<p>There are two components to take into account for this question</p>
<ul>
<li>the earth rotation rate <strong>on itself</strong></li>
<li>the orbiting period <strong>around the sun</strong></li>
</ul>
<p>A day back then was shorter than today's earth day. The earth was rotating quicker in the past, but its rotating period around the sun didn't change much because of Kepler's law.</p>
<p>I cannot say how it was 5 billion years ago (anyway, did the earth had something you could call a <em>surface</em> to track the day from then?), but you could find data derived from fossils analysis for the last billion years, e.g. <a href="https://spacemath.gsfc.nasa.gov/earth/6Page58.pdf" rel="nofollow noreferrer">here</a> from Nasa's SpaceMath, on how long a day was back then - <em>I realize now the figures below assume the orbiting period of the earth around the sun (~8766 hours) stayed constant over this period of time. <a href="https://astronomy.stackexchange.com/a/19857/11264">This assumption is wrong as explained here</a> - however stating it &quot;has been close to its current value for the last 2-3 billion years&quot;, so below figures are an approximation</em></p>
<div class="s-table-container">
<table class="s-table">
<thead>
<tr>
<th>Period</th>
<th>Age(years)</th>
<th>*Days/year</th>
<th>*Hours/day</th>
<th>ratio</th>
</tr>
</thead>
<tbody>
<tr>
<td>Current</td>
<td>0</td>
<td>365</td>
<td>24.0</td>
<td>1.000</td>
</tr>
<tr>
<td>Upper Cretaceous</td>
<td>70   million</td>
<td>370</td>
<td>23.7</td>
<td>0.988</td>
</tr>
<tr>
<td>Upper Triassic</td>
<td>220  million</td>
<td>372</td>
<td>23.5</td>
<td>0.979</td>
</tr>
<tr>
<td>Pennsylvanian</td>
<td>290  million</td>
<td>383</td>
<td>22.9</td>
<td>0.954</td>
</tr>
<tr>
<td>Mississippian</td>
<td>340  million</td>
<td>398</td>
<td>22.0</td>
<td>0.917</td>
</tr>
<tr>
<td>Upper Devonian</td>
<td>380  million</td>
<td>399</td>
<td>22.0</td>
<td>0.917</td>
</tr>
<tr>
<td>Middle Devonian</td>
<td>395  million</td>
<td>405</td>
<td>21.6</td>
<td>0.900</td>
</tr>
<tr>
<td>Lower Devonian</td>
<td>410  million</td>
<td>410</td>
<td>21.4</td>
<td>0.892</td>
</tr>
<tr>
<td>Upper Silurian</td>
<td>420  million</td>
<td>400</td>
<td>21.9</td>
<td>0.913</td>
</tr>
<tr>
<td>Middle Silurian</td>
<td>430  million</td>
<td>413</td>
<td>21.2</td>
<td>0.883</td>
</tr>
<tr>
<td>Lower Silurian</td>
<td>440  million</td>
<td>421</td>
<td>20.8</td>
<td>0.867</td>
</tr>
<tr>
<td>Upper Ordovician</td>
<td>450  million</td>
<td>414</td>
<td>21.2</td>
<td>0.883</td>
</tr>
<tr>
<td>Middle Cambrian</td>
<td>510  million</td>
<td>424</td>
<td>20.7</td>
<td>0.863</td>
</tr>
<tr>
<td>Ediacarin</td>
<td>600  million</td>
<td>417</td>
<td>21.0</td>
<td>0.875</td>
</tr>
<tr>
<td>Cryogenian</td>
<td>900  million</td>
<td>486</td>
<td>18.0</td>
<td>0.750</td>
</tr>
</tbody>
</table>
</div>
<p><a href="https://i.sstatic.net/QPiPI.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/QPiPI.png" alt="enter image description here" /></a></p>
<p>From which I would roughly interpolate: <strong>600 <strike>days</strike> <em>earth self-rotation</em> per <strike>year</strike> <em>full rotation around the sun</em> 2 billion years ago</strong></p>
<p><a href="https://i.sstatic.net/pJdlg.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/pJdlg.png" alt="600 days per year 2 billion years ago" /></a></p>
<h1>Edit</h1>
<p>We are saying here that earth was rotating quicker in the past:</p>
<blockquote>
<p>Detailed  studies  of  fossil  shells,  and  the  banded  deposits  in  certain  sandstones,  reveal a much different length of day in past eras! These bands in sedimentation and  shell-growth  follow  the  lunar  month  and  have  individual  bands  representing  the  number  of  days  in  a  lunar  month.</p>
<p>By  counting  the  number  of  bands,  geologists can work out the number of days in a year, and from this the number of hours  in  a  day  when  the  shell  was  grown,  or  the  deposits  put  down.  The  table  above shows the results of one of these studies.</p>
</blockquote>

CC BY-SA 3.0
2017-07-18T13:59:47.880Z
This still isn't in current Earth days, its just showing how many days there would be in a year if the Earth span faster and a day was shorter.
Dean
CC BY-SA 3.0
2017-07-18T15:08:01.497Z
I see now where I was wrong. I'll try to make it clearer.
J. Chomel
CC BY-SA 3.0
2017-07-18T23:19:11.277Z
Just wrong. The orbital period of the Earth must have been shorter in the past, since the Sun was more massive.
ProfRob
CC BY-SA 3.0
2017-07-19T07:41:49.713Z
@RobJeffries, as you mentionned it, it *"has been close to its current value for the last 2-3 billion years."*, so I'll make note of it.
J. Chomel
CC BY-SA 4.0
2023-10-14T19:01:54.570Z
The day was six hours when the earth formed  https://link.springer.com/article/10.1556/AGeod.39.2004.1.3   .
Maxwell
CC BY-SA 4.0
2017-07-18T12:35:52.173Z
2020-12-16T19:34:31.060Z
2020-12-16T19:34:31.060Z
J. Chomel
<p>Roughly how long did it use to take for the early Earth 4.5~ billion years ago to complete one revolution around the Sun?</p>

<p>(I know the Earth was spinning much faster, so a day would only take 2-3 hours, so I'd like to have the answer in terms of current Earth days for better understanding - if possible.)</p>

<p>(I Google'd the heck out of this question and couldn't find an answer.)</p>

CC BY-SA 3.0
2017-07-18T16:22:22.877Z
The accepted answer does not answer the question. The duplicate does.
David Hammen
CC BY-SA 3.0
2017-07-20T17:19:24.013Z
I must downvote for accepting an answer to another question.
Walter
CC BY-SA 3.0
2017-12-22T06:40:49.830Z
@cheekbanana (and David), why did you un accept my answer?  Did it became wrong?
J. Chomel
CC BY-SA 3.0
2017-12-23T00:19:41.693Z
I am torn between giving you due diligence and following the website's format. Here, have it back. And no, it didn't "become wrong". It is helpful as ever, and thank you for that.
Faruk D.
<the-sun><earth>
How long was an early Earth year?
Faruk D.
# How long was an early Earth year?

(Asked by: username_0 on 2017-07-18)

Roughly how long did it use to take for the early Earth 4.5~ billion years ago to complete one revolution around the Sun?

(I know the Earth was spinning much faster, so a day would only take 2-3 hours, so I'd like to have the answer in terms of current Earth days for better understanding - if possible.)

(I Google'd the heck out of this question and couldn't find an answer.)

**Tags:** the-sun, earth
> **Comments:**
> **username_1**: The accepted answer does not answer the question. The duplicate does.  
> **username_2**: I must downvote for accepting an answer to another question.  
> **username_3**: @cheekbanana (and David), why did you un accept my answer?  Did it became wrong?  
> **username_0**: I am torn between giving you due diligence and following the website's format. Here, have it back. And no, it didn't "become wrong". It is helpful as ever, and thank you for that.  

---

## 1 answers

### Answer by username_3 on 2017-07-18. Score: 1 | Accepted answer

There are two components to take into account for this question

* the earth rotation rate **on itself**
* the orbiting period **around the sun**

A day back then was shorter than today's earth day. The earth was rotating quicker in the past, but its rotating period around the sun didn't change much because of Kepler's law.

I cannot say how it was 5 billion years ago (anyway, did the earth had something you could call a *surface* to track the day from then?), but you could find data derived from fossils analysis for the last billion years, e.g. [here](https://spacemath.gsfc.nasa.gov/earth/6Page58.pdf) from Nasa's SpaceMath, on how long a day was back then - *I realize now the figures below assume the orbiting period of the earth around the sun (~8766 hours) stayed constant over this period of time. [This assumption is wrong as explained here](https://astronomy.stackexchange.com/a/19857/11264) - however stating it "has been close to its current value for the last 2-3 billion years", so below figures are an approximation*

| Period | Age(years) | \*Days/year | \*Hours/day | ratio |
| --- | --- | --- | --- | --- |
| Current | 0 | 365 | 24.0 | 1.000 |
| Upper Cretaceous | 70 million | 370 | 23.7 | 0.988 |
| Upper Triassic | 220 million | 372 | 23.5 | 0.979 |
| Pennsylvanian | 290 million | 383 | 22.9 | 0.954 |
| Mississippian | 340 million | 398 | 22.0 | 0.917 |
| Upper Devonian | 380 million | 399 | 22.0 | 0.917 |
| Middle Devonian | 395 million | 405 | 21.6 | 0.900 |
| Lower Devonian | 410 million | 410 | 21.4 | 0.892 |
| Upper Silurian | 420 million | 400 | 21.9 | 0.913 |
| Middle Silurian | 430 million | 413 | 21.2 | 0.883 |
| Lower Silurian | 440 million | 421 | 20.8 | 0.867 |
| Upper Ordovician | 450 million | 414 | 21.2 | 0.883 |
| Middle Cambrian | 510 million | 424 | 20.7 | 0.863 |
| Ediacarin | 600 million | 417 | 21.0 | 0.875 |
| Cryogenian | 900 million | 486 | 18.0 | 0.750 |

[![enter image description here](https://i.sstatic.net/QPiPI.png)](https://i.sstatic.net/QPiPI.png)

From which I would roughly interpolate: **600 days *earth self-rotation* per year *full rotation around the sun* 2 billion years ago**

[![600 days per year 2 billion years ago](https://i.sstatic.net/pJdlg.png)](https://i.sstatic.net/pJdlg.png)

# Edit

We are saying here that earth was rotating quicker in the past:

> Detailed studies of fossil shells, and the banded deposits in certain sandstones, reveal a much different length of day in past eras! These bands in sedimentation and shell-growth follow the lunar month and have individual bands representing the number of days in a lunar month.
>
> By counting the number of bands, geologists can work out the number of days in a year, and from this the number of hours in a day when the shell was grown, or the deposits put down. The table above shows the results of one of these studies.

> **Comments:**
> **username_4**: This still isn't in current Earth days, its just showing how many days there would be in a year if the Earth span faster and a day was shorter.  
> **username_3**: I see now where I was wrong. I'll try to make it clearer.  
> **username_5**: Just wrong. The orbital period of the Earth must have been shorter in the past, since the Sun was more massive.  
> **username_3**: @RobJeffries, as you mentionned it, it *"has been close to its current value for the last 2-3 billion years."*, so I'll make note of it.  
> **username_6**: The day was six hours when the earth formed  https://link.springer.com/article/10.1556/AGeod.39.2004.1.3   .  

---

