If you gaze at the night sky for a long time, you'll see that the stars move in a circular path around the North Star. This movement suggests either that the Earth spins on its axis (an imaginary line connecting the North and South Poles) with respect to the stars, or that the stars orbit the earth.

Curiously, it was not until the middle of the neneteenth century that Jean Bernard Léon Foucault (1819-1868), a French physicist, proved that the Earht spins on its axis. He made this discovery by settinga heavy pendulum, attached to a long string, in motion. As the pendulum continued to swing for many days, Foucault noted that the plane in which it oscillated (a plane perpendicular to the Earth's surface) appeared to rotate around a vertical axis (a line perpendicular to the Earth's surface.) If Newton's first law of motion—objects in motion remain in motion, objects at rest remain at rest—was correct, then the only exlanation for this phenomenon was that the Earth rotated under the pendulum while the pendulum continued to swing in the same plane (Fig. 1.4a, b). Foucalt displayed his discover beneath the great dome of the Pantheon in Paris, to much acclaim.

We now know that, in fact, the earth's spin axis is not fixed in orientation; rather, it wobbles. This wobble, known as precession corresponds to the wobble of a top as it spins. We'll see later in this book that the precession of the Earth's axis, which takes 26,500 years, may affect the planet's climate.

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