Mapping the Universe - Mapping Scientific Frontiers: The Quest for Knowledge Visualization - page 60

Fig. 2.12 Constellations in the northern Hemisphere in 1795s. The Constellations of Eratosthenes

Ptolemy was familiar with the science of map projection through his work
in terrestrial geography. In Planisphaerium , he described the polar stereographic
projection that is ideal for star charts. This projection divides the heavens into
northern and southern hemispheres and spreads each onto planes centered on the
celestial poles. Celestial latitude is stretched progressively away from the poles
toward the equator, and all the stars in one hemisphere can be positioned correctly.
Islamic scholars picked up Ptolemy's science between the eighth and twelfth
centuries. They described the brightest stars, modeled on Ptolemy's Almagest ,and
illustrated each constellation with charts. They also made beautiful, precise celestial
globes. Islamic astronomers perfected a sophisticated scientific instrument called the
astrolabe, which was an essential tool of astronomers until the seventeenth century.
The astrolabes are used to show how the sky looks at a specific place at a given time.
Only two kinds of star maps have survived from the centuries of classical
and medieval astronomy - that embodied by the astrolabe and the image of the
single constellation. Until the fifteenth century, European scientists and scholars