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 the growth."So we make a cross section through this shell. You see the layers' growth in the shell a bit like the layers in a tree.And that allowed us to really count the number of days in the year that far back in time."Niels de Winter, paleoclimatologist 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 that there were actually not 365 days in the year; 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 mollusk's shell, the team was able to put a precise number on the history of this process.Seventy million years ago, the faster rotation of the Earth meant that there were just 23 and a half hours in a day.
