Do We Really Need the Moon? Page #6

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'This is a fossil coral from the Devonian era.

'It's 400 million years old

'but still beautifully preserved.

'As well as monthly growth bands, there are annual bands

'and even daily bands, a quarter of a millimetre apart.'

So this coral grew about a quarter of a millimetre every day 400 million years ago.

And if we took the time to count up all these daily growth bands,

within the year we'd find...

not 365 days

but in fact in this coral there are 400 bands every year.

- 400 bands?

- Per year.

- So that means that there were an extra 35 days every year?

- That's right.

That's quite mind-boggling.

'If there were really 400 days in the year back then, how long was each day?'

If you do the sums, and take the total number of hours in a year

and divide by 400 days, then you come to the conclusion

that in the Devonian period, when this fossil was alive,

a day actually lasted 21 hours and 55 minutes.

Now I must admit I find that really weird.

The fact that in the past, a day wasn't 24 hours.

The length of a day is simply the time it takes for the Earth to spin once

and go from one sunrise to the next.

If, in the past, days were shorter,

then the Earth must have been spinning faster.

In fact, back in time, back billions of years,

the planet was spinning so fast

that each day lasted just five hours.

But why should the spin of the Earth have changed over time?

Because of the Moon.

When the Moon formed, it was so close to the Earth,

and pulling so hard that it acted as a brake on our planet.

The gravitational pull of the Moon was slowing the Earth's spin

and it's still doing so.

As the Earth spins,

the effect of friction between the ocean bulge and ocean floor

causes the Earth's spin to slow down.

It means days have been getting longer.

What was once was five hours now lasts 24.

We humans have been around for such a short time,

about 200,000 years,

that we've only ever known 24 hour days.

Our body clocks are completely geared for that length of day.

And yet, we only have 24-hour days because of the Moon.

It's amazing to think that the very rhythms of our planet

have been set by this ball of rock out in space.

But what about the Moon itself?

How has it been affected by the spin of the Earth?

One of the first things you learn in physics is that for every action

there is an equal and opposite reaction.

As the Earth has been slowing down all these years,

something else has been accelerating,

and that's the Moon.

And to compensate for its acceleration,

something's been happening to its orbit at the same time.

Imagine the centre of this roundabout is the Earth, and I'm the Moon in orbit.

As we speed up, I get slung outwards,

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