How old is the moon?

The planet Earth's moon is around 4.5 billion years old. Some estimates place the Moon's age at 4.47 billion years old, while others suggest a date for the formation of Earth's moon closer to 4.6 billion years ago. Astronomers generally agree on one theory of how the Moon came to be. Around 4.5 billion years ago, a planet or massive asteroid slammed into the Earth, sending debris into space. Some of this debris, caught up in Earth's gravitational pull, coalesced together into the body we now know as the Moon. Interestingly, this is about the same time that our solar system was being formed, so it is quite possible that a planet drawn in by the Sun's gravitational pull knocked into our own planet.

Extraterrestrial rock, like that collected on the surface of the moon, can be dated by measuring the decay of radioactive elements contained in the rock material. Rubidium-strontium isochron dating of moon rock suggests that the Moon may be anywhere from 3.9 to 4.5 billion years old, but we should bear in mind that younger rock material may have been incorporated into the structure of the Moon at a later date. It is also possible for younger or older rock material to have arrived on the moon during collisions with other celestial bodies.