One of the most understated Apollo experiments continues to conduct scientific research without the need for a battery, a radio, or any moving components.
It is not a lunar telescope. It is not an embedded device or an overlooked antenna. It consists of a series of small corner-cube reflectors, placed on the Moon’s surface by Apollo astronauts, allowing light from Earth to be reflected back.
The fundamental concept is straightforward enough to be illustrated with a stopwatch. Emit a brief laser pulse toward the Moon. Wait for a minuscule portion of that light to bounce back. Record the time taken for the round trip. Given that light travels at a constant speed, this time translates into a distance measurement.
The outcome represents one of the longest-standing physical measurements in space science: lunar laser ranging, a method that has transformed the distance between the Earth and Moon into a clock-like metric that has been repeated over more than fifty years.
These mirrors are not typical mirrors.
In July 1969, the Apollo 11 astronauts installed the first lunar laser ranging retroreflector on the Moon. Subsequently, Apollo 14 and Apollo 15 added larger arrays. These are frequently referred to as mirrors in everyday discussion, but they are far more accurate than standard bathroom mirrors.
Each array is composed of corner-cube reflectors. A corner cube directs incoming light back towards its source, even if the reflector is not perfectly aligned. This capability is what facilitates the experiment. A laser beam projected from Earth spreads out significantly by the time it contacts the Moon, and only a tiny fraction of photons return. The reflectors assist in making those returning photons significantly easier to detect than light scattered from the rough lunar terrain.
The first successful returns from the Apollo 11 reflector were documented in 1969. A paper published in Science in 1973 detailed the lunar laser ranging experiment and its initial outcomes. Subsequent reviews, including a 1994 Science article by J. O. Dickey and colleagues, regarded lunar laser ranging as an ongoing legacy of Apollo rather than a finished mission relic.
How a pulse translates into distance
The average distance from the Earth to the Moon is approximately 384,400 kilometers, but this actual distance is perpetually fluctuating. The Moon orbits elliptically. The Earth rotates. Observatories shift with the motion of the ground. Tides, atmospheric delays, lunar orientation, and the specific position of the reflector are all significant factors.
This is why lunar laser ranging is not simply a matter of “shining a laser and dividing by two.” The fundamental measurement is the round-trip time of light traveling from an observatory to a reflector and back again. Converting that into a useful Earth-Moon distance necessitates intricate models of Earth’s rotation, lunar movement, observatory positioning, atmospheric conditions, and relativistic influences.
The contemporary iteration of the experiment can achieve astonishing precision. The Apache Point Observatory Lunar Laser-ranging Operation, known as APOLLO, commenced science-quality observations in 2006. In a dataset paper from 2023, James Battat and colleagues indicated that APOLLO determined the Earth-Moon separation by recording the travel time of photons returning from five lunar retroreflector arrays, encompassing the Apollo arrays and two Soviet Lunokhod arrays.
That paper, accessible as an arXiv preprint, noted a median nightly precision of 1.7 millimeters for APOLLO measurements. This does not imply that the Moon itself remains stationary within a millimeter. It indicates that the ranging system can measure the line-of-sight distance to the reflector with remarkable accuracy, once all geometry and timing are accounted for.
The Moon is drifting away
The well-known figure is roughly 3.8 centimeters annually. This is the average rate at which the Moon is distancing itself from Earth, due to tidal interactions between the two celestial bodies.
The analogy to fingernail growth is not a perfect biological comparison. Fingernail growth varies among individuals. However, as a comparing image, it serves: the Moon’s average outward migration is approximately on par with the yearly growth of human fingernails.
The underlying reason is not that the Moon is “escaping” in a dramatic fashion. It is due to angular momentum shifting throughout the Earth-Moon system. Earth’s gravity creates tides in both the oceans and the solid Earth. As the Earth rotates faster than the Moon orbits, the tidal bulge is slightly ahead of the Earth-Moon alignment. This displaced mass exerts a pull on the Moon, imparting a small amount of orbital energy. Consequently, the Moon moves outward while Earth’s rotation gradually slows.
This process does not unfold at a rapid pace when measured against human timescales. In one year, the alteration is less than the width of two fingers. Over the course of millions and billions of years, it becomes significant.
Not literally every observatory every night
The phrase “every night since” in the title encapsulates the ongoing nature of the experiment, but the operational reality is more pragmatic. Lunar laser ranging is contingent upon weather conditions, telescope availability, lunar phases, equipment functionality, and the spatial relationship between Earth, Moon, and reflector. Various observatories have contributed at different intervals.
What holds true is that the measurement has persisted through the decades. Observatories have consistently transmitted laser pulses to lunar reflectors.