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Until the mid‐16^th century, it was accepted as an established fact by most civilizations that Earth lay at the center of the universe.² Like the axle of a wheel, everything else rotated around it.

Figure 1.1 The relative sizes of the orbits of the “planets” visible to the naked eye and recognized by ancient astronomers. All the orbits are slightly elliptical and nearly in the same plane as Earth's orbit (the ecliptic). The diagram is from a view above the ecliptic plane and away from the perpendicular axis that goes through the Sun.

(Lunar and Planetary Institute)

The reasons for this thinking seemed self evident. All the celestial objects, including the Sun, moved across the sky from east to west (with the occasional exception of a comet or shooting star). However, since no one experienced any of the sensations that would be expected if Earth was continually spinning, it seemed logical to believe that it was the heavens which were in motion around Earth.

According to this geocentric theory, the Sun, Moon, and planets were carried by invisible, crystalline spheres which were centered on the Earth. A much larger celestial sphere carried the fixed stars around the central Earth once every day.

Although early civilizations accepted the visual evidence that Earth is (more or less) flat, this idea was contradicted by several lines of evidence (see Chapter 3). For example, different star patterns or constellations are visible from different places. However, if Earth is flat, then the same constellations should be visible everywhere at a certain time.

One key piece of evidence was the curved outline of Earth's shadow as it drifted across the face of the full Moon during a total lunar eclipse. This was the case no matter where the observation was made or at what time it took place. Since only a spherical body can cast a round shadow in all orientations, it seemed clear that Earth was round.

Similarly, observations of a sailing ship disappearing over the horizon showed that, instead of simply becoming smaller and smaller, its hull disappeared from view before the sails and mast. This could only be explained on a curved ocean.