Mural instrument
Wall-mounted angle measurer for meridian astronomy.
A mural instrument is a device for measuring angles that is either mounted on or built into a wall. For astronomical use, these walls were aligned exactly with the meridian. When such an instrument measured angles from 0 to 90 degrees, it was known as a mural quadrant. These tools were used in ancient Egypt and ancient Greece. The mural quadrant became known as the "quintessential instrument" of 18th-century observatories and gained importance in positional astronomy during that period.
**Construction**
Older mural quadrants were often made by marking divisions directly onto the wall surface. Later versions used a precisely built frame that was permanently fixed to the wall. The arc of the instrument is marked with divisions, almost always in degrees and fractions of a degree. In the oldest designs, a fixed indicator sat at the center of the arc. An observer could move a device with a second indicator along the arc until the line of sight from the movable indicator through the center indicator aligned with an astronomical object. The angle was then read, giving the object's elevation or altitude. For smaller instruments, an alidade could be used. More modern mural instruments employed a telescope with a reticle eyepiece for observation. Many mural quadrants were built to measure a 90-degree range of elevation, though mural sextants that read 60 degrees also existed. Mural quadrants from the 1600s were notably expensive: Flamsteed's 1689 quadrant cost £120 (equivalent to £26,377 in 2025), and Edmond Halley's 1725 quadrant cost over £200 (equivalent to £35,630 in 2025). These large fixed quadrants were pricier than typical portable ones; for example, a Bird 2-foot quadrant cost 70 guineas, or £73.50 (equivalent to £13,094 in 2025).
**Usage**
To measure the position of a star, an observer needed a sidereal clock alongside the mural instrument. Using the clock to track time, the observer watched the star through the instrument until it crossed an indicator, signaling it was transiting the meridian. At that moment, the time on the clock and the star's angular elevation were recorded. This gave the star's position in the instrument's coordinates. If the arc was not marked relative to the celestial equator, the elevation was adjusted to find the star's declination.
- Field
- Positional astronomy
- Known for
- Measuring the elevation or altitude of astronomical objects as they transit the meridian
- Earliest known use
- Ancient Egypt and ancient Greece
- Notable examples
- Tycho Brahe's mural quadrant, Ptolemy's mural quadrant
Lore & Background
Many older mural quadrants were 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 for observation. Notably, during Edmond Halley's tenure at Greenwich, no transit instrument with a single vertical wire was used—such an instrument was installed later by his successor, James Bradley. Additionally, Ulugh Beg's Fakhri Sextant is not a mural instrument; it is a sextant built into a trench, designed to measure angles up to 60 degrees.
Reader's Guide
Mural instruments, particularly mural quadrants, were central to the development of positional astronomy during the 18th century. By being fixed to a wall aligned precisely on the meridian, they allowed astronomers to measure the elevation of stars and planets at the moment they crossed the meridian. Combined with a sidereal clock, this yielded the star's declination and right ascension, providing fundamental data for celestial cartography and navigation. The instrument's fixed, large-scale construction improved accuracy over portable devices, though at considerable expense. Notable examples include the mural quadrant at the Royal Observatory, Greenwich, used by Edmond Halley and later by James Bradley, and the massive Fakhri Sextant built by Ulugh Beg in Samarkand. The mural quadrant's prominence in observatories of the 1700s earned it the description of the 'quintessential instrument' of that era. Its legacy is also preserved in the obsolete constellation Quadrans Muralis, which represents a mural quadrant.
Did You Know?
- A mural instrument that measured angles from 0 to 90 degrees was called a mural quadrant.
- Edmond Halley used a mural quadrant built by George Graham at the Royal Observatory, Greenwich, after its erection in 1725.
- Ulugh Beg constructed the 'Fakhri Sextant' in Samarkand with a radius of 40 meters.
- The obsolete constellation Quadrans Muralis represents a mural quadrant.
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