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How a little shell tells us the day was 23½ hours long, once upon a time

It has long been known that Earth’s spin has slowed over time. A new study looks at daily and annual variations in the mollusc shell.

Towards the end of the time when dinosaurs ruled Earth, a little mollusc lived for nine years in a shallow seabed. Seventy million years later, scientists picked up its fossil from what is now dry land in the mountains of Oman. Their analysis provided new clues about the behaviour of a much warmer Earth.

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 scientists said.

The study is published in the American Geophysical Union’s journal Paleoceanography and Paleoclimatology.

Faster Earth in the olden days

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. The new study looked at daily and annual variations in the mollusc shell.

The ancient mollusc, Torreites sanchezi, belonged to an extinct group called rudist clams. At 70 million years ago, it belonged to the Late Cretaceous — it was around the time this epoch ended, some 65 million years ago, that dinosaurs went extinct.

Torreites sanchezi grew very fast, laying down daily growth rings. 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.

It is important to note that the period of Earth’s orbit has remained the same. In other words, one year 70 million years ago was as long as as one year today. However, if there were a calendar then, the year would have been 372 “days” long, with each “day” half-an-hour shorter than one day today.

Today, Earth’s orbit is not exactly 365 days, but 365 days and a fraction, which is why our calendars have leap years, as a correction. In the new study, the most accurate estimate that can be made is in a whole number of days a year, said lead author Neils de Winter, a geochemist from Vrije Universiteit Brussel,

“We are pretty sure this number (372) is very accurate because of our new method of looking at multiple chemical records and multiple years. However, the exact number could be, for example, 372.25 or 371.75, just like it is approximately 365.25 days nowadays (when we count the leap days),” de Winter said, by email.

The Moon’s retreat

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. The authors say their study helps reconstruct that history.

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