What is the name of our planet's only natural satellite?

The Moon, and the definite article is doing a lot of work - every other moon in the Solar System is named, and ours is simply the one the word was invented for. It is unusually large relative to its planet, about a quarter of Earth's diameter, which is why some astronomers prefer to describe the pair as a double planet.

It almost certainly came from a collision. The leading account has a Mars-sized body striking the young Earth, throwing molten debris into orbit that coalesced within a few thousand years. The evidence is chemical: Moon rock is strikingly similar to Earth's mantle in its oxygen isotopes and strikingly poor in iron and in volatile elements, exactly as material blasted off a mantle and left to cool in vacuum should be.

It is also stabilising us. The Moon's gravity holds the Earth's axial tilt within a narrow range, which keeps the seasons predictable over geological time; Mars, with no large moon, wobbles across tens of degrees and has a chaotic climate record. And it is leaving, at about 3.8 centimetres a year - which means total solar eclipses are a temporary coincidence. The Moon currently appears almost exactly the same size as the Sun, and in a few hundred million years it will be too small to cover it.