Book III covers the mediaeval Quadrivium, the four subjects that supplemented the Trivium being mathematics, geometry, music, and astronomy. He argues that there are infinitely many numbers, as you can always add one (or any other number) to whatever number you think is the limit. He attributes geometry to Ancient Egypt, arguing that because the River Nile flooded and covered the land with mud, geometry was needed to mark out people's land "with lines and measures". Isidore distinguishes astronomy from astrology and covers the world, the sky and the celestial sphere, the zodiac, the sun, moon, stars, Milky Way, and planets, and the names of the stars. He derives the curved ("curvus") vault of the heavens from the Latin word for "upside-down" ("conversus"). He explains eclipses of the sun as the moon coming between the earth and the sun and eclipses of the moon as happening when it runs into the shadow of the earth. He condemns the Roman naming of the planets after their gods: Jupiter, Saturn, Mars, Venus, and Mercury.