Slow Orbit · Episode 2 · 13 min

The Moon

Our only natural companion: its one face, its seas of old lava, its long days and nights, the tides, the visitors who walked there, and the mirrors they left behind. Read by a synthetic voice. Script written and fact-checked by the show's team.

The script

The companion

Every night, somewhere on the Earth, the Moon is rising.

It comes up over the edge of the world, large and pale, and climbs slowly into the dark. It has done this for longer than there have been eyes to see it... longer than there have been oceans for its light to fall on.

It is our only natural companion in space. Everything else in the night sky is far away... unimaginably far. The Moon is close. Close enough that its mountains and its plains can be seen without any instrument at all... as patches of light and shadow on its face.

On average, it is about three hundred and eighty-four thousand kilometres away. Light, travelling from its surface, reaches the Earth in a little more than one second.

The same face

The Moon turns as it travels. But it turns at exactly the pace of its journey around the Earth... once for every orbit.

And so, it always shows us the same face.

The dark patches, the bright patches, the rayed craters... every person who has ever looked up has seen the same arrangement. The same shapes. Different peoples have found different pictures in them: a face, a rabbit, a woman carrying wood.

The other side is always turned away. For all of human history, no one saw it... until 1959, when a small spacecraft swung around behind the Moon and sent back the first blurred pictures of a face no one had ever seen.

It is not darker than the near side. It has its days and its nights, just the same. It is simply... always looking away.

How it began

The Moon is old... about four and a half billion years old. Almost as old as the Earth itself.

The most widely held idea of its beginning is a violent one. Very early in the history of the Solar System, a body about the size of Mars struck the young Earth a glancing blow. Rock from both worlds was thrown out into space, glowing, molten... and gathered, over time, into a ring around the Earth.

From that ring, the Moon formed.

That is the leading idea. It explains a great deal... and yet, scientists still argue about its details. The rocks of the Moon and the rocks of the Earth are remarkably alike... more alike than a simple collision should leave them.

The question is still open.

A smaller world

The Moon is about a quarter as wide as the Earth.

Across its middle, it is about three thousand four hundred and seventy kilometres.

Its pull is weak. On the Moon, everything weighs about one sixth of what it weighs on Earth. A stone thrown upward rises slowly, hangs for a long moment, and falls back as if through water.

There is no air here. Nothing to carry sound. No wind. No weather. No blue sky: even in full sunlight, the sky above the Moon is black, and the stars are there, all day long, if the sunlight is not too bright to see them.

The seas that are not seas

The dark patches on the Moon's face have names. The Sea of Tranquillity. The Sea of Serenity. The Ocean of Storms.

They were named long ago, by people who thought they might be water.

They are not water. They are plains of dark rock... lava, which welled up from inside the Moon billions of years ago, filled the great basins left by ancient impacts, and slowly cooled and hardened.

The brighter land around them is older, higher, and rougher... the highlands, covered in craters on top of craters on top of craters.

Over everything lies a layer of fine grey dust. It is made of rock that has been broken and broken again by tiny meteorites over billions of years. It is soft underfoot, and it clings to everything.

Day and night

A day on the Moon is long.

From one sunrise to the next takes about twenty-nine and a half Earth days. So the Sun rises slowly over the grey plains... and shines for about two weeks without setting.

There is no air to hold the heat or to spread it. In the long sunlight, the ground grows hotter than boiling water. In the long night, it grows colder than anywhere on Earth... far colder than the coldest winter.

And at the poles, at the bottom of certain deep craters, there are places where the sunlight never reaches at all. They have been in shadow for billions of years. There, it seems, ice has gathered... frozen, untouched, waiting.

The tides

The Moon is far away, and yet it reaches the Earth every day.

Its gravity pulls on the oceans... very gently. The water on the side nearest the Moon is pulled toward it, a little. And on the far side of the Earth, the water bulges outward too. As the Earth turns beneath these two gentle swellings, the sea along every coast rises... and falls... and rises again.

Twice a day, on most shores, the tide comes in. Twice a day, it goes out.

The Sun pulls too, more weakly. When the Sun and the Moon line up, at the new Moon and the full Moon, the tides are higher than usual. When they pull at right angles, the tides are gentler.

On the beaches of the Earth, the water keeps time with the Moon.

The visitors

For a few short years, people walked on the Moon.

Between 1969 and 1972, twelve astronauts stepped down onto its surface... two at a time, from small landing craft that had travelled four days across space to reach it.

They walked in the slow, bounding steps that the weak pull of the Moon allows. They collected rocks and dust... several hundred kilograms in all... and carried them home. They drove small electric cars across the plains, raising arcs of dust that fell back without drifting, because there is no air to hold them up.

And they left things behind. Flags. Instruments. The lower halves of their landing craft. Footprints.

With no wind and no rain, those footprints will last a very long time. The tiny meteorites that fall on the Moon will smooth them... but slowly. Very slowly.

The mirrors

Among the things they left, there are mirrors.

Small panels of special reflectors, set down on the dust and pointed back toward the Earth. They need no power. They simply wait.

From observatories on Earth, scientists fire pulses of laser light at them. The light crosses the gap to the Moon... strikes the mirrors... and comes back. Only a tiny fraction returns... a few particles of light from each pulse. But it is enough.

By timing that journey, there and back, to a tiny fraction of a second, scientists measure the distance to the Moon... to within a few millimetres.

And from those measurements, they have learned something quiet and surprising.

Drifting away

The Moon is slowly moving away from us.

About three point eight centimetres every year. About as fast as a fingernail grows.

It is the tides that do it. As the oceans are pulled into their bulges and the Earth turns beneath them, a little of the Earth's spin is passed to the Moon... and the Moon climbs, very slowly, into a wider orbit. At the same time, the Earth's day grows very slightly longer.

Long ago, the Moon was much closer, and the days on Earth were shorter. In the far future, it will be a little smaller in the sky.

But the change is slow. Over a whole human lifetime, the Moon moves away by about three metres.

Earthrise

In December 1968, three astronauts were the first people to travel around the Moon.

As their spacecraft came around from the far side, they saw something no one had seen before. Over the grey, cratered horizon of the Moon... the Earth was rising.

Small. Blue and white. Half in light, half in shadow. Alone in the black.

They took a photograph. It has been printed and shared countless times since then. A whole world... every ocean, every city, every person who had ever lived... small enough to cover with a thumb held out at arm's length.

Still there

New visitors are on their way. Spacecraft from several countries have landed on the Moon in recent years... one of them, in 2019, on the far side, the side that always looks away. Others are planned.

And the Moon goes on as it always has.

It rises over the edge of the world. It climbs slowly into the dark. It pulls, very gently, at the oceans... and the tide comes in, and goes out.

It turns the same face toward us, as it has for longer than anyone has watched.

Its light, which is the Sun's light, reflected from grey dust and ancient lava, falls quietly on the hills, on the sea, on the roofs of every town... a little more than one second after it left the Moon.

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