Why July Arrives a Second Later?

Science Fields

Earth’s rotation slowing, but it’s not as simple as that…

A digital clock would normally show the passage to a new day in  the sequence 23:59:59 – 00:00:00. But the displays of (imaginary) clocks will read 23:59:60 just before the time slips from June 30, 2015 into July 1. That is, just as we have a leap year every four years to make our calendars conform to the orbital period of the Earth by appending an extra day to the end of February, we’ll now have a leap second – to add to the ones we had in past. But this re-adjustment, connected with the Earth’s rotation period around its axis, is not as regular. It involves roles by a host of factors and complex calculations. Daniel MacMillan of NASA’s Goddard Space Flight Center summarises thus:

The “Standard Time” or “Coordinated Universal Time”, based on the extremely precise vibrations, or transition between the energy levels of cesium atoms, divides the day to 86,400 seconds. This atomic clock is so sensitive that it deviates by only one second in 1,400,000 years.

But the fact is, sensitive measurements show that the Earth completes one rotation in 86,400.002 seconds, the reason being its rotation slowing over time. In fact, no “standard day” of 86,400 seconds has been recorded since 1820.

This “real” or “astronomical” time is based on sensitive observations of quasars made with powerful radiotelescopes across the world dubbed Very Large Based Interferometry (VLBI). Quasars are extremely active supermassive black holes at the cores of galaxies billions of light years away in early universe, guzzling gas around them voraciously. Before falling into the black hole, the gas attains temperatures of millions of degrees, outshining the galaxies in radiation red shifted to radio wavelengths. Their immense distances make them virtually motionless in the sky, and since the light which started  out billions of years ago from these “fixed” points arrive at different radio telescopes at slightly different times, scientists can accurately gauge the precise rotation period of the Earth.

While the two-microsecond (two seconds in a thousand) deviation from the standard time looks too miniscule to be given a thought, if it materializes every day, it will add up to nearly a second at the end of the year. So, why don’t we have a leap second every year? The answer lies in the small irregularities in the Earth’s rotation period, caused primarily by atmospheric dynamics on the short term, and longer-termed other factors such as dynamics of the Earth’s core, oceanic circulation, and thickness of the ice cover. These counteract the slowdown in Earth’s rotation. All these bar the regularity of leap second accumulation. Indeed, the leap seconds, first began to be implemented in 1972 and added annually until 1999, were required only four times since the turn of the century.

Since it hardly affects our lives, there have been calls for the abolishment of the practice, on which the International Telecommunications Union will have to take a decision, in late 2015 at the earliest. 


  • 1. “NASA Explains why June 30 will get extra second” , NASA/Goddard Space Flight Center, 26 june 2015