November 1, 2011
The Moon has changed phase thousands of times since the beginning of the cosmos, but only once did it shine on the night you arrived.
Hammer Versus Feather on the Moon
Birthday · November 1, 2011
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APOD published November 1, 2011 · Video still. Full image credits are on the original APOD entry below. A publication-date match, not a photo taken on your date. Independent of NASA.
If you drop a hammer and a feather together, which reaches the ground first? On the Earth, it's the hammer, but is the reason only because of air resistance? Scientists even before Galileo have pondered and tested this simple experiment and felt that without air resistance, all objects would fall the same way. Galileo tested this principle himself and noted that two heavy balls of different masses reached the ground simultaneously, although many historians are skeptical that he did this experiment from Italy's Leaning Tower of Pisa as folklore suggests. A good place free of air resistance to test this equivalence principle is Earth's Moon, and so in 1971, Apollo 15 astronaut David Scott dropped both a hammer and a feather together toward the surface of the Moon. Sure enough, just as scientists including Galileo and Einstein would have predicted, they reached the lunar surface at the same time. The demonstrated equivalence principle states that the acceleration an object feels due to gravity does not depend on its mass, density, composition, color, shape, or anything else. The equivalence principle is so important to modern physics that its depth and reach are still being debated and tested even today.