Cartographica Helvetica 23 (2001) 19–25
The instrument known as the Astrolabium planisphaerium has been known throughout the late antiquity, during the Middle Ages and up until the 18th century. It allowed astronomical and astrological computations, served in demonstrating celestial movements and could also be used as an observation and measuring instrument. With the aid of a double alidade with sights, it was possible to measure angles whose accuracy, however, did not exceed ¼ degree, even when using the best instruments.
The astrolabe is based on Ptolemy's idea of a geocentric world order with the stars and sky revolving around a fixed earth as opposed to Copernicus' conception of today's rotating stellar maps.
A variation of the instrument, the marine astrolabe, was designed exclusively for measuring vertical angles to the stars and was limited to an alidade and a circle of degrees. In order to make measuring on a pitching boat possible, such instruments were constructed to be very heavy but openwork so as to offer as little wind resistance as possible.