Celestial globe

This ambiguity is famously evident in the astronomical ceiling of New York City's Grand Central Terminal, whose inconsistency was deliberately left uncorrected though it was noticed shortly after installation.

[2][3] Working under the incorrect assumption that the cosmos was geocentric the second-century Greek astronomer Ptolemy composed the Almagest in which "the movements of the planets could be accurately represented by means of techniques involving the use of epicycles, deferents, eccentrics (whereby planetary motion is conceived as circular with respect to a point displaced from Earth), and equants (a device that posits a constant angular rate of rotation with respect to a point displaced from Earth)".

[4] Guided by these ideas astronomers of the Middle Ages, Muslim and Christian alike, created celestial globes to "represent in a model the arrangement and movement of the stars".

What is known is that in book VIII, chapter 3 of Ptolemy's Almagest he outlines ideas for the design and production of a celestial globe.

This includes some notes on how the globe should be decorated, suggesting ‘the sphere a dark colour resembling the night sky’.

[11] Similarly, it was "instrumental in displacing the traditional Bedouin constellation imagery and replacing it with the Greek/Ptolemaic system which ultimately came to dominate all astronomy".

[13] In the 13th century, a celestial globe, now housed in the Mathematisch-Physikalischer Salon in Dresden, was produced at one of the most important centres of astronomy in intellectual history, the Ilkhanid observatory at Maragha in north-western Iran constructed in 1259 and headed by Nasir al-Dln TusT (d. 1274), the renowned polymath.

All forty-eight classical constellations used in Ptolemy's Almagest are represented on the globe, meaning it could then be used in calculations for astronomy and astrology, such as navigation, time-keeping or determining a horoscope.

While they are no longer used in astronomy today, they are called "ecliptic latitude circles" and help astronomers of the Arabic and Greek worlds find the co-ordinates of a particular star.