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Mollusc shell indicates shorter length of days long ago

Date: 17 March 2020 Tags: Miscellaneous


Scientists picked up its fossil from dry land in the mountains of Oman. Their analysis provided new clues about the behaviour of the Earth.



It has long been known that Earth’s spin has slowed over time. Previous climate reconstructions, however, have typically described long-term changes over tens of thousands of years.



  • Earth spun 372 times a year 70 million years ago, compared to the current 365. This means the day was 23½ hours long, compared to 24 today. This new measurement, in turn, informs models of how the Moon formed and how close it has been to Earth over their 4.5-billion-year gravitational relationship

  • The ancient mollusc, Torreites sanchezi, belonged to an extinct group called rudist clams. At 70 million years ago, it belonged to the Late Cretaceous and was around the time this epoch ended that dinosaurs went extinct.

  • Using lasers on a single individual, scientists sampled tiny slices and counted the growth rings accurately. This allowed them to determine the number of days in a year 70 million years ago, and more accurately calculate the length of a day.

  • The period of Earth’s orbit has remained the same. One year 70 million years ago was as long as as one year today. However, the year would have been 372 days long, with each day half-an-hour shorter than one day today.

  • Friction from ocean tides, caused by the Moon’s gravity, slows Earth’s rotation and leads to longer days. And as Earth’s spin slows, the Moon moves farther away, at 3.82 cm per year.

  • If this rate is projected back in time, however, the Moon would be inside the Earth only 1.4 billion years ago. Which cannot be, for the Moon has been with us much longer. Which means the Moon’s rate of retreat has changed over time.


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