How do we know how far away the moon is?

The exact distance between the Moon and Earth varies since the Moon follows an elliptical (oval) orbit, but on average the Moon is 384,399 km away. If you had a piece of string that long, you could wrap it all the way around the Earth’s equator almost ten times.

Ancient Greek astronomers were able to make rough estimates of the distance to the Moon using information from eclipses to make geometric calculations. Since the 1960s, we have however had access to a much more accurate way of measuring this distance.

Astronauts on the Apollo 11, 14 and 15 missions positioned retro-reflectors (a bit like the cat’s eyes you see on roads) on the surface of the Moon. Physicists bounce a pulse of laser off one of these reflectors, timing how long it takes for the laser beam to make the return trip - about two and a half seconds. Since we know the speed of light, they can then work out how far away the Moon is to within a few millimetres.

This project is called the Lunar Laser Ranging experiment and it took measurements  every year between 1969 and 2009. The information collected has led for instance to the discovery that the Moon creeps 38 mm further away from us every year. It has also allowed physicists to deduce that the Moon’s core is liquid.

When it comes to calculating how far away the other planets in the solar system are, we don’t have the luxury of reflectors. Instead, astronomers use a similar technique which relies on bouncing a radar signal off a planet’s surface. For planets visited by space probes, they can also time how long a radio signal sent by the probe takes to travel back to Earth.