Bouncing Light Off the Moon: The Geometry of Lunar Retroreflectors

When the Apollo astronauts walked on the moon, they left behind several unassuming panels covered in small, geometric silica prisms. Decades later, these Lunar Ranging Retroreflectors remain the only Apollo experiments still actively gathering data, and they are the key to our understanding of celestial mechanics. Unlike a standard mirror that scatters light, a retroreflector is mathematically designed to bounce light back in the exact, precise direction it came from. Every day, observatories on Earth fire powerful green lasers across the void of space to hit these specific prisms. By measuring the exact time it takes for the photons to return, scientists can calculate the distance between the Earth and the Moon down to the millimeter. This book explores the optical physics of the retroreflectors and the astonishing discoveries they have yielded—most notably, the definitive proof that the Moon is slowly spiraling away from the Earth at a rate of 3.8 centimeters per year. Explore the longest-running experiment in human history. Discover how a few panes of specialized glass in the lunar dust continue to anchor our understanding of the cosmos.