Earth's Rotational Velocity Anomalies

National Physical Laboratory, UK

Earth spins at fastest rate for 80 years: If it feels to you like the days are getting shorter and life is getting more and more hectic, I might be able to give you the reason. Recent data from the International Earth Rotation Service (IERS) imply that the Earth's rotation is speeding up and that it is spinning at its fastest rate since 1929. The chart indicates the speed of the Earth's rotation between the years 1700 and 1997 by showing the length of the day over the last 297 years compared to the length we expect - 24 hours. This chart shows that the rotation of the Earth speeds up and slows down in an apparently random fashion. When the data is above the zero axis it means that the length of the day is longer than 24 hours, signifying that the Earth's rotation is slower than that expected, and if it goes below, the Earth is rotating faster than expected. The Earth was recorded spinning at its slowest rate ever in 1912 and at its fastest, since reliable recordings have been made, in 1871. The fact that we can resolve variations in the length of the day indicates that the rotating Earth is not a very accurate time keeper. 

In 1955, caesium atomic clocks were developed, based on a more accurate time keeper than the Earth - the caesium atom. The world now operates to Coordinated Universal Time (UTC) which is based on atomic clocks but corrected by inserting 'leap' seconds to keep track with the Earth's time. There have been twenty-two leap seconds inserted since the current time scale was introduced in 1972. Recent data released by the IERS shows that in July 1999, the length of day will shorten to approximately 0.5 milliseconds (0.0005 seconds) less than the length of a standard day (see Chart 2 for the length of day from April 1999 to April 2000). At first this data seems to contradict the popular belief that the Earth is on average slowing down. However, from Chart 2 it is clear that this period of rapid rotation is short lived, and afterwards the Earth's rotation will begin to slow down again. Note that periodic variations can be seen in Chart 2 below, with a periods of less than 35 days, these are produced by the tides of the oceans and solid earth. They cannot be seen in Chart 1 as this data has been averaged over each year, so oscillations with a period of less than a year cannot be seen.

 

The changes in the Earth's rotation are very difficult to predict in the long term (although they are predicted over a short time scale with acceptable accuracy by the IERS), as they result from a complex series of gravitational and geological interactions. As well as changes in the speed of the Earth's rotation, the axis on which the Earth rotates also wobbles. This precession of the Earth's axis of rotation is caused by the gravitational torque exerted by the Moon, Sun and other planets and by displacements of matter in different parts of the planet. The displacement effects include changes in the centre of the mass of the Earth with respect to the crust, mantle elasticity, properties of the core-mantle boundary and the coupling between other layers of the planet, underground water, oceanic variability (tides) and atmospheric and climate effects.

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