Throughout his life, Galileo always sought empirical evidence to support or refute the scientific ideas of his time. This was Galileo's approach to the revolutionary theory put forth by Copernicus in the sixteenth century, which said that Earth orbited the Sun. Even though astronomical observations already suggested to Galileo that heliocentrism, as the theory was known, was correct, he continued to look for tangible, earthly evidence that would support it further. While aboard a boat traveling to Venice, he surmised that ocean tides might provide the explanation he was seeking.

Galileo argued that the tides resulted from variations in the surface speed of Earth, which, according to the heliocentric view, is both turning on its axis and orbiting the Sun. He said that at any given time certain points on Earth's surface are moving in the direction of orbit, while other points are moving in the opposite direction, resulting in a difference in surface speed. This slowing down and speeding up, Galileo concluded, would cause the oceans to move back and forth, just as water in a basin sloshes when the basin is rocked back and forth. Despite being based on empirical observations, the conclusion Galileo made about the motion of Earth causing tidal variations was wrong, and it became known as his biggest scientific mistake.

Physicists now know that just as Earth's gravity pulls at the Moon, keeping it in orbit around our planet, the Moon also pulls at Earth. This pull causes ocean water to bulge on the side of Earth nearest the Moon. It also makes the water bulge on the other side of Earth at the same time. This is because the Moon's gravitational pull is strongest on the water closest to the Moon, a bit weaker on the solid Earth and weaker still on the water on the far side of Earth. In essence, the Moon is both pulling the water that is closest to it away from Earth and pulling Earth away from the water on the far side. As Earth spins, the water surrounding land masses experiences these bulges at regular intervals, causing the tides to rise and fall.