His idea was that the moon’s orbit was a kind of never-ending fall to the Earth. But unlike a falling apple, the falling moon doesn’t crash to the ground because it’s also simultaneously cruising sideways due to inertia. It’s like one of Galileo’s cannonballs, gliding sideways and falling at the same time, tracing a curved path, except that it’s gliding so fast that it never reaches the surface of the spherical Earth curving away beneath it. As its orbit deviates from a straight line, the moon accelerates—not in the sense that its speed changes, but its direction of motion changes. What pulls
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