<blockquote>
<p>I find it bizarre that the earth rotates at nearly exactly 365 1/4 times per year, requiring an extra day every 4 years.</p>
</blockquote>
<p>The Earth does not rotate nearly exactly 365 1/4 times per year. That's the Julian calendar. The Gregorian calendar, which also is not perfect, omits leap years on years that are divisible by 100 but not by 400. The Julian calendar, with one leap year every four years, is off by about 11 minutes per year. The Gregorian calendar is off by only 27 seconds per year.</p>
<blockquote>
<p>Leap seconds imply that this estimate is only off by about a second per year.</p>
</blockquote>
<p>Leap seconds have nothing to do with the calendar. Leap seconds are designed to keep the 86400 atomic time seconds in sync with the Earth's rotation rate. The intent of leap seconds is to keep mean (average) solar noon in sync with observed. Leap years are a very different phenomenon; they are designed to keep the calendar in sync with the Earth's seasons.</p>

CC BY-SA 4.0
2022-03-29T21:27:41.830Z
@user17791008 I added "nearly" in front of "exactly". That there are **currently** about 365 1/4 days per year is random luck. The Earth's rotation rate is slowing down. 600 million years ago there were over 400 days per year. In a few million years, maybe even sooner, there will be exactly 365 days per year.
David Hammen
CC BY-SA 4.0
2022-03-29T21:32:35.210Z
So basically, leap years keep the seasons from drifting ... and leap seconds keep noon time from drifting.
user17791008
CC BY-SA 4.0
2022-03-29T21:32:58.807Z
@user17791008 Exactly.
David Hammen
CC BY-SA 4.0
2022-03-29T21:09:33.147Z
2022-03-29T21:15:36.140Z
2022-03-29T21:15:36.140Z
David Hammen
<p>I find it bizarre that the earth rotates at <strong>nearly</strong> 365 1/4 times per year, requiring an extra day every 4 years.</p>
<p>I would have thought that this is just a rough estimate, but looking into leap seconds ...</p>
<p><a href="https://en.wikipedia.org/wiki/Leap_second" rel="nofollow noreferrer">Leap seconds</a> imply that this estimate is only off by about a second per year.</p>
<p>I would have expected it to be random, but it appears to be off by 6 hours per year.  I would have expected variation in not just the hours, but also the minutes, and in the seconds.</p>
<p>Here is a <a href="https://www.programmr.com/blogs/what-every-developer-should-know-about-leap-seconds" rel="nofollow noreferrer">programming perspective</a>.</p>

CC BY-SA 4.0
2022-03-29T21:02:44.377Z
The Earth does not rotate exactly 365 1/4 times per year.
David Hammen
CC BY-SA 4.0
2022-03-31T06:01:36.663Z
The sun and earth largely formed from the same material, an accretion disk, oriented in similar directions.
LazyReader
<astronomy><time>
Why does the earth rotate in correlation with it's rotation around the sun to such precision?
user17791008
# Why does the earth rotate in correlation with it's rotation around the sun to such precision?

(Asked by: username_0 on 2022-03-29)

I find it bizarre that the earth rotates at **nearly** 365 1/4 times per year, requiring an extra day every 4 years.

I would have thought that this is just a rough estimate, but looking into leap seconds ...

[Leap seconds](https://en.wikipedia.org/wiki/Leap_second) imply that this estimate is only off by about a second per year.

I would have expected it to be random, but it appears to be off by 6 hours per year. I would have expected variation in not just the hours, but also the minutes, and in the seconds.

Here is a [programming perspective](https://www.programmr.com/blogs/what-every-developer-should-know-about-leap-seconds).

**Tags:** astronomy, time

> **Comments:**
> **username_1**: The Earth does not rotate exactly 365 1/4 times per year.  
> **username_2**: The sun and earth largely formed from the same material, an accretion disk, oriented in similar directions.  

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## 1 answers

### Answer by username_1 on 2022-03-29. Score: 6 | Accepted answer

> I find it bizarre that the earth rotates at nearly exactly 365 1/4 times per year, requiring an extra day every 4 years.

The Earth does not rotate nearly exactly 365 1/4 times per year. That's the Julian calendar. The Gregorian calendar, which also is not perfect, omits leap years on years that are divisible by 100 but not by 400. The Julian calendar, with one leap year every four years, is off by about 11 minutes per year. The Gregorian calendar is off by only 27 seconds per year.

> Leap seconds imply that this estimate is only off by about a second per year.

Leap seconds have nothing to do with the calendar. Leap seconds are designed to keep the 86400 atomic time seconds in sync with the Earth's rotation rate. The intent of leap seconds is to keep mean (average) solar noon in sync with observed. Leap years are a very different phenomenon; they are designed to keep the calendar in sync with the Earth's seasons.

> **Comments:**
> **username_1**: @username_0 I added "nearly" in front of "exactly". That there are **currently** about 365 1/4 days per year is random luck. The Earth's rotation rate is slowing down. 600 million years ago there were over 400 days per year. In a few million years, maybe even sooner, there will be exactly 365 days per year.  
> **username_0**: So basically, leap years keep the seasons from drifting ... and leap seconds keep noon time from drifting.  
> **username_1**: @username_0 Exactly.  

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