  "El Cielo de Salamanca" ("The Sky of Salamanca") is a quarter-sphere-shaped
vault 8.70 metres in diameter. It was painted sometime between 1480 and 1493
and shows five zodiacal constellations, three boreal and six austral. The Sun
and Mercury are also represented. It formed part of a three times larger vault
depicting the 48 Ptolemaic constellations and the rest of the planets known at
the time. This was a splendid work of art that covered the ceiling of the first
library of the University of Salamanca, one of the oldest in Europe having
obtained its royal charter in 1218. But it was also a pioneering scientific
work: a planetarium used to teach astronomy, the first of its kind in the
history of Astronomy that has come down to us in the preserved part that we now
call "The Sky of Salamanca". We describe the scientific context surrounding the
chair of Astrology founded around 1460 at the University of Salamanca, which
led to the production of this unique scientific work of art and to the
flourishing of Astronomy in Salamanca. We analyse the possible dates compatible
with it, showing that they are extremely infrequent. In the period of 1100
years from 1200 to 2300 that we studied there are only 23 years that have
feasible days. We conclude that the information contained in "El Cielo de
Salamanca" is not sufficient to assign it to a specific date but rather to an
interval of several days that circumstantial evidence seems to place in August
1475. The same configuration of the sky will be observable, for the first time
in 141 years, from the 22nd to the 25th of August 2022. The next occasion to
observe it live will be in 2060.
