While others enthusiastically projected their divine myths onto the firmament, the Greeks focused on identifying meaningful patterns. To the ancient Greeks, the cyclical drama of the heavens contained objects that could be studied and measured. Observing the grinning silver disc of the moon in its many phases, ancient Greeks had a hunch it was a real celestial body located at a fixed distance from where they stood.
The Greeks’ extraordinary insights laid the foundation for the science of lunar distance, or “LD.” Hellenistic thinkers estimated astronomical intervals using only the naked eye and some basic math, yet their reasoning ultimately proved sound: the Iron Age estimate of the space separating moon from mankind — 240,000 miles — was surprisingly accurate.
We now know the distance to the moon is, on average, about 384,400 kilometers (km) or 238,857 miles. That’s equal to thirty times the width of Earth at its widest girth.
Of course, the moon doesn’t sit still. As Earth’s only natural satellite completes its daily elliptical orbit, the distance from earth to moon varies continuously with the moon’s orbital location. Lunar distance ranges from 353,300 km at the moon’s nearest point, or perigee, to 406,700 km at its furthest point, or apogee.
From Logic to LasersBefore there was Copernicus, there was Aristarchus of Samos, a precocious Greek astronomer who accurately estimated lunar distance in the 3rd century BCE, long before the invention of telescopes or powerful computers. A physics student trained at the prestigious University of Alexandria, he hailed from the island of Samos, birthplace of the mathematical genius Pythagoras.
Watching lunar eclipses, Aristarchus realized the phases of the moon corresponded to its visual displacement by Earth’s shadow. That is, the moon is visible only to the extent the Earth isn’t blocking the sun’s light from hitting it. Based on his observations, Aristarchus recorded the first approximate measure of lunar distance in his only written work, On the Sizes and Distances (of the Sun and Moon). He calculated the stretch from moon to earth at about 240,000 miles, or 60 Earth radii.
Aristarchus wasn’t far off the mark; today’s average hovers around 60.3 Earth radii. But today’s scientists demand laser precision in these matters. Decades ago during its Apollo heyday, NASA slyly stashed a handful of retroreflectors on the moon’s surface. This way, earthbound observatory geeks could simply aim lasers at the orb any time they wanted to know the updated lunar distance. Those retroreflectors boomerang photon streams right back to Earth’s surface, allowing LD calculation to the millimeter.
When our 4.6 billion-year-old moon was in its earliest stages of formation, it was much closer to the Earth — about three or four thousand times closer. Today, the moon is actually slowly hurtling away from our planet, at a rate of nearly four centimeters annually. Earth and its moon are connected by the pull of gravity, but our planet spins faster than her moon. This speed discrepancy means Earth’s tidal bulge has to drag the slower satellite along each day.
The drag transfers energy from Earth to the moon, gradually expanding the latter’s orbit. The result is the slow expansion of lunar distance over time. As the lunar cycle increases, the Earth’s rotation will also slow down a tad. In a century, days will be longer by about two milliseconds.
Bonus Facts to Arouse Your Curiosity
Man in the MoonLunar cartographers named a thirty-mile-wide crater on the near side of the moon after Aristarchus. It’s the most active part of the moon, having been associated with a third of all Transient Lunar Phenomena, or TLPs—periodic lunar flashes or glowing spots of unknown origin. Famous for its brilliant luminosity, the highest peak in Aristarchus is the most reflective spot on the moon, in contrast to the flat darkness of most of its surface.
Aristarchus was also the first individual to challenge the Aristotelian notion that the Earth is the center of the solar system. His prescient heliocentric hypothesis would not enjoy mainstream legitimacy for another couple millennia. Though Copernicus is usually credited with first conceiving of heliocentrism, it was actually Aristarchus that inspired his postmedieval theory.
The moon and the sun seem to be about the same size from an Earthling’s perspective. Given that the sun is as large as 72 million moons, why do they look the same size? It’s because they are at different distances from our eyeballs. The sun’s diameter is 400 times greater, but it’s also 400 times further away. Because the larger sun is much further away than our little moon, their mismatched sizes appear to balance out, much like an apple held in front of your eye will appear bigger than a whole orchard seen at a great distance.
Eclipsing the Red Giant
Many moons ago, there were only total eclipses of the sun by the moon because Earth’s satellite had always appeared bigger in the sky. In our era, the sun and moon look about the same size, and sometimes we have annualar, or ringed, eclipses. In the future, total eclipses will become ever rarer. The process of lunar ‘shrinking’ will eventually max out 15 billion years from now. Of course, by that time the sun may have already become a blustery Red Giant and swallowed its closest neighbors—including us—making eclipses moot.
How long would it take to travel from Earth to the moon? In a modern passenger jet traveling at 1,000 km/hr, you’re looking at a 16-day journey with no layovers. It would only take eight-and-a-half hours with a rocket launcher giving you a 58,000 km/hr head start, such as the speed of NASA’s Pluto probe in 2006. Then again, the breakneck pace of light makes this seem slow. It takes reflected moonshine 1.26 seconds to lap the LD, compared to the 8 minutes it takes light to journey to Earth from the Sun.