# How long was an early Earth year? [duplicate]

This question already has an answer here:

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.)

## marked as duplicate by David Hammen, Rob Jeffries, James K, adrianmcmenamin, Rory AlsopJul 23 '17 at 17:40

• The accepted answer does not answer the question. The duplicate does. – David Hammen Jul 18 '17 at 16:22
• I must downvote for accepting an answer to another question. – Walter Jul 20 '17 at 17:19
• @cheekbanana (and David), why did you un accept my answer? Did it became wrong? – J. Chomel Dec 22 '17 at 6:40
• 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. – cheekybanana Dec 23 '17 at 0:19

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 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 - 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 p*|*Hours p*|
|                  |              |  year  | 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|


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

# 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.

• 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 Jul 18 '17 at 13:59
• This seems a little shaky to me because orbital period is related to orbital distance via R^3/T^2 being constant, unless you're saying the Sun's mass has changed enough to change the R^3/T^2 constant? – barrycarter Jul 18 '17 at 14:37
• 365 days today to 486 days 900 million years ago seems like a fairly large change to me, at least if the R^3/T^2 identity holds (ie, the Sun's mass remains relatively constant). – barrycarter Jul 18 '17 at 15:00
• I'm missing something. Your comment earlier says "My answer is 486 days (implicating today earth days)" and you even emphasize "in current earth days". – barrycarter Jul 18 '17 at 15:05
• I see now where I was wrong. I'll try to make it clearer. – J. Chomel Jul 18 '17 at 15:08