The Sphere of Johannes de Sacrobosco

DE SACROBOSCO CAP. I.

its rising or setting. But we see the opposite happening. The Sun, or another star existing in the east or west, appears larger than when in the middle of the heaven. But since the truth of the matter is not so, the cause of this appearance is that in winter or rainy weather, certain vapors rise between our sight and the Sun or another star; and since those vapors are a corpus diaphanum transparent body, they scatter our visual rays so that they do not comprehend the object in its natural and true size, just as is clear with a coin cast into the bottom of clear water, which, because of a similar scattering of rays, appears of a greater size than its true one.

A collection of five astronomical and optical diagrams demonstrating principles of vision and refraction:
1. Top: A triangular framework with stars at its vertices. An eye at the bottom vertex looks upward through the triangle toward a small sphere.
2. Middle Left: A sunburst emits rays. One ray strikes a crescent moon with a face, while another passes it to reach an eye. A third ray goes directly from the sun to the eye.
3. Middle Center: A moon with a face is positioned above a sphere (the Earth), with lines connecting them to illustrate a field of view or illumination.
4. Middle Right: Within a rectangular frame, an eye looks down into a vessel of water. At the bottom lies a coin inscribed "MONETA NOVA" (New Money) with a central cross. Lines from the eye bend at the water's surface, illustrating the refraction that makes the coin appear larger or in a different position.
5. Bottom: A large sphere on the left (the Sun) casts a conical shadow behind a smaller sphere (the Earth). An eye on the far right is positioned at the apex of this shadow cone, representing an eclipse or the limits of a shadow.

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