Long ago, a year used to have 500 days

Published at: Jan. 20, 2023

The earth's spin is slowing down, and that has effect on the length of our days. Over the last century, the average day has grown by about 2 milliseconds because of this phenomenon. This might not seem like much, but it can add up over time.

Earths spin is slowing down

What makes the earth spin go slower?

The Earth's rotation is affected by a number of factors, including the gravitational pull of the moon and the sun, changes in the atmosphere due to weather patterns, and even human-made structures. The slowing down of earth is caused by a phenomenon called the tidal effect. The is the effect the moon has on the earth’s rotation. In the long term this effect causes the rotation to slow down. As a result, we experience changes in day length over long periods of time.

How did the day length change over the course of history?

Give or take 1 billion years ago, a year had 500 days. Gradually, with the passing of millions of years, the Earth's rotation slowed down leading to longer and longer days. This continued until today where we have 365 days for every trip around the Sun.

source: https://spacemath.gsfc.nasa.gov/earth/6Page58.pdf
Period Age (years) Days per year Hours per day
Current 0 365 24
Cretaceous 70 370 23.7
Triassic 220 372 23.5
Pennsylvanian 290 383 22.9
Mississippian 340 398 22.0
Upper Devonian 380 399 22.0
Middle Devonian 395 405 21.6
Lower Devonian 410 410 21.4
Upper Silurian 420 400 21.9
Middle Silurian 430 413 21.2
Lower Silurian 440 421 20.8
Upper Ordovician 450 414 21.2
Middle Cambrian 510 424 20.7
Ediacarin 600 417 21.0
Cryogenian 900 486 18.0

What about the future? Will earth come to a full stop?

It is projected that the tidal effect on earth will cause it to come to a complete standstill in 50 billion years. Unfortunately, if humanity is unable to move beyond our solar system, we may not exist at this point in time. In just 5 billion years, the sun will enter its final stage of life and become a red giant star and swallow our planet in the process.

How does this affect us in our daily lives?

To keep things simple in our daily live we all agreed on basing our local times on the Universal Standard Time or UTC. Modern atomic clocks keep track of UTC for us making sure each day has precis

Time in our daily live is dictated by UTC which is based on the modern atomic clock. Atomic clocks don’t take variations in the earths rotating speed into account. Therefore we also keep track of time using the stars and planets as out reference, this time system is called UT1. Whenever the difference between UT1 and UTC is more than a second, UTC can be adjusted by adding a leap second to the time. This has happened 28 times in the past 50 years. Did you notice it?

The exception that proves the rule

For the past few billion years, Earth has been gradually slowing down its rotation. But recently, over the last few years, we've witnessed the opposite; there have been multiple new records set for historically short days (since we started measuring in 1960). There is still a mystery as to why the speedup of the Earth's rotation has been happening. If it continues, it might be necessary to add a negative leap second - something that has never been done before. Despite this, there is no need to panic. Because we've only been tracking data since the 1960s there's a good chance that some acceleration has also taken place sometime in the past 2 Billion years.