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The Experiment
Galileo's landmark work, "Two New Sciences," describes the results of such an experiment, explains Stanford University's Gravity Probe B. One of the participants of the scientific dialogue announces, "I, Simplicio, who have made the test, can assure you that a cannon ball weighing one or two hundred pounds, or even more, will not reach the ground by as much as a span ahead of a musket ball weighing only half a pound, provided both are dropped from a height of 200 cubits." His assistant Viviani later said that Galileo himself did this experiment at the Leaning Tower "in front of all the faculty and students assembled."
The Repetition
An experimental result must be repeatable to be considered valid, warns Anthony Carpi of the City University of New York. Scientists and students have repeated Galileo's experiment many times, with a variety of objects. The University of Virginia's lesson plan suggests modern alternatives, a bowling ball and a tennis ball. Astronaut David Scott tried it on the moon using a feather and a hammer, since the lack of air resistance meant he did not need items with the same basic shape.
The Theory
The results -- that items of different weight hit the ground at the same time -- seems surprising. Aristotle had predicted an opposite result in explaining his theory of gravity. This refinement of theories about gravity is now known as the Equivalence Principle, which, according to the Stanford Gravity lab, means, basically, that "all bodies fall with the same acceleration under the influence of gravity." Author and lecturer Benjamin Crowell explains that the differences in rate of fall of some light objects, like paper or features, is due to their shape, which resists the air flow, not to their mass.
The Facts
While Galileo certainly did perform experiments on gravity, he may not have done so in the way Viviani claimed. Indeed, the account by this student is the only record of the Leaning Tower experiment. The University of Tennessee's Stephen J. Daunt and Christian Cardell note that, based on Galileo's writings, he "probably did similar experiments," regardless of whether he did them the way Viviani described. Also contrary to popular belief, Galileo was not the first to drop things to disprove Aristotle's ideas. Previous scientists had performed similar experiments, including the Byzantine philosopher, Iohannes Philiponus, about 1,000 years before Galileo.
The Legacy
Why, then, is Galileo's experiment so famous? Crowell suggests, "Galileo was the one who changed the course of history because he was able to assemble the observations into a coherent pattern, and also because he carried out systematic quantitative (numerical) measurements." Galileo's work inspired others to follow up on it, and his experiments became foundations for the work of Isaac Newton. The Equivalence Principle also informed the work of Albert Einstein, who used it in formulating the landmark Theory of Relativity.