The recent and tragic earthquake that has left so much devastation in Japan has also highlighted an interesting aspect about the measurement of time and the rotation of the Earth.
So powerful was the 9.0 magnitude earthquake, it actually shifted Earth axis by 165mm (6½ inches) according to NASA.
The quake, one of the most powerful felt on Erath over the last millennia, altered the distribution of the planet’s mass, causing the Earth to rotate on its axis that little bit faster and therefore, shortening the length of every day that will follow.
Fortunately, this change is so minute it is not noticeable in our day to day activities as the Earth slowed by less than a couple of microseconds (just over a millionth of a second), and it isn’t unusual for natural events to slow down the speed of the Earth’s rotation.
In fact, since the development of the atomic clock in the 1950’s, it has been realised the Earth’s rotation is never continual and in fact has been increasing very slightly, most probably for billions of years.
These changes in the Earth’s rotation, and the length of a day, are caused by the effects of the moving oceans, wind and the gravitational pull of the moon. Indeed, it has been estimated that before humans arrived on Earth, the length of a day during the Jurassic period (40-100 million years ago) the length of a day was only 22.5 hours.
These natural changes to the Earth’s rotation and the length of a day, are only noticeable to us thanks to the precise nature of atomic clocks which have to account for these changes to ensure that the global timescale UTC (Coordinated Universal Time) doesn’t drift away from mean solar time (in other words noon needs to remain when the sun is highest during the day).
To achieve this, extra seconds are occasionally added onto UTC. These extra seconds are known as leap seconds and over thirty have been added to UTC since the 1970’s.
Many modern computer networks and technologies rely on UTC to keep devices synchronised, usually by receiving a time signal via a dedicated NTP time server (Network Time Protocol).
NTP time servers are designed to accommodate these leap seconds, enabling computer systems and technologies to remain accurate, precise and synchronised.