The first of five clues. Each one below gives a little more away.
I am slowly leaving you — about four centimetres every year.
I am the reason the sea breathes twice a day.
Twelve people have walked on me, all of them between 1969 and 1972.
You have only ever seen one of my faces.
Full, half, crescent, new.
The Moon
In roughly 600 million years it will have drifted far enough that total eclipses stop happening altogether. We live in the era that gets to see them.
The leading explanation for the Moon is a collision. A body roughly the size of Mars struck the young Earth, and the debris that failed to fall back settled into orbit. The retreat is not a guess: the Apollo crews left mirrors on the surface, and observatories have been bouncing lasers off them ever since, measuring the distance to within a few centimetres. The same tidal drag that pushes it away long ago slowed its spin until one rotation matched one orbit, which is why the far side stayed unseen until a Soviet probe photographed it in 1959.
Reading five clues at once is not the game. In the real thing they arrive one at a time and every wrong guess costs an hour. Play mystery #3 · today's mystery · the vault
Previous: mystery #2. Next: mystery #4. Or try #23, #24, #25, #26, #27, #28.