A Day Was Half an Hour Shorter When Dinosaurs Were Alive

A day feels like one of the few truly fixed things in life. The sun comes up, the sun goes down, and twenty-four hours later it happens again. But the length of a day has not always been the same. According to a study reported by the American Geophysical Union in 2020, about 70 million years ago a day lasted only around 23 and a half hours. The Earth was spinning a little faster, and a year held 372 days instead of the 365 we are used to.

That was a world that still had dinosaurs in it. National Geographic places Tyrannosaurus rex around 68 million years ago, and describes the species being wiped out about 66 million years ago when an asteroid or comet struck the Earth. So the shorter days of 70 million years ago belonged to the final stretch of the age of dinosaurs. Any dinosaur alive then enjoyed afternoons about half an hour shorter than ours, and seven more of them each year.

The evidence comes from an unlikely witness: a fossil shell. The study examined Torreites sanchezi, a member of an extinct and very varied group known as rudist clams. ScienceDaily describes these animals as looking like tall pint glasses with lids shaped like bear-claw pastries. They grew quickly and laid down growth rings every day, so a well-preserved shell holds a record of individual days, stacked up across the seasons of a year.

Counting those rings is delicate work. The researchers, led by Niels de Winter of the Vrije Universiteit Brussel, used lasers to sample minute slices of the shell. According to the AGU, this let them count the growth rings more accurately than human researchers could with microscopes. From that count they worked out how many days fitted into one year, and from there they calculated how long each day must have been. The findings were published in the journal Paleoceanography and Paleoclimatology.

Why have days grown longer since then? The AGU explains that the length of a day has been growing steadily as friction from ocean tides, caused by the Moon’s gravity, slows the Earth’s rotation. The same interaction nudges the Moon along in its orbit so that it slowly moves away from us. According to the AGU, the Moon is pulling away from the Earth at about 3.82 centimetres a year, roughly the speed at which fingernails grow, although that comparison is ours rather than the scientists’.

So the days of the dinosaurs were genuinely shorter, and the proof was sitting in a clam shell for 70 million years. It is a lovely reminder that even the most dependable rhythms of our lives are slowly changing. The next time an afternoon seems to fly by, you can blame the dinosaurs for having it easier. Their days really were shorter, and their Moon was a little closer to home.