USING THE ASTROLABE

The astrolabe was used to make a number of different astronomical calculations. Users could find things such as the altitude of a star or the sun, the position of the sun along its annual path through the zodiacal constellations (the ecliptic) as viewed from earth, the time of sunrise and and sunset and the length of day. Here are a few instructions for some of these calculations:

Finding the altitude of a star or sun:

Suspend the astrolabe by its ring and rotate the alidade until you see the Sun's shadow or the star through its sight. Note its altitude on the altitude scale.

Finding the position of the sun along the ecliptic (the sun's annual path through the zodiacal constellations as seen from earth):

Use the two circles on the back of the astrolabe which give the position of the sun for every date.

Finding the time of sunrise:

Determine the sun's position in the ecliptic. Rotate the rete until that point of the ecliptic is on the eastern horizon of the latitude plate. Read the time where the rule meets the limb.

 Astrolabe c. 1600 Maker Unknown England? Brass Presented by A.E. Gunther This astrolabe was first donated to the University in 1659 by Nicholas Greaves, the brother of John Greaves the Savilian Professor of Astronomy at Oxford. The rete is similar to that of an astrolabe signed by the well-known British instrument maker Humfrey Cole, now at the British Museum.

 Astrolabe, Hispano-Moorish c. 1300 Maker Unkown Spain? Brass Billmeir Collection Islamic designs for the astrolabe, like the bird shaped star pointers on this rete, were introduced into Europe through Moorish Spain. The pin, rule and alidade may be later replacements.

 Linear Astrolabe 1943 Made by Henri Michel Brussels Ebony and Ivory Billmeir Collection Modern reconstruction of a linear astrolabe described by the 13th century Iranian mathematician al-Tusi. The baton represents a specific latitude, while the threads and scales can be used to represent the circles on a standard astrolabe.

 Geographical Astrolabe 1560 Made by Gillis Coignet Antwerp Brass Lewis Evans Collection This astrolabe’s only plate is engraved with a map from the Tropic of Capricorn to the north Pole, encompassed by a circle of latitudes. The back includes an unusual Rojas-style sundial.

 Universal & Geographical Astrolabe c. 1600 Made by Morillard Narbonne and Paris, France Brass & Blued Steel Billmeir Collection The Parisian maker Sevin added a Rojas-style universal projection and a set of rules to this astrolabe in the late 17th century.

 Astrolabe Quadrant 1804 Made by 'Abdallah Ahmad, born 'Alî al-Andalusî Moorish? Brass and Stone Purchased by the Museum This astrolabe-quadrant is in the style of the 13th century polymath Prophatius Judaeus. It was made for a “jurist and noble scribe” and features lines for the Muslim prayer times. The back features a sine and cosine graph. It is inscribed ‘for the region of Meknes’ [in present day Morocco] and is for the latitude of 34°.