Today at precisely 10am Australian Eastern Standard time, something chronologically peculiar will take place: there’ll be an extra second between 09:59:59 and 10:00:00.
This will make 1st July 2015 (or 30th June 2015 in many other parts of the world) one second longer than the length of a standard day. The culprit is a “leap second”, although it’s far from unique. In fact, it’ll be the 26th one we’ve had since they were first introduced in 1972.
How long is a piece of day?
Why do we need to add an extra second to the day? Historically the second had been defined as a fraction of the day: one 86,400th of the total time for the sun to return to the same position in the sky.
That was precise enough for most purposes, but by the early twentieth century, astronomers had determined that the Earth’s rotation was not constant. It was actually slowing down. This meant that a second defined in this fashion would slowly lengthen over time.
The development of atomic clocks in the 1950s allowed the second to be defined with incredible accuracy, with a variance of only one part in 1014.
Thus was the second redefined in 1967 by the International Committee for Weights and Measures in 1967. It was no longer pegged to the Earth’s rotation. Instead it was defined in terms of a very particular physical property of a caesium-133 atom.
This mechanical definition has disconnected the second from the length of the solar day. In fact, the tables turned and the day was subsequently redefined in terms of this newly established atomic second: 86,400 seconds make up a standard day.
The length of the solar day – or the time it actually takes the Earth to complete a rotation – is no longer precisely as long as a standard day, and it has not been for a century. This is because the Earth’s rotation continues to slow.
The main reason it’s lagging is tidal friction from the Moon, which by itself would increase the length of the day by 2.3 milliseconds each century.
However, other geological process on Earth that shift mass around will also have an effect on the rotation rate, since the system mus conserve its total angular momentum. This can end up increasing the Earth’s rotation rate as well as decreasing it.
For example, the 2005 earthquake in Indonesia that caused the tsunami also decreased the length of the day by 2.68 microseconds.
So we have to keep adding leap seconds to keep the time of noon at Greenwich (Greenwich Mean Time) in line with noon as measured by the atomic clock (International Atomic Time). This guarantees that the solar time (the rise and fall of the sun) doesn’t fall too far out of sync with our clocks.
Taking time