Every Then & Theory plate comes from a true story, and this card tells the one behind yours.

The Then & Theory team

The Moon is leaving

A great sphere hangs over a dark sea, with one red sail below it. Our own Moon pulls on the whole Earth and draws the oceans into two bulges, one on the side facing it and one on the far side. Those bulges are the high tides. The Moon is also slowly moving away. Apollo astronauts left reflectors on its surface, and lasers fired at them from Earth show the distance growing by about 3.8 centimetres a year. The leading explanation for the Moon itself is a collision about 4.5 billion years ago between the young Earth and something roughly the size of Mars.

Note. The giant impact is a theory. Whether it was one Mars-sized body or a series of impacts is still open.

Sources

1. NASA, “The Apollo experiment that keeps on giving”
2. NASA Science, “Tides”
3. NASA Science, “Moon formation”