- The moon is moving away from Earth at a rate of 3.8 centimeters (1.5 inches) per year, but the speed of its retreat has varied over time.
- A new animation by planetary scientist James O’Donoghue shows the moon‘s 4.5-billion-year journey from a fiery ball of magma looming over Earth to the cold, distant rock it is today.
- Some of the moon’s fastest retreat speeds line up with major geological changes on Earth, like supercontinents breaking up and the mass melting of glaciers.
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The moon is slowly moving away from us.
About 4.5 billion years ago, a Mars-sized object (or perhaps a series of many smaller objects) crashed into Earth, sending bits of Earth’s crust into space. They fell into the planet’s orbit and eventually coalesced, forming our moon. That newborn moon — a ball of molten rock covered in a magma ocean — was nearly 16 times closer to Earth than it is today.
As it cooled, the moon backed away, retreating thousands of miles into the distance. A new animation depicts that process with unprecedented clarity.
The video’s creator, James O’Donoghue, works as a planetary scientist at the Japan Aerospace Exploration Agency (JAXA). He told Business Insider that he set out to create an accurate picture of the moon’s creation, but got carried away and wound up animating its entire history.
The sequence starts with the moon’s current position and follows it back in time to its birth, tracking its distance from Earth, apparent size relative to our planet, and the speed of its retreat over time.
O’Donoghue said he only recently learned how to create scientific animations like this — his first were for a NASA news release about Saturn’s vanishing rings. After that, he moved on to animating other difficult-to-grasp space concepts, like the torturously slow speed of light.
“My animations were made to show as instantly as possible the whole context of what I’m trying to convey,” O’Donoghue previously told Business Insider, referring to those earlier videos. “When I revised for my exams, I used to draw complex concepts out by hand just to truly understand, so that’s what I’m doing here.”
The moon’s retreat has been inconsistent
Today, the moon is pulling away from Earth at about 3.8 centimeters (1.5 inches) per year. Scientists refer to this as “lunar retreat.” The pace of this motion hasn’t always been constant: The moon started out moving away at 20.8 centimeters (8.2 inches) per year, and its drift has fluctuated from between 0.13 centimeters (0.05 inches) per year to 27.8 centimeters (10.9 inches) per year.
“I didn’t appreciate how many different past rates of lunar ‘retreat’ there were until this week,” O’Donoghue said, adding, “this is the most researched animation I’ve made to date.”
But you can’t see every rate of retreat in his video, he noted, because it quickly moves through millions of years.
“I use the main average rates to avoid it flickering over values that people can’t read in time,” he said.
Much of the variation in the moon’s pace of movement comes from lunar meteorite impacts and major geological changes on Earth.
Such events have coincided with three notable spikes in the pace of lunar retreat. One spike came around the same time as some of the earliest evidence of ocean tides — about 3.2 billion years ago. At that time, the moon started retreating at 6.93 centimeters per year.
Similarly, about 900 million years ago, the moon’s speed of retreat spiked to 7 centimeters per year as it got bombarded with meteors. It continued racing away at that rate as the supercontinent Rodinia broke apart on Earth.
The third spike was roughly 523 million years ago: As life was exploding on Earth following millions of years of fluctuation between ice ages and hothouse conditions, the moon retreated at 6.48 centimeters per year.
The fates of Earth’s oceans and the moon’s location in space are connected because the moon’s gravity pulls on ocean water, creating a “tidal bulge” that stretches slightly towards the moon. In turn, Earth’s tidal bulge exerts gravity on the moon. The Earth spins faster than the moon orbits it, so as the bulge rotates away, it pulls the moon along with it.
The moon pulls back, and that slows the Earth’s rotation. All this dragging back and forth creates friction around Earth’s tidal bulge, which pushes the moon outward and makes its orbit larger.
That’s why researchers look to the imprints of ancient ocean tides to determine how fast the moon has retreated at different periods in time.