The day lasts 24 hours ...... but was that always true?

The Earth appears unchanging, but it has been around for a long time.
In the 4.6 billion years since it formed, a lot has happened.
It is difficult to realise how different the Earth has been in the past.

Even the days were different lengths.
In the Devonian the year was around 400 days.

The Earth rotated more quickly....so the days were shorter.

The friction between the oceans and the rocks of the Earth is enough to slow the spin.

How can scientists tell the Earth had 400 days in a year so long ago?
Corals in the sea add a new layer to their structure each day, and these add up to make annual bands, like rings on a tree.
So coral fossils are full of information about the world in the time they were growing.
So we know there were around 400 days in each Devonian year.

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