Ptolemy is standing with his back to the viewer
in the right foreground of Raphael's fresco. he is holding an
earth sphere and is facing Zoroaster who is holding a celestial
sphere.
Ptolemy is a controversial figure. Some regard him
as a great mathematician and astronomer and others (including
the great scientist Newton) accused him of fraud. He is credited
with developing a mathematical theory of the motion of the sun,
moon and planets and his famous treatise on the subject survived
as an authoritative source until the publication of the theories
of Copernicus in the 16th century.
To read more about Pytolemy and to study his writings,
examine the following web pages: