How the Earth and Moon “fell in love”: a park service tale of the Moon's origin

The Moon's story begins about 4.5 billion years ago, when a smaller rock ran headfirst into a bigger one. A National Park Service article, illustrated by NPS's Kat Gardiner, tells it as a love story. It sits among Acadia National Park's pages.

A cartoon of the collision (NPS / Kat Gardiner).

The collision

The science behind the joke: the prevailing theory says the Moon formed when a Mars-sized asteroid slammed into Earth and flung bits of both into space. The article notes that many theories came before and new ones may follow, which it calls the beauty of the scientific method. After the impact the Moon's surface was a magma ocean, and the story has it staying put for four billion years while it gathered what was left of itself.

A faster Earth, then a steadier one

At first the Earth rotated too quickly, completing one rotation in just 6 hours, with fast and furious winds. Over many millions of years it slowed. The tale also claims Earth had been a fairly unlivable place before the Moon arrived because of its wobble, so that only the hardiest bacteria could survive weather swinging from winter to summer overnight. Over time the Moon's gravity made the wobble more controlled and predictable. That, the article says, is why Acadia now has loons, beavers and sea stars. It adds a reminder that life developed slowly, from microscopic to multicellular creatures, not all at once.

Earth and Moon fall in love (NPS / Kat Gardiner).

Facing the Earth

The Moon now orbits in "synchronous rotation": it turns once on its axis in the same time it takes to go around the Earth once, about 27 days, so it always shows the same face to Earth.

The two are not perfectly in step. The Earth still rotates slightly faster than the Moon, and tugs it along, like someone running while holding your hand. The tug pushes the Moon outward, so each year it moves a few centimeters farther from Earth.

Acadia's animals, as seen from the Moon (NPS / Kat Gardiner).

Sources

• National Park Service, Acadia National Park, How the Earth and Moon Fell in Love; rewritten in hubnx's own words.