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The astrolabe (from the ancient Greek ἀστρολάβος, astrolabos, via the medieval Latin astrolabium, "star taker") is an astronomical instrument for observation and analog calculation. An instrument with multiple functions, it notably makes it possible to measure the height of stars, including the sun, and thus to determine the time of observation and the direction of the star. Its design, whose Greek origins go back to Antiquity, much later perfected by the Arabs, is based on a flat projection of the celestial vault and the local sphere, called stereographic projection.

A simplified adaptation, the nautical astrolabe, was used for maritime navigation.

Various components

Classical astrolabes are almost all built on the same model.

A summary technical analysis of the instrument makes it possible to visualize its layout and to fix the reference vocabulary used.


- The mother, sometimes called “matrix”, is the main disk on which the other elements are centered. Its limb is graduated. On its upper part is the throne, often worked, which supports the ring of suspension via a bail.

- The alidade (on the back) allows you to aim for the stars. The eardrum is an added disc, integrated into the mother. On its visible face there are plots of line networks necessary for calculating the time, which depend on the latitude of the place. There are most often several eardrums per astrolabe, each corresponding to a latitude.

- The spider, is the projection of the simplified map of the sky. There are placed the main stars and the circle of the ecliptic, place of the Sun in its annual movement. It rotates, like the sky, around its axis, in 24 hours.

- The ruler, or “ostensor”, allows, by rotation, the alignment of points of the spider with marks on the mother or the eardrum. An axle, whose design has varied over the centuries, ensures the assembly of the whole.

The astrolabe superimposes two different main functions which can be associated: measuring the height of a star on the one hand and determining the time of observation on the other. The implementation of these two functions makes it possible to describe the constitution and the elementary layouts of the instrument.

On the back, measurement of a height

On modern astrolabes, the measurement of the height of a celestial body (star or Sun or planet) - or of any object whatsoever - is carried out on the back of the instrument. This operation is the simplest use that can be made with the astrolabe. This is the only function performed by a nautical astrolabe, which does not have an abacus on the front.

On the front, determination of the time

The front of the astrolabe with its main parts makes it possible to determine, among other things, the solar time also called equinoctial time, from the previous data.

Stereographic projection

The "planispheric" astrolabe is obtained by projection of the celestial sphere associated with the local sphere. These two spheres can be imaged by a modeled upright representation of the armillary sphere. The projection used, called stereographic projection, has the essential property of transforming the circles of the sphere into other circles, which can easily be traced in the projection plane when certain points are known. The stereographic projection greatly expands the regions far from the center, therefore here those between the celestial tropics, to the detriment of the polar region. This "inconvenience" when it comes to representing the constellations becomes an advantage in the case of an astrolabe, since it is precisely the sightings located between the celestial tropics that make it possible to determine the time and the direction with the most accuracy. precision.

Astronomical use

On a complete tympanum, we find the almucantarats, the beam of the azimuths and the tracing of the temporary hours. Any problem concerning the association of these elements can be treated with the astrolabe: finding the azimuth and the orientation of the observer on the one hand and, on the other hand, the temporary time of an observation, qu it is arbitrary or carried out at the instants of sunrise or sunset of the stars, at dusk, etc...

The astrolabe today

The astrolabe is an aesthetic and rare object, therefore of great value. It is mainly found in museums, among collectors and specialized art dealers. Due to its rarity, we sometimes find counterfeits on the market that can be revealed by analyzes and non-destructive tests.