The astrolabe (or Almincantarat, Almicantarat, of the Arab المقنطرة: Al-Muqantara) is a double plane projection (generally a polar projection) which makes it possible to represent the movement of stars on the vault of heaven. The principle of its construction is known since the Greek time; but its current use was widespread only by the astronomers of the Islam, as from the 7th century. As use limited for the astronomical observations, it is used especially for astrology, the teaching of astronomy, and the calculation of the hour the day by the observation of the Sun or during the night by the observation of stars. In its simplified form, the “nautical astrolabe”, it were the principal instrument of Navigation since the 16th century until the 18th century, at the moment when the Sextant was invented.
Operation
The astrolabe superimposes two scientific instruments. ; Levelling of the stars An astrolabe is composed of a Cercle graduated in degree S (Rapporteur) with a rotating arm attached to its center, the Alidade. When it is suspended by its higher ring, the mark 0° on the circle is aligned with the horizon. If a star, or very other celestial body, is aimed at the end of the mobile arm, the position of star can be read (“taken”) on the graduated dial. The Greek etymology of the name comes from this action: astro = star, labe = to take.This function is the only one realized by the “nautical astrolabes”, used for sea transport, and which do not introduce the central part.
; Calculation of the hour The center of the astrolabe is an abacus making it possible to determine the hour starting from the height of the star, and, its direction.
- On the plate ( to subdue ) is engraved lines which represent the stereographic Projection of the celestial Sphère, only valid for a geographical Latitude given.
- On this grid of coordinates turns the rete , which is a framework with points representing fixed stars.
The height of the star aimed being known, one makes turn the rete until the reference mark of the rete corresponding to the star coincides with the graduation height on the to subdue . In this position, the astrolabe is regulated at local time, and the direction of the axis can be read on the other family of graduation of the to subdue . For a correct reading, it should of course be known if the star concerned is ascending (with the East) or going down (in the occident), which hardly poses problem with the trained observer.
; Use The astrolabe is before a whole instrument intended to read the solar or stellar hour in a given place, to carry out astronomical observations or astrological. When it is intended to be used on a journey, it has a set of several plates, making it possible to adjust the representation of the local sky according to the latitude of the place.
Construction
The interest of such a stereographic Projection of the vault of heaven is primarily practical and esthetic: with this projection, all the circles (heights, of azimuth,…) are transformed into circles, which facilitates the layout largely of it, and led to a result much more esthetic. Without this constraint of realization, an arbitrary central projection could be retained.The rete is not that a reinforcement intended to support matérialisation of the zodiac on the one hand, and the pointers of principal stars on the other hand. This reinforcement is often a true work of art to it only. It tends to appear symmetrical compared to the axis of the solstices, whereas the stellar positions are naturally not it.
The astrolabe is generally dimensioned so that the circle of the zodiac is practically tangeant with the external edge. The stars represented restricted with those are thus located at the top of the tropic of Capricorn. Sirius thus is always represented, often Antarès, but generally not Fomalhaut.
; Dating of an astrolabe The astrolabe is a sufficiently precise instrument so that the Précession of the equinoxes is manifest there: even the astrolabes of the XV° century are not usable any more nowadays. On engraving, one can thus notice that the point more in top on the right, which closes the external circle, and marks the position of Antarès, is appreciably in the alignment of the graduation 28° of the Scorpion (238°). This star is currently (Y2000) with 247°, that is to say 7° of Sagittarius. The precession of the equinoxes being of 1° for 72 years, the difference of 10° corresponds to an age of about 700 years (the original is given for 1208, the difference probably comes from what engraving introduced of small additional variations).
In the astrolabe of the XVI° century below, the position of Antarès is given to 0.5° of Sagittarius, that is to say a shift of 6.5° and an apparent age of about 470 years. The astrolabe being gone back to 1569, the variation (40 years, that is to say a half-degree) appears simply due to the errors of realization and reading.
History and origin
The astrolabe was probably invented by Hipparque. The brass astrolabe is originating in Perse (Iran) and improved in the Islamic world, before reaching the Europe towards 970, via the monk Gerbert d' Aurillac, who brought back it from Spain, from where it brought back number of scientific knowledge transmitted by the Arabs, who occupied the Iberian peninsula partly. The author English Geoffrey Chaucer (v. 1343 - 1400) wrote a treaty on the astrolabe for his son. At the 15th century, the manufacturer of French instrument Jean Fusoris (v. 1365 - 1436) started to sell them in his store with Paris, with sundials portable and other popular scientific instruments at that time.The astrolabe is the name of one of the oldest instruments of navigation which measures the angle formed by the horizon and a celestial body thus giving the exact latitude - but not longitude. The astrolabe exists since Antiquity (). Conceived by Hipparque, Ptolémée studied it later 3 centuries. the astrolabe is used initially for the astrologers, Moslems in particular (10th century). They are Portuguese who ends as from 1485 to decisive progress by adapting the astrolabe to sea transport and by drawing up tables (regimientos) allowing to calculate the magnetic variation. The problem of longitude will be solved only with the invention of the Chronomètre (2nd half of the 18th century). This tool - whose name comes from two Greek words meaning star and to take consists of a circular part, a brass disc or bronzes from 10 to 50 cm in diameter, graduated in degree S on the circumference and a Alidade provided with sights in rotation on the center. The azimuth reading device swivels on its axis and is pointed towards the sun or a star in order to read the angle representing the height of the sun or a known major étoie compared to the horizon, on the reference marks of the disc. The astrolabe is held vertically with the hand by a ring; the stars are aimed while turning the sight until one of them is seen by the two ends. The value in degrees obtained by the sight on the arc can be converted into degrees of latitude of the point of observation.
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