Galileo's experiment on the moon

Aristotle (384-322 BC) was one of the great Greek philosophers, who speculated on many fields of human experience, including motion. Although one of the most important names in western philosophy, his scientific thinking was sometimes fatally marked by a lack of experimental work. This led him to make curious blunders, such as the assertion that the human male has more teeth than the female! However, he laid the foundations of logical reasoning, made some rigorous observations of marine creatures, and is rightly regarded as one of the great thinkers of all time.

Such was his fame that for nearly 2000 years, if anyone disagreed with Aristotle, their ideas never took root. The earth was accepted as being at the centre of the universe, even though some of Aristotle's contemporaries believed otherwise. Again, according to Aristotle, a falling objects gains speed, based on its weight. A heavier object will fall faster, and a lighter one will fall slower.

Sometime around 1589, an Italian scientist named Galileo Galilei advanced a theory that contradicted Aristotle, and stated that "the velocity of moving bodies of the same composition, but of different weights, moving through the same medium, do not attain the proportion of their weight as Aristotle decreed, but move with the same velocity."

Essentially, what he meant was that bodies of different weights would fall at the same velocity. Bodies of different materials, though, initially fell at slightly different rates, but attained the same velocity. Galileo is then believed to have gone on and dropped a cannonball and a musket ball from the Leaning Tower of Pisa, demonstrating the truth of his theory.

Apparently Galileo was not the first to challenge Aristotle. But he was the first to muster both theory and experiment in his cause. As for Galileo's observation about objects with different densities, there are two points of view. One is that denser objects would be less affected by air. The second is that human error while holding and dropping a heavy load would ensure that the heavier object was dropped slightly after the lighter one.

Whatever the exact truth, Aristotle's assertion now belonged only in scientific history.

If you try this experiment with the lighter object being something like paper or a feather, air resistance would slow it down severely. So what would happen in a vacuum? Theory, as well as experiments in vacuum chambers tell us that a feather and a steel weight would fall at the same rate. Imagine then, the excitement when this experiment was tried on the Moon which, as you know, has no atmosphere!

Transcript of broadcast from the lunar surface, on 2 August, 1971

Commander David R Scott: "Well, in my left hand I have a feather; in my right hand, a hammer. And I guess one of the reasons we got here today was because of a gentleman named Galileo, a long time ago, who made a rather significant discovery about falling objects in gravity fields. And we thought: 'Where would be a better place to confirm his findings than on the Moon?'

[Camera zooms in on Scott's hands. One is holding a feather, the other a hammer. The camera pulls back to show the Falcon the Apollo 15 landing craft and the lunar horizon.]

Scott: "And so we thought we'd try it here for you. The feather happens to be, appropriately, a falcon feather for our Falcon. And I'll drop the two of them here and, hopefully, they'll hit the ground at the same time." [Scott releases hammer and feather. They hit the ground at about the same time.]

Scott: "How about that! Mr Galileo was correct in his findings."