The term astrolabe designates several related observational instruments used primarily for astronomical and navigational calculations, the most common and historically significant of which is the planispheric astrolabe. Others include the rare linear and spherical astrolabes and the simplified mariner’s astrolabe. The astrolabe’s basic function was observing the angular distance between objects, such as between a star and the horizon. Planispheric astrolabes were also designed to make calculations and predictions based on a rotating planar projection of the celestial sphere, which enabled the user to, among other things, tell time. These astrolabes illustrated the annual path of the sun and the daily rotation of the stars, which could be related to the current time or predicted for any specific time. Thus, the planispheric astrolabe was, in some sense, an observational tool, a calculator, a clock, and a calendar all in one. In fact, it was the design of the planispheric astrolabe, with its two-dimensional depiction of the celestial rotation, that inspired the familiar circular face of the mechanical clocks that began to emerge in the 14th century. On astronomical clocks, popular in the 15th and 16th centuries, the planispheric projection used on the astrolabe was retained inside the center of a 24-hour clock face.
Although the exact origin of the astrolabe is unknown, it is thought to have developed in Greek antiquity, perhaps as early as Hipparchus but at least by the time of Ptolemy in the 2nd century CE. Although the astrolabe fell out of use in the West during the latter portion of the 1st millennium CE, its popularity flourished in Arabic culture for centuries. Particularly noteworthy is the work of 10th-century Islamic astronomer Abd al-Rahman al-Sufi, who is said to have described some 1,000 uses of the astrolabe. The instrument reentered Western astronomy during the 11th century through contact with Islamic culture, alongside the rediscovery and translation of classical texts. Hermann the Cripple (1015-1054), a Benedictine monk, wrote of the astrolabe in Latin around this time; Geoffrey Chaucer wrote the first English treatise on the subject 3 centuries later.
Planispheric astrolabes are similar in shape to modern pocket watches. Typically made of brass or other metals, the instrument is built around a circular base plate called a mater. The back of the astrolabe is engraved around the outside of the mater with a scale of degrees for measuring angles, as well as other scales to indicate the position of the sun on the ecliptic. Attached to the center is a rotating sighted bar called an alidade, which is used for measuring angles in the sky. Such observations can be taken by holding the astrolabe vertically by an attached ring, moving the alidade to align a celestial object with both sights, and reading the degrees indicated by the alidade on the scale. The front of the astrolabe is used for representing the observed rotation of the heavens in a planispheric projection. The top layer of the front consists of the rete, a skeletal rotating plate that represents the positions of the stars and the ecliptic path of the sun. Beneath the rete is a detachable plate marked with celestial coordinates that can match the position of the rete to the altitudes observed with the alidade. These plates are necessarily specific to particular latitudes; hence most astrolabes were provided with multiple interchangeable plates. The front of the astrolabe typically also included a scale of hours around the outside and a pivoting ruler to aid in measurements. The mariner’s astrolabe was a simplified version of this instrument used only for observing celestial altitudes at sea.
The versatility and ingenuity of the astrolabe made it popular among astronomers from antiquity until the beginnings of modernity. A sighting instrument, a calculator, a star map, a clock, or a calendar, the astrolabe had numerous applications to astronomy and navigation as well as astrology. Because it recreates and measures the rotation of the heavens, the astrolabe has also much connection with the measurement of time. Astrolabes could be marked to indicate both uneven and even hours of the day and could relate the time of year based on the sun’s position in the ecliptic. A notable symbol of classical and medieval astronomy, the astrolabe is also the primary precedent for the circular clock format used around the world today.
See also Chaucer, Geoffrey; Clocks, Mechanical; Measurements of Time
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