Astrolabe

The astrolabe is a historical astronomical instrument and analog computer. Its many uses include finding and predicting the positions of the stars and the sun; determining local time given local longitude, and vice-versa; and surveying and triangulation.

No one knows for certain who invented the astrolabe, but it was was the chief navigational instrument until the invention of the sextant in the 16th century. Some historians credit the invention of the astrolabe to Hipparchus (2nd century BC) while others credit the Persian Fazari (Richard Nelson Frye: Golden Age of Persia. p163). In the 15th Century, a metal astrolabe was developed by Abraham Zacuto in Lisbon, which improved on the accuracy of its wooden Arabian precursors.

An astrolabe consists of a hollow disk, called the mater, which is deep enough to hold one or more flat plates called tympans, or climates. A tympan is made for a specific latitude and is engraved with a stereographic projection of lines of equal azimuth and altitude representing the portion of the celestial sphere which is above the local horizon. The rim of the mater is typically graduated into hours of time, or degrees of arc, or both. Above the mater and tympan, the rete, a framework bearing a projection of the ecliptic and several pointers indicating the positions of stars, is free to rotate. Some astrolabes have a narrow rule which rotates over the rete, and may be marked with a scale of declinations.

As the rete is rotated, the stars and the ecliptic move over the projection of the sky coordinates on the tympan. A complete rotation represents the passage of one day. The astrolabe is therefore a predecessor of the modern planisphere.

On the back of the mater there will often be engraved a number of scales which are useful in the astrolabe's various applications; these will vary from designer to designer, but might include curves for time conversions, a calendar for converting the day of the month to the sun's position on the ecliptic, trigonometric scales, and a graduation of 360 degrees around the back edge. Another ruler, called the alidade, is attached to the back face. When the astrolabe is held vertically, the alidade can be rotated and a star sighted along its length, so that the altitude (in degrees) of the star could be read ("taken") from the graduated edge of the astrolabe; hence "astro" = star + "labe" = to take.

The astrolabe reached the Islamic world in the 8th or 9th century, and it was re-introduced to Europe via Islamic Spain in the 11th century (early Christian recipients of Arab astronomy included Gerbert of Aurillac and Hermannus Contractus). The mathematical background was established by Al-Battani in his treatise Kitab az-Zij (ca. 920), which was translated into Latin by Plato Tiburtinus (De Motu Stellarum).

The English author Geoffrey Chaucer (ca. 1343 - 1400) compiled a treatise on the astrolabe for his son, mainly based on Messahalla. The same source was translated by the French astronomer and astrologer Pelerin de Prusse and others. The first printed book on the astrolabe was Composition and Use of Astrolabe by Cristannus de Prachaticz, also using Messahalla, but relatively original.

In the 15th century, the French instrument-maker Jean Fusoris (ca. 1365 - 1436) started selling astrolabes in his shop in Paris, along with portable sundials and other popular scientific gadgets of the day.

In the 16th century, Johannes St?er published Elucidatio fabricae ususque astrolabii, a manual of the construction and use of the astrolabe.

See also

• Antikythera mechanism
• Armillary sphere
• Astrology
• Astronomical clock
• Orrery
• Planetarium
• Prague Orloj
• Torquetum

References

• Critical edition of Pelerin de Prusse on the Astrolabe (translation of Practique de Astralabe). Edtiors Edgar Laird, Robert Fischer. Binghamton, New York, 1995, in Medieval & Renaissance Texts & Studies. ISBN 0866981322

External links

• The Astrolabe (http://www.astrolabes.org)