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.[1]

Tycho Brahe's mural quadrant
Mural quadrant constructed as a frame mounted on a wall. This instrument was made by John Bird in 1773 and is in the Museum of the History of Science, Oxford.

The mural quadrant has been called the "quintessential instrument" of 18th century (i.e 1700s) observatories.[2] It rose to prominence in the field of positional astronomy at this time.[2]

Construction
A Bird Mural Quadrant was for many years the main instrument of the Mannheim Observatory in Germany, shown here installed.

Many older mural quadrants have been constructed by marking directly on the wall surfaces. More recent instruments were made with a frame that was constructed with precision and mounted permanently on the wall.

The arc is marked with divisions, almost always in degrees and fractions of a degree. In the oldest instruments, an indicator is placed at the centre of the arc. An observer can move a device with a second indicator along the arc until the line of sight from the movable device's indicator through the indicator at the centre of the arc aligns with the astronomical object. The angle is then read, yielding the elevation or altitude of the object. In smaller instruments, an alidade could be used. More modern mural instruments would use a telescope with a reticle eyepiece to observe the object.

Many mural quadrants were constructed, giving the observer the ability to measure a 90° range of elevation. There were also mural sextants that read 60°.

Mural quadrants of the 17th century were noted for their expense, with Flamsteed's costing 120 pounds (1689), and Edmund Halley's costing over 200 pounds (1725).[3] 120 pounds of 1689 converted to the 2019 U.S. dollars can be estimated at over 30,000 USD, and 200 pounds of 1725 to over 50,000 USD.[4] The large fixed quadrants were more expensive than a typical portable quadrant, with a Bird 2-foot quadrant costing 70 guineas.[3]

Usage

In order to measure the position of, for example, a star, the observer needs a sidereal clock in addition to the mural instrument. With the clock measuring time, a star of interest is observed with the instrument until it crosses an indicator showing that it is transiting the meridian. At this instant, the time on the clock is recorded as well as the angular elevation of the star. This yields the position in the coordinates of the instrument. If the instrument's arc is not marked relative to the celestial equator, then the elevation is corrected for the difference, resulting in the star's declination. If the sidereal clock is precisely synchronized with the stars, the time yields the right ascension directly.[5]

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

References
  1. Robert Grant, History of Physical Astronomy from The Earliest Ages to the Nineteenth Century (1852) pp. 483-484.
  2. "2002JHA....33..373T Page 373". adsabs.harvard.edu. Retrieved 2019-11-14.
  3. "2002JHA....33..373T Page 373". adsabs.harvard.edu. Retrieved 2019-11-14.
  4. "Currency Converter, Pounds Sterling to Dollars, 1264 to Present (Java)". www.uwyo.edu. Retrieved 2019-11-14.
  5. Sidereal Time
  6. O'Connor, John J.; Robertson, Edmund F., "Abu Mahmud Hamid ibn al-Khidr Al-Khujandi", MacTutor History of Mathematics archive, University of St Andrews.
  7. Ulugh Beg, Dictionary of Scientific Biography.
  8. Hoyle, Fred, Astronomy, A history of man's investigation of the universe, Crescent Books, Inc., London 1962, p 37.
  9. Mannheimer Observatory quadrant
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.