The Moon Was Born Close. It Is Still Leaving.

This is Vishala Space. On clear nights, observatories in New Mexico, France, Italy and Germany fire lasers at the Moon, at mirrors that were put there on purpose. The light comes back in about two and a half seconds. And every year, it takes a tiny bit longer. Because the Moon is leaving us, about one and a half inches a year, a little slower than your fingernails grow. It was born close. So close that it would have looked up to about thirty times wider than it does today.

Here is how it was born, how we measure it leaving, and why the simple version of this story does not add up. Last time, we squeezed Earth's whole life into one day, and at about ten minutes past midnight, the Moon was born. Here is what happened that day, as far as anyone can tell. About four and a half billion years ago, the young Earth was hit by another body, roughly the size of Mars. Scientists call it Theia. This is the leading model, not a proven fact. In the classic computer runs, most of the Moon is made from Theia.

Yet Moon rocks are almost identical to Earth rocks, isotope after isotope, and for decades that has been a puzzle. A study in twenty twenty-five found that Earth and the Moon share the same iron isotopes, and that Theia most likely formed in the inner Solar System, perhaps even closer to the Sun than Earth. So the match may be less strange than it looked. But no single version of the impact explains everything yet. What you see is a model, drawn as rings and particles, not a photograph. The debris spreads into a ring around the Earth, and the ring gathers itself into the Moon. In the models, it forms just outside the distance where Earth's pull would tear it apart: about eleven thousand five hundred miles from Earth's centre.

Today, it is about two hundred and thirty-nine thousand miles. How long ago, exactly? Moon rocks give ages from about four point three five to four point five one billion years, and scientists still argue about which of those ages marks the birth. Now stand on the young Earth and look up. You could not really stand there. The surface was most likely molten. But the geometry is simple.

At that distance, the newborn Moon would have spanned about sixteen degrees of sky. Hold your fist out at arm's length. It covers about ten degrees. The newborn Moon was wider than that. Today's Moon spans about half a degree. That is up to about thirty times wider, in the models. The exact number depends on where it formed.

Start it a little farther out, and it is fifteen or twenty times wider. Either way, it filled a big part of the sky. And from that day on, it has been moving away. Today, the Moon is about two hundred and thirty-nine thousand miles away, on average. To watch it leave, you need something on the Moon to aim at. So we put mirrors there. Apollo 11 left the first one in nineteen sixty-nine: a panel of one hundred glass corner cubes.

Apollo 14 left another hundred. Apollo 15 left three hundred, and because it is the biggest, it sends back the most light. Two Soviet robot rovers, Lunokhod 1 and Lunokhod 2, carried mirrors too. That makes five classic mirrors. And in twenty twenty-five, a sixth arrived, on a robotic lander called Blue Ghost, and within days, lasers found it. Lunokhod 1's mirror was lost for decades, until a camera orbiting the Moon spotted the rover, and in twenty ten a laser found it again. Each Apollo corner cube is one and a half inches wide: about the same as one year of the Moon's drift.

This is a real image, taken from orbit by NASA's Lunar Reconnaissance Orbiter. The Apollo 15 landing site, near a long, winding valley called Hadley Rille. Each pixel is about twenty inches across. And this small bright spot is the mirror the lasers aim at. Hitting it is the hard part. Even a tight laser beam spreads to more than a mile wide by the time it reaches the Moon. Only about one photon in twenty-five million even lands on the Apollo 11 mirror.

On the way back, the light spreads again, and the telescope catches only a sliver of it. In total, about one photon in a billion billion makes the round trip and gets counted. A single pulse carries about three hundred thousand trillion photons. Multiply it out, and most shots come back empty. The good ones bring back one photon, sometimes a few. Many of those single photons, timed and averaged, pin the distance down to within a few millimetres. Do that for decades, and the answer comes out clear.

The Moon moves away by thirty-eight point three millimetres a year, on average. That is one and a half inches, or three point eight centimetres. Your fingernails grow about forty-two millimetres a year, so the Moon is a little slower than your fingernails. Day by day, it is about a tenth of a millimetre. It moves away by the height of an adult, about five foot seven, in about forty-four years. In a century, about twelve and a half feet. In a million years, at today's pace, about twenty-four miles.

So why is it leaving? Tides. The Moon's gravity pulls Earth's oceans into a bulge. But Earth spins much faster than the Moon goes around it, so the spin carries that bulge a couple of degrees ahead of the Moon. The bulge pulls the Moon forward, just a little, all the time. A forward pull lifts the Moon into a wider orbit. And the energy comes from Earth's spin, which slowly winds down.

Every tide on every beach is part of this trade. Most of the energy is lost where seas are shallow. About a quarter is lost in the deep ocean, over rough sea floor. Now run the film backwards. Here is the simple version. Today's distance, divided by today's speed, gives about ten billion years, more than twice the age of the Moon. So the simple arithmetic says nothing is wrong.

But it is the wrong arithmetic. Tides get much stronger when the Moon is closer. Run today's tidal physics backwards, and the Moon ends up right next to the Earth less than one point six billion years ago. And the Moon is about four and a half billion years old. That is the real puzzle. Today's Moon is leaving too fast. The rocks back this up.

In South Australia, layered tidal sediments from about six hundred and twenty million years ago record the tides of their time, layer by layer. Since then, the Moon has moved away by only about two centimetres a year on average, not three point eight. Older rocks agree. In China, layers from one point four billion years ago put the Moon about two hundred and twelve thousand miles away. In Western Australia, rocks from two point four six billion years ago put it at about two hundred thousand miles: already eighty-four percent of today's distance. So the Moon did not leave at a steady pace. The leading idea is that the strength of the tides depends on the shape of Earth's oceans, which changes as continents drift, and on how fast Earth spins.

Only sometimes are they tuned to make strong tides. Today, they happen to be tuned very well. That is a model, but it fits both the rocks and the Moon's age. Which means one popular picture is wrong. Dinosaurs did not see a giant Moon. Sixty-six million years ago, the Moon was less than one percent closer than it is today. It would have looked the same.

The giant Moon belongs only to the very beginning, long before anyone was there to see it. And the future? The Moon will not escape. As it moves out, the push gets weaker. In the models, Earth's spin and the Moon's orbit slowly come into step, until a day and a month are the same length: about forty-seven of today's days, with the Moon about one point four times farther away than now. Earth would then keep one face turned to the Moon, the way the Moon already does to us. But it is so slow that the Sun will change first.

In about five billion years, the Sun will begin to swell into a red giant, long before Earth and the Moon could ever lock together. Before any of that, something else ends. Today, the Moon is only just big enough in our sky to cover the Sun, and only when it is near its closest. That is why some eclipses are total and some leave a ring of Sun. As it moves away, it shrinks. In roughly six hundred million years, total eclipses start to become rare. The very last one may come more than a billion years from now.

These are models too, but the direction is certain. Here is the part we skipped. The Moon gets its push from Earth's spin. So when the Moon was closer, Earth must have been spinning faster, and a day was shorter. How much shorter? Rocks and fossils kept the count. And is today's day still getting longer?

That is next. Next: how long a day used to be.