Astrolabe

An astrolabe is an astronomical instrument once widely used to measure stars or planets to determine latitude and time, primarily for navigational purposes. The original meaning of the word in Greek is “star-taker.” The astrolabe was probably invented by astronomers in the second century BC.

At least two forms of the astrolabe are known to have existed. The older form, known as the planispheric astrolabe, consists of two circular metal disks, one representing Earth and the other the celestial sphere at some particular location (latitude) on the Earth’s surface. The first of these disks, called the plate

or tympan, is fixed in position on a supporting disk known as the mater. It shows the great circles of altitude at the given latitude. Any given plate can be removed and replaced by another plate for some other altitude with the appropriate markings for that latitude.

The second of the disks, called the rete or spider, is attached to the plate and the mater by a metal pin through its center. The metal disk that makes up the rete is primarily cut out so that it consists of a complex series of curved lines ending in points. The points indicate the location of particular stars in the celestial sphere. The rete can be rotated around the central pin to show the position of stars at various times of the day or night, as indicated by markings along the circumference of the mater.

To use the astrolabe, an observer hangs the instrument from a metal ring attached at the top of the mater. A sighting device on the back, the alidade, is then lined up with some specific star in the sky. As the alidade is moved to locate the star, the rete on the front of the astrolabe is also pivoted to provide the correct setting of the celestial sphere for the given time of day. That time of day can then be read directly off the mater.

A much simpler form of the astrolabe was invented in about the fifteenth century by Portuguese navigators. It consisted only of the mater and the alidade, suspended from a ring attached to the mater. The alidade was used to determine the elevation of a star above the horizon and, thus, the latitude of the ship’s position. This form of the astrolabe, known as the mariner’s astrolabe, later evolved into the instrument known as the sextant.

More elaborate forms of the mariner’s astrolabe were later developed and are still used for some specialized purposes. One of these, known as the impersonal astrolabe, was invented by the French astronomer André Danjon (1890-1967). The modern prismatic astrolabe is based on Danjon’s concept. In this form of the astrolabe, two light rays from the same star are passed through a prism, one directly and one after reflection from the surface of a pool of mercury. The star is observed as it rises (or sets) in the sky. During most of this period, the two light rays passing through the prism are out of phase with each other. At some point, the specific latitude for which the astrolabe is designed is attained and the two star images coincide with each other, giving the star’s precise location at that moment.

See also Celestial coordinates.

Astrolabe

An astrolabe is an astronomical instrument once used widely to measure stars or planets in order to determine latitude and time , primarily for navigational purposes. The original meaning of the word in Greek is "star-taker." The astrolabe was probably invented by astronomers in the second century b.c.

At least two forms of the astrolabe have existed. The older form, known as the planispheric astrolabe, consists of two circular metal disks, one representing Earth and the other, the celestial sphere at some particular location (latitude) on the Earth's surface. The first of these disks, called the plate or tympan, is fixed in position on a supporting disk known as the mater. It shows the great circles of altitude at the given latitude. Any given plate can be removed and replaced by a plate for some other altitude with the appropriate markings for that latitude.

The second of the disks, called the rete or spider, is attached to the plate and the mater by a metal pin through its center. The metal disk that makes up the rete is primarily cut out so that it consists of a complex series of curved lines ending in points. The points indicate the location of particular stars in the celestial sphere. The rete can be rotated around the central pin to show the position of stars at various times of the day or night, as indicated by markings along the circumference of the mater.

To use the astrolabe, an observer hangs the instrument from a metal ring attached at the top of the mater. A sighting device on the back of the astrolabe, the alidade, is then lined up with some specific star in the sky. As the alidade is moved to locate the star, the rete on the front of the astrolabe is also pivoted to provide the correct setting of the celestial sphere for the given time of day. That time of day can then be read directly off the mater.

A much simpler form of the astrolabe was invented in about the fifteenth century by Portuguese navigators. It consisted only of the mater and the alidade, suspended from a ring attached to the mater. The alidade was used to determine the elevation of a star above the horizon and, thus, the latitude of the ship's position. This form of the astrolabe, known as the mariner's astrolabe, later evolved into the instrument known as the sextant .

More elaborate forms of the mariner's astrolabe were later developed and are still used for some specialized purposes. One of these, known as the impersonal astrolabe, was invented by the French astronomer André Danjon (1890-1967). The modern prismatic astrolabe is based on Danjon's concept. In this form of the astrolabe, two light rays from the same star are passed through a prism , one directly and one after reflection from the surface of a pool of mercury. The star is observed as it rises (or sets) in the sky. During most of this period, the two light rays passing through the prism are out of phase with each other. At some point, the specific latitude for which the astrolabe is designed is attained and the two star images coincide with each other, giving the star's precise location at that moment.

See also Celestial coordinates.

Astrolabe

The astrolabe is an ancient astronomical instrument, dating back more than 2,000 years, used to observe the positions of the stars. With modifications it has also been used for time-keeping, navigation, and surveying.

Astrolabes depict the visual reference points of stars on the night sky as a function of time. As such, an observer can also set the time to predict the visible star pattern expected. The most common type of astrolabe, the planispheric astrolabe, consists of a star map (the rete) engraved on a round sheet of metal. With regard to the rete, only the angular relationship of the stars needs to be accurate to ensure proper functioning of the astrolabe. A metal ring is moved across the map to represent the position of the local horizon. An outer ring is adjusted to allow for the apparent rotation of the stars around the North Star, using prominent stars as reference points.

Astrolabes were forerunners of mechanical clocks, and looked somewhat like watches. With a set of tables, the observer could determine the day and hour for a fixed location by the position of the stars. With the addition of a sighting-rule, called an alidade, an astrolabe could be used as a surveying instrument. The rule could be moved across a scale to measure elevation. Navigational astrolabes marked celestial altitudes (the altitude in degrees above the horizon).

Although there is evidence to support the assertion that ancient Greek culture had astrolabes, it is certain that the Arabs perfected and made regular use of the astrolabe. With the clear desert sky at their constant disposal, the Arab people excelled in astronomy and used the stars to navigate across the seas of sand . Regular use of astrolabes continued into the 1800s. The newer prismatic astrolabe continues to be used for precision surveying.

Modern versions of stellar charts and bowls with adjustable time and date markings on sliding rings are based upon earlier astrolabe construction and design principles.

See also Celestial sphere: The apparent movements of the Sun, Moon, planets, and stars; History of exploration I (Ancient and classical); History of exploration II (Age of exploration)

astrolabe Early astronomical instrument for showing the appearance of the celestial sphere at a given moment and for determining the altitude of celestial bodies. The basic form consisted of two concentric disks, one with a star map and one with a scale of angles around its rim, joined and pivoted at their centres (rather like a modern planisphere), with a sighting device attached. Astrolabes were used from the time of the ancient Greeks until the 17th century for navigation, measuring time, and terrestrial measurement of height and angles.

as·tro·labe / ˈastrəˌlāb/ • n. chiefly hist. an instrument formerly used to make astronomical measurements, typically of the altitudes of celestial bodies, and in navigation for calculating latitude, before the development of the sextant.