The most notable feature of the Main Concourse is the great astronomical mural,
from a design by the French painter Paul Helleu, painted in gold leaf on
cerulean blue oil. Arching over the 80,000 square-foot Main Concourse,
this extraordinary painting portrays the Mediterranean sky with
October-to-March zodiac and 2,500 stars. The 60 largest stars mark the
constellations and are illuminated with fiber optics, but used to be lit
with 40 watt light bulbs that workers changed regularly by climbing above
the ceiling and pulling the light bulbs out from above. Soon after the
Terminal opened, it was noted that the section of the zodiac depicted by
the mural was backwards. For several decades lively controversy raged over
why this was so. Some of the explanations offered were that it just looked better,
or it didn’t fit into the ceiling any other way. The actual reason is that
Paul Helleu took his inspiration from a medieval manuscript, published in an
era when painters and cartographers depicted the heavens as they would have
been seen from outside the celestial sphere.

from: grandcentralterminal