The speed of the Earth’s rotation is not constant. It changes over the short and long terms, due to different factors. Short-term changes are produced by things like:

Large Earthquakes
* Freezing and melting of polar ice caps – as snow melts, the ground underneath rebounds and changes the shape of the Earth.
* Plate tectonics, which changes the distribution of landmasses and oceans.

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The most important long-term effect is due to the moon, which causes a tidal bulge on the Earth on the side facing towards the moon. This is most pronounced in the oceans, and friction produced by the movement of large volumes of water causes the Earth’s rotation to slow down.

Short term effects are random, so they need to be constantly monitored to get exact estimates of how fast the Earth’s rotation is speeding up or slowing down in any given year. However, the long-term effect due to the moon is relatively steady, at approximately 1.4 ms per day per century.

On the assumption that short term changes probably cancel each other out over the long term, let’s see how long term changes due to the moon have affected Earth’s rotation.

Moon’s Effect on Rotation of Earth

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The rate of slowing of the Earth’s rotation is 1.4 milliseconds per year per century. Let’s convert that to hours per year.

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The length of the mean solar day at time x years in the past would be mean solar day today minus rate of slowdown times x.

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Let’s check some past dates and see what we get.

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So we have the length of the day at 23.6 hours about a hundred million years ago, 22.6 hours at 350 million years ago, 20.11 hours at a billion years ago, and 6.11 hours at 4.6 billion years ago.

Yes, I know that the Earth couldn’t be slowed by the moon before the moon was formed, and that happened at some point after the formation of the Earth. However, our numbers are very approximate to begin with, so we’re not bothered with these issues, we’re just using this to get a rough idea.

How does this check out with independent physical evidence of the length of the day? According to this article, coral growth cycles in the fossil record show that 350 million years ago, the day was “less than 23 hours long”. Our result is 22.64 hours, which fits the evidence. It also says that 620 million years ago, the day was 21.9 hour or even less, according to the evidence of tidal mineral deposition on rocks. Our calculated figure is 21.59 days, which fits this as well. And it says that when the moon was formed about 4.5 billion years ago, the length of the day was about 6 hours. Our calculation for 4.6 billion years shows a length of 6.11 hours.

So barring more precise figures from actual physical evidence that can be reliably dated, I’m going to say this approximation is good enough.

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Length of Day for Geological Periods

Length of Day for Geological Periods

Eon Era Period Epoch Date Day
Hadean Cryptic 4.53b 6.38
Basin Groups 4.23 b 7.57
Nectarian 3.90 b 8.83
Early Imbrian 3.83 b 9.13
Archean Eoarchean 3.70 b 9.61
Paleoarchean 3.40 b 10.78
Mesoarchean 3.00 b 12.33
Neoarchean 2.65 b 13.69
Proterozoic Paleoproterozoic Siderian 2.40 b 14.67
Rhyacian 2.18 b 15.54
Orosirian 1.93 b 16.51
Statherian 1.70 b 17.39
Mesoproteroizoic Calymmian 1.50 b 18.17
Ectasian 1.30 b 18.94
Stenian 1.10 b 19.72
Neoproterozoic Tonian 925 m 20.40
Cryogenian 743 m 21.11
Ediacaran 589 m 21.71
Phanerozoic Paleozoic Cambrian 516 m 21.99
Ordovician 466 m 22.19
Silurian 429 m 22.33
Devonian 387 m 22.50
Carboniferous Miss. 339 m 22.68
Penn. 309 m 22.80
Permian 275 m 22.93
Mesozoic Triassic 225 m 23.13
Jurassic 172 m 23.33
Cretaceous 105 m 23.59
Cenozoic Paleogene Paleocene 60.5 m 23.76
Eocene 45.0 m 23.83
Oligocene 28.5 m 23.89
Neogene Miocene 14.2m 23.94
Pliocene 3.96 m 23.98
Pleistocene 1.3 m 23.99
Holocene 5850 24.00

The table above shows the length of day during the geological time scale. The “date” in the 5th column represents the number of years before present. Rather than showing the start and end of each era/period/epoch, the midpoint date has been selected to represent that period. The 6th column is the length of the day in hours.

As we can see, the change is very gradual. For the entire period of human evolution, the day has changed by about half a minute. At the dawn of the dinosaurs, the day was only about 45 minutes shorter than today. During the big radiation of multicellular life in the Ediacaran and Cambrian, the day was about 2 hours shorter than today.