The Moon is about 384,400 kilometres, or 238,855 miles, from Earth on average. That is the number you will see in most textbooks, but it is only an average, because the Moon's orbit is an ellipse, not a circle, and the distance changes by tens of thousands of kilometres through the month.

At perigee, its closest point, the Moon can come within about 356,500 kilometres of Earth. At apogee, its farthest point, it can be more than 406,700 kilometres away. The difference is about 50,000 kilometres, which is why a full moon near perigee, a supermoon, looks slightly larger and brighter than a full moon near apogee, a micromoon.

Light takes about 1.3 seconds to travel from the Moon to Earth, so when you look at the Moon you are seeing it as it was 1.3 seconds ago. That sounds trivial, but it is the same physics that makes the Sun eight light-minutes away and the nearest star more than four light-years away. The Moon is the only object in the sky close enough that its light delay is measured in seconds.

Astronomers measure the distance with lasers. Reflectors left on the lunar surface by the Apollo missions bounce laser pulses back to Earth, and timing the round trip gives the distance to within a few centimetres. Those measurements show the Moon is drifting away from Earth by about 3.8 centimetres per year, roughly the rate your fingernails grow.

The changing distance has real consequences. When the Moon is near perigee, tides are slightly stronger. When a new moon coincides with perigee, it can produce a total solar eclipse, because the Moon is close enough to cover the Sun completely; when it coincides with apogee, the Moon is too small and the result is an annular eclipse, a ring of fire. So the next time you hear supermoon or eclipse in the news, the distance is the story.