Long before recorded history, humans observed the world around them, taking note of the rise and fall of tides, the phases of the Moon, and countless other physical and astronomical phenomena. Great thinkers such as Aristotle recorded their observations of these phenomena. One of Aristotle's conclusions was that heavier objects fall faster than lighter objects. Aristotle also surmised that the rate at which an object falls toward Earth when dropped is directly proportional to its mass, i.e., an object with twice the mass of another would fall twice as fast.

But Aristotle was wrong. Certainly more massive objects sometimes fall faster than lighter objects. A bowling ball, for example, will fall faster than a feather. But heavier objects will not always fall faster than lighter ones, and Galileo sought to illustrate why.

Through a "thought experiment," Galileo showed that falling objects with different masses could not possibly "know" their mass relative to other falling objects, whether they were attached to those objects or not. Galileo therefore concluded that all objects accelerate toward Earth at a set rate.

The reason feathers fall more slowly than bowling balls is because of air resistance. This force resists the acceleration of falling objects, and the larger an object's surface area, the greater the resistant force. In the absence of air resistance, such as in a vacuum or on the Moon (where there is almost no atmosphere), all objects accelerate toward the ground at the same rate.