Jean-Roch’s astrolabe

This particular astrolabe is a modern rendition of the historical tool used by philosophers, astronomers, mathematicians and builders alike for hundreds of years and still functions in the same way. In short, it is a projection of the sky on a two dimensional surface specific to the viewers geographical coordinates, that can be used to compute various information. Since the location of the different constellations is in constant movement in the sky as the Earth rotates around the sun and on itself, the astrolabe can easily compute the location of various celestial bodies at a precise date and time. Furthermore, since the projection of the sky varies greatly in regards to the user’s latitude and longitude, different sky maps called Plates are placed on the astrolabe, which makes it a site specific tool.
On the Plate of the astrolabe are the projections of different astronomical concepts which are fixed: the Tropic of Capricorn is represented by the outer ring, the Tropic of Cancer by the inner ring and the Equator by the middle ring (the Tropics would be reversed if the astrolabe was to be used in the southern hemisphere). The Almucantar and Azimuth lines are drawn in concordance to the latitude of the user: they represent a grid that is used to locate objects in relation to the horizon, the zenith (at the centre of the Almucantar rings), east and west as well as north and south. On the circumference of the Plate are the 24 hours of the day numbered in clockwise direction.
On top of the Plate is the Rete, which represents a projection of the celestial sphere that can be rotated on the fixed Plate. It consists of the Ecliptic of the sun’s path represented by an offset circle and the most important stars and constellations represented by arrows or dots. Over the Rete is the Rule, which is used to relate the hours of the day to the location of the sun in the sky.
In order to correctly read the astrolabe, you have to position the top of the Plate called the Throne due south, and look at it as you would at a map. In order to know the location of celestial objects at a specific date and time, turn the Rule the the corresponding Zodiac Scale of today’s date (usually found on the backside of the astrolabe) along the sun’s Ecliptic. Turn both Rule and Rete so that the Rule points at the correct time of the day. The end result is a map of the current sky, where the position of objects is read by their degree above the horizon, and degree north or south of east or west. For example, on October 4th (Libra 11) at 4pm, Altair is located at 35 degrees above the horizon, 30 degrees south of east.


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