Corpus of Electronic Texts Edition
An Irish Astronomical Tract (Author: [unknown])

Caibidil 18

Concerning the size of the sun.

Rationes geometricae, etcetera.

The geometrical calculations of Ptolemy, the astrologer, prove the size of the sun. He says: The size of the sun must be equal to, lesser, or greater than the size of the earth ; and if the sun and the earth were equal, the shadow of the earth, i.e., the darkness co-extensive with the earth itself, would travel out to the sphere of the fixed stars and obscure them, and an eclipse, i.e., deficiency of light in the moon, would occur every month throughout the year; for the earth's mass, which would be as large as the mass of the body of the sun, would deprive the moon and the stars of the sun's light, and there would then never be a moon, but constant darkness from the beginning of the night until the end. Therefore, since the moon is present and the stars are seen illuminated in the night, let it be understood from this that the sun and the earth are not equal to each other.

To make it clearly understood, I will make a figure of three circles around each other; the outer circle for the orbit of the fixed stars, the middle circle for the orbit of the sun, and the smallest circle for the orbit of the moon; and the earth in the middle, and the sun down beneath the earth in its own sphere co-extensive with the earth, and the shadow of the earth opposite the sun on the other side of the earth, and co-extensive with the earth passing out straight to the sphere of the stars.

to face p. 84 (2)