical or in any chosen plane.

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 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.

Constantinople Observatory of Taqi ad-Din

The Constantinople observatory of Taqi ad-Din, founded in Constantinople by Taqi ad-Din Muhammad ibn Ma'ruf in 1577, was one of the largest astronomical observatories in medieval world. However, it only existed for a few years and was destroyed in 1580.

Triquetrum (astronomy)

The triquetrum was the medieval name for an ancient astronomical instrument first described by Ptolemy in the Almagest. Also known as Parallactic Rulers, it was used for determining altitudes of heavenly bodies. Ptolemy calls it a "parallactic instrument" and seems to have used it to determine the zenith distance and parallax of the Moon.

Jacobs staff

The term Jacob's staff, also known as cross-staff, a ballastella, a fore-staff, or a balestilha, is used to refer to several things. 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:

Octant (instrument) Measuring instrument used primarily in navigation; type of reflecting instrument

The octant, also called reflecting quadrant, is a measuring instrument used primarily in navigation. It is a type of reflecting instrument.

Meridian circle

The meridian circle is an instrument for timing of the passage of stars across the local meridian, an event known as a culmination, while at the same time measuring their angular distance from the nadir. These are special purpose telescopes mounted so as to allow pointing only in the meridian, the great circle through the north point of the horizon, the north celestial pole, the zenith, the south point of the horizon, the south celestial pole, and the nadir. Meridian telescopes rely on the rotation of the sky to bring objects into their field of view and are mounted on a fixed, horizontal, east–west axis.

Celestial cartography

Celestial cartography, uranography, astrography or star cartography is the fringe of astronomy and branch of cartography concerned with mapping stars, galaxies, and other astronomical objects on the celestial sphere. Measuring the position and light of charted objects requires a variety of instruments and techniques. These techniques have developed from angle measurements with quadrants and the unaided eye, through sextants combined with lenses for light magnification, up to current methods which include computer-automated space telescopes. Uranographers have historically produced planetary position tables, star tables, and star maps for use by both amateur and professional astronomers. More recently computerized star maps have been compiled, and automated positioning of telescopes is accomplished using databases of stars and other astronomical objects.

Mariners astrolabe

The mariner's astrolabe, also called sea astrolabe, was an inclinometer used to determine the latitude of a ship at sea by measuring the sun's noon altitude (declination) or the meridian altitude of a star of known declination. Not an astrolabe proper, the mariner's astrolabe was rather a graduated circle with an alidade used to measure vertical angles. They were designed to allow for their use on boats in rough water and/or in heavy winds, which astrolabes are ill-equipped to handle. In the sixteenth century, the instrument was also called a ring.

Mural instrument

A mural instrument is an angle measuring device mounted on or built into a wall. For astronomical purposes, these walls were oriented so they lie precisely on the meridian. A mural instrument that measured angles from 0 to 90 degrees was called a mural quadrant. They were utilized as astronomical devices in ancient Egypt and ancient Greece. Edmond Halley, due to the lack of an assistant and only one vertical wire in his transit, confined himself to the use of a mural quadrant built by George Graham after its erection in 1725 at the Royal Observatory, Greenwich. Bradley's first observation with that quadrant was made on 15 June 1742.

Dividing engine

A dividing engine is a device employed to mark graduations on measuring instruments to allow for reading smaller measurements than can be allowed by directly engraving them. The well-known vernier scale and micrometer screw-gauge are classic examples that make use of such graduations.

Quadrant (instrument) Navigation instrument

A quadrant is an instrument that is used to measure angles up to 90°. Different versions of this instrument could be used to calculate various readings, such as longitude, latitude, and time of day. It was originally proposed by Ptolemy as a better kind of astrolabe. Several different variations of the instrument were later produced by medieval Muslim astronomers.

Transversal (instrument making)

Transversals are a geometric construction on a scientific instrument to allow a graduation to be read to a finer degree of accuracy. Transversals have been replaced in modern times by vernier scales. This method is based on the Intercept theorem.

Reflecting instruments are those that use mirrors to enhance their ability to make measurements. In particular, the use of mirrors permits one to observe two objects simultaneously while measuring the angular distance between the objects. While reflecting instruments are used in many professions, they are primarily associated with celestial navigation as the need to solve navigation problems, in particular the problem of the longitude, was the primary motivation in their development.

An Elton's quadrant is a derivative of the Davis quadrant. It adds an index arm and artificial horizon to the instrument. It was invented by John Elton a sea captain who patented his design in 1728 and published details of the instrument in the Philosophical Transactions of the Royal Society in 1732.

Nonius (device)

Nonius is a measuring tool used in navigation and astronomy named in honour of its inventor, Pedro Nunes, a Portuguese author, mathematician and navigator. The nonius was created in 1542 as a system for taking finer measurements on circular instruments such as the astrolabe. The system was eventually adapted into the Vernier scale in 1631 by the French mathematician Pierre Vernier.

References

  1. 1 2 O'Connor, John J.; Robertson, Edmund F., "Abu Mahmud Hamid ibn al-Khidr Al-Khujandi", MacTutor History of Mathematics archive , University of St Andrews .
  2. Tekeli, Sevim (1958), 'Nasiruddin, Takiyuddin ve Tycho Brahe'nin Rasat Aletlerinin mukayesesi'. Ankara Universitesi Dil ve Tarih-Cografya Fakültesi Dergesi, XVI, p. 4. (in Turkish)