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This is Scientific American's 60 Second Science. I'm Adam Levy.

Open up a clam and you might see the making of a tasty snack.

You probably wouldn't expect to find the history of our planet.

But that's what researchers have been deducing from a 70 million-year-old fossil clam.

They opened up this mollusk's secrets by drilling a small hole in its shell with a laser

and analyzing the pattern of growth.

So we make a cross-section through the shell.

When you see the layers, the growth layers, the shell, a bit like the layers in the tree.

And that allowed us to really count the number of days in a year that far back in time.

Niels de Winter, paleo-climatologist at the Free University of Brussels in Belgium.

Using a laser to carefully count the number of growth rings in a 70 million-year-old fossil

might seem like quite a laborious way to find out the number of days in a year.

After all, we all know the

answer is 365 and 366 in leap years, right?

That's interesting because what we found is actually that there were not 365 days ago. There

were a few more. You see, Earth's spin on its axis gradually slows down over time. The reduction is due to the friction from Earth's tides, which are driven by the moon's orbit.

Back to that ancient clam. By measuring the rings on this mollusks shell, the team was able to put a precise number on the history of this process.

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