Perhaps this Dialogue throws more light on the views about the solar system accepted or under discussion in the first century of our era than a scientific treatise could have done. No reference is made to the great astronomical work of Ptolemy, which belongs to the second century, and closed most questions until the sixteenth. The estimate, e.g. of the moon’s distance (56 earth’s radii) is not Ptolemy’s (59). Some of the geographical details, as that of the Caspian Sea, seem to show that Ptolemy’s geographical work was not known to the Author.
It may be useful to enumerate some of the simpler of the accepted views about the heavens :
(1) That the earth is a Sphere was known to Pythagoras and allowed by Plato (Phaedo 110 B), and affirmed by Aristotle, De Caelo, 2, 14, 297 b 18. The moon, and, according to Aristotle, the stars, are also spherical.
(2) That the moon derived her light from the sun was a discovery due to Anaxagoras (fifth century B.C.).
(3) The true cause of eclipses was known to the Pythagoreans, and is stated by Aristotle, and, with more precision, by Posidonius.
(4) The inclination of the equator to the sun’s path is stated by Oenopides of Chios (a little after Anaxagoras).
(5) That the moon revolves round the earth at a moderate distance is stated by Empedocles.
(6) The other planets (including the sun) revolve round the earth at a distance vastly less than that of the fixed stars. (No actual estimate of the distances or sizes is given even by Ptolemy, who is not able to state a parallax for any, or an angular diameter.)
(7) That the planets share in the (apparent) daily motion of the stars, and also have an (apparent) motion of their own in the reverse direction was held by Pythagoras.
All these refer to physical facts and can be stated without the use of mathematical language, though many of the discoverers were expert mathematicians. Gradually, and certainly from the time of the great astronomer Hipparchus (about 130 B.C.), attention came to be fixed upon the accurate mathematical interpretation of observed apparent facts; in a favourite phrase, the object was ‘to save the phenomena’, irrespective of physical and actual fact.