How do we know how far away the Moon is?

Scientists time how long a laser pulse takes to travel from Earth to the Moon and back, then multiply by the speed of light and halve the result. The pulses bounce off reflectors placed on the lunar surface. The average distance is about 384,403 kilometers, or 238,857 miles, according to the National Radio Astronomy Observatory ([public.nrao.edu](https://public.nrao.edu/ask/what-is-the-distance-to-the-moon-and-how-is-it-measured/)). The distance is not constant: it ranges from about 351,000 to 406,000 kilometers over the Moon's orbit ([newapo.apo.nmsu.edu](https://newapo.apo.nmsu.edu/mainpage/apollo/lunarranging/)). [1] [2]

Apollo astronauts left retroreflector arrays on the Moon. These are special mirrors that send light straight back to its source. The first array held 100 corner cube prisms. On August 1, 1969, a laser pulse from the 3-meter telescope at Lick Observatory hit it for the first time. The round trip took about 2.5 seconds, and the distance was fixed to within 25 centimeters ([nist.gov](https://www.nist.gov/history/moon-and-back-25-seconds)). [3]

Modern systems reach roughly 1 millimeter, a large improvement on the precision of 1969 ([nist.gov](https://www.nist.gov/history/moon-and-back-25-seconds)). Round-trip times run from about 2.34 to 2.71 seconds, and the timing is precise to a few trillionths of a second. Measuring to a defined reflector is far better than measuring to the rough lunar surface. Reflectors also include one on a Soviet rover and, as of 2025, a NASA-built one ([newapo.apo.nmsu.edu](https://newapo.apo.nmsu.edu/mainpage/apollo/lunarranging/)). [3] [2]

A single photon cannot give that precision. The Apache Point team says each photon carries 30 to 50 millimeters of uncertainty, because the laser pulse is about 2.5 centimeters long and the reflector array is slightly tilted. Averaging many returning photons sharpens the result. The team must also correct for the telescope's own motion, since tides from the Moon and Sun raise the site by about a foot twice a day, and the continental plate drifts ([newapo.apo.nmsu.edu](https://newapo.apo.nmsu.edu/mainpage/apollo/lunarranging/)). [2]

These measurements show the Moon receding by about 3.8 centimeters (1.5 inches) per year, which the Apache Point team attributes to tidal friction ([public.nrao.edu](https://public.nrao.edu/ask/what-is-the-distance-to-the-moon-and-how-is-it-measured/)). [1] [2]

Around 270 BC, Aristarchus estimated the distance from the duration of a lunar eclipse. He assumed the Moon circles Earth at constant speed, and compared the time it takes to cross Earth's shadow (about 3 hours at longest) with its monthly orbit. That gave a distance of about 60 Earth radii, which matches the figure accepted today ([pwg.gsfc.nasa.gov](https://pwg.gsfc.nasa.gov/stargaze/Shipprc2.htm)). [4]

Parallax is another method: observing how an object's position shifts against distant stars when the viewpoint changes. A bigger shift means a closer object. NIST says Hipparchus formally described the method in the second century B.C.E. ([nist.gov](https://www.nist.gov/how-do-you-measure-it/how-do-you-measure-distance-moon-planets-stars-and-beyond)). [5]