# Aberration in the earth's rotational speed.

**URL:** <https://boards.straightdope.com/t/aberration-in-the-earths-rotational-speed/800387>\
**Category:** Factual Questions\
**Created:** [November 1, 2017, 3:17pm UTC](https://boards.straightdope.com/t/aberration-in-the-earths-rotational-speed/800387 "2017-11-01T15:17:23Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![bizerta](https://avatars.discourse-cdn.com/v4/letter/b/3be4f8/32.png) [@bizerta](https://boards.straightdope.com/u/bizerta)\
**Post date:** [November 1, 2017, 3:17pm UTC](https://boards.straightdope.com/t/aberration-in-the-earths-rotational-speed/800387/1 "2017-11-01T15:17:23Z")

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Aberration in the earth’s rotational speed.

I am aware of the fact that the moon and tides are slowing the rotation of the earth. This is the reason for leap seconds.

[http://blog.cloudera.com//wp-content/uploads/2012/07/leapsecond.png](http://blog.cloudera.com//wp-content/uploads/2012/07/leapsecond.png) This is a graph of the earth’s rotational speed and leap seconds. Leap seconds occurred where there is a vertical spike in the line. When the line is sloping downward, the earth is rotating slower than it was in 1900. The steeper the line, the slower is the earth’s rotation. Between 1985 and 1999 there were nine leap seconds, one every 18 months or so. However, the next leap second was not added for seven years. What caused the rotation to increase?

What’s most unusual is that since 1998, there are visual aberrations in the graph from June to September. This is most pronounced during the summers of 2001-2005. During this period (when the line slope is upward), the earth’s rotation increased to the point that it was rotating faster than it was in 1900. What was the cause?

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**Author:** ![markn\_1](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@markn\_1](https://boards.straightdope.com/u/markn_1)\
**Post date:** [November 1, 2017, 3:53pm UTC](https://boards.straightdope.com/t/aberration-in-the-earths-rotational-speed/800387/2 "2017-11-01T15:53:04Z")

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For an overview, you can check the Wikipedia leap second article:

> **[Leap second | Slowing rotation of the Earth](https://en.wikipedia.org/wiki/Leap_second#Slowing_rotation_of_the_Earth)**
>
> Leap seconds are irregularly spaced because the Earth's rotation speed changes irregularly. Indeed, the Earth's rotation is quite unpredictable in the long term, which explains why leap seconds are announced only six months in advance.
> A mathematical model of the variations in the length of the solar day was developed by F. R. Stephenson and L. V. Morrison, based on records of eclipses for the period 700 BCE to 1623 CE, telescopic observations of occultations for the period 1623 until 1967 and a...

Basically there are a number of more or less chaotic processes that affect the Earth’s rotation, including convective movements of the mantle, earthquakes which move large volumes of crust, and even human activities, mainly dam building and dam destruction which move large amounts of water around the surface.