<p>Here is the suitable publication that calculates the length of day over the entire Earth history.
<a href="https://www.researchgate.net/publication/26541355_The_Length_of_the_Day_A_Cosmological_Perspective" rel="nofollow noreferrer">https://www.researchgate.net/publication/26541355_The_Length_of_the_Day_A_Cosmological_Perspective</a>
Thanks.
Maxwell</p>

CC BY-SA 4.0
2021-08-16T09:46:05.930Z
Link only answers are not good for this site due to the possibility of link rot. A summary of the information in the paper would significantly improve this answer.
Fred
CC BY-SA 4.0
2021-08-16T20:13:34.610Z
@Fred, totally agree. That said, it's a bit fairer, as it's a paper, so it won't disappear so easily (though better to give author\journal rather than just a link!), and it may be a bit complex to boil down into a quick answer.
JeopardyTempest
CC BY-SA 4.0
2021-08-16T20:13:42.127Z
Though skimming through it does look like it boils down to one equation, exponential decay, fairly linear over our timescale, and he (the PI) would do great to try to help explain it for a broader audience here :-)  I think that's more the reason for a longer answer... to help those who aren't versed in advance science enough so they don't have to struggle to figure it out themselves from journal articles :-)
JeopardyTempest
CC BY-SA 4.0
2021-08-16T08:51:09.347Z
2021-08-16T08:51:09.347Z
Arbab Arbab
<p>I want to know how to calculate the number of days since an event in Earth's past, and am using the Cambrian explosion as an example.</p>

<p>I define a day to be one rotation of Earth with respect to the sun. The key point is that Earth's rotation has been slowing down over millions of years, so it will not be as simple as multiplying the time in years by 365.</p>

<p>Is there a formula for the number of days x years ago that I can simply integrate to find the answer? Or some other way to approximate?</p>

CC BY-SA 4.0
2020-03-10T00:08:54.960Z
The complications you mention make make this question impossible to answer. Daylength has not always been the same, and we don't know exactly how many days there were in a year for the early Cambrian. We don't even know  on what day the Cambrian period started, or exactly how many years it lasted, never mind how many days!
Michael Walsby
CC BY-SA 4.0
2020-03-10T03:50:09.290Z
@MichaelWalsby I disagree. There are estimates for a day's length that go back in time (which I don't remember now, but they exist). You can also assume that the Cambrian explosion was the day exactly 541 million years ago. You are missing the point of the question. This is a good question.
Gimelist
CC BY-SA 4.0
2020-03-10T04:29:48.997Z
@MichaelWalsby I am looking for an approximate answer. Of course you cannot know exactly how long it has been since the Cambrian started.
Perry Ainsworth
CC BY-SA 4.0
2020-03-10T07:51:52.393Z
@Gimmelist How many years approximately did the Cambrian last, and exactly how many days were there in a Cambrian year? And what about the Ordovician and Silurian? Once we know this we can give a roughly correct answer.
Michael Walsby
CC BY-SA 4.0
2020-03-10T08:29:58.093Z
@Perry Ainsworth You will be lucky to get an answer accurate to within 50 million years. There are a lot of days in 50 million years, especially if they are short days as they would surely be. Any answer you get will be little more than a wild guess.
Michael Walsby
CC BY-SA 4.0
2020-03-10T09:27:27.177Z
I think the point about the Cambrian explosion muddies the waters here - if we aren't sure to the exact year (and we aren't) then errors in the number of days will be small in comparison. You'll be saying something like 987,654,321,123,456 days ago give or take 500,000 days (numbers completely made up for illustrative purposes) - there is no point having the precision of the last five digits if we've got that large an uncertainty. What you actually seem to be asking, however, is if there is an equation/estimate of day length going back in time.
Lio Elbammalf
CC BY-SA 4.0
2020-03-10T12:44:53.240Z
It's not answerable if you want an answer which is more than a wild guess, You don't seem to have any idea of the sort of unknowns and unknowables which need to become knowns in order to give an even roughly correct answer.
Michael Walsby
CC BY-SA 4.0
2021-08-20T00:59:40.267Z
given that the exact time of Cambrian explosion has a huge margin of error, any error created by changes in the length of day  will be swamped by that much larger error. it is a bit like asking how many seconds has it been since the human civilization began.
John
<geophysics><earth-history><measurements><mathematics>
How many days has it been since the Cambrian explosion?
Perry Ainsworth
# How many days has it been since the Cambrian explosion?

(Asked by: username_0 on 2020-03-09)

I want to know how to calculate the number of days since an event in Earth's past, and am using the Cambrian explosion as an example.

I define a day to be one rotation of Earth with respect to the sun. The key point is that Earth's rotation has been slowing down over millions of years, so it will not be as simple as multiplying the time in years by 365.

Is there a formula for the number of days x years ago that I can simply integrate to find the answer? Or some other way to approximate?

**Tags:** geophysics, earth-history, measurements, mathematics

> **Comments:**
> **username_1**: The complications you mention make make this question impossible to answer. Daylength has not always been the same, and we don't know exactly how many days there were in a year for the early Cambrian. We don't even know  on what day the Cambrian period started, or exactly how many years it lasted, never mind how many days!  
> **username_2**: @username_1 I disagree. There are estimates for a day's length that go back in time (which I don't remember now, but they exist). You can also assume that the Cambrian explosion was the day exactly 541 million years ago. You are missing the point of the question. This is a good question.  
> **username_0**: @username_1 I am looking for an approximate answer. Of course you cannot know exactly how long it has been since the Cambrian started.  
> **username_1**: @Gimmelist How many years approximately did the Cambrian last, and exactly how many days were there in a Cambrian year? And what about the Ordovician and Silurian? Once we know this we can give a roughly correct answer.  
> **username_1**: @username_0 You will be lucky to get an answer accurate to within 50 million years. There are a lot of days in 50 million years, especially if they are short days as they would surely be. Any answer you get will be little more than a wild guess.  
> **username_3**: I think the point about the Cambrian explosion muddies the waters here - if we aren't sure to the exact year (and we aren't) then errors in the number of days will be small in comparison. You'll be saying something like 987,654,321,123,456 days ago give or take 500,000 days (numbers completely made up for illustrative purposes) - there is no point having the precision of the last five digits if we've got that large an uncertainty. What you actually seem to be asking, however, is if there is an equation/estimate of day length going back in time.  
> **username_1**: It's not answerable if you want an answer which is more than a wild guess, You don't seem to have any idea of the sort of unknowns and unknowables which need to become knowns in order to give an even roughly correct answer.  
> **username_4**: given that the exact time of Cambrian explosion has a huge margin of error, any error created by changes in the length of day  will be swamped by that much larger error. it is a bit like asking how many seconds has it been since the human civilization began.  

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## 1 answers

### Answer by username_5 on 2021-08-16. Score: 0

Here is the suitable publication that calculates the length of day over the entire Earth history.
<https://www.researchgate.net/publication/26541355_The_Length_of_the_Day_A_Cosmological_Perspective>
Thanks.
Maxwell

> **Comments:**
> **username_6**: Link only answers are not good for this site due to the possibility of link rot. A summary of the information in the paper would significantly improve this answer.  
> **username_7**: @username_6, totally agree. That said, it's a bit fairer, as it's a paper, so it won't disappear so easily (though better to give author\journal rather than just a link!), and it may be a bit complex to boil down into a quick answer.  
> **username_7**: Though skimming through it does look like it boils down to one equation, exponential decay, fairly linear over our timescale, and he (the PI) would do great to try to help explain it for a broader audience here :-)  I think that's more the reason for a longer answer... to help those who aren't versed in advance science enough so they don't have to struggle to figure it out themselves from journal articles :-)  

---

