At the end of this day, June 30, clocks will show something they rarely do, the hour 23:59:60. The extra second is inserted into our standard definition of time to account for the fact that the length of the solar day is increasing.
This means that the Earth is beginning to take longer and longer to spin around its axis. The difference is minute, and will continue to grow imperceptibly for the foreseeable future. Current technological means at our disposal enable us to measure the length of a day with extreme precision.
Measurements are conducted using a high-precision technique called Very Long Baseline Interferometry (VLBI), which is put to use daily by an international consortium of scientists, operating a network of ground stations spread out around the globe.
Each day, the results of all these studies are correlated and compared, so scientists have access to extreme-fidelity data on precisely how long it takes for our planet to spin around its own axis.
“The solar day is gradually getting longer because Earth's rotation is slowing down ever so slightly,” explains scientist Daniel MacMillan, who holds an appointment at the NASA Goddard Space Flight Center (GSFC), in Greenbelt, Maryland.
The GSFC is responsible for coordinating the international VLBI efforts. Experts here also process and archive all data that result from these measurements, for future reference. This enables us to discover any potential variations in Earth's speed.
The American space agency is currently running a so-called Space Geodesy Project that, under the supervision of the GSFC, is trying to improve the accuracy of VLBI measurements even further, primarily by creating next-generation radio antennas.
According to planetary scientists, tidal forces between Earth and the Moon are the primary reason why the planet is slowing down. Currently, solar days are increasing in length at a rate of 1.4 milliseconds per century.
“At the time of the dinosaurs, Earth completed one rotation in about 23 hours. In the year 1820, a rotation took exactly 24 hours, or 86,400 standard seconds. Since 1820, the mean solar day has increased by about 2.5 milliseconds,” says MacMillan, also a member of the GSFC VLBI team.
“Normally, the clock would move from 23:59:59 to 00:00:00 the next day. Instead, at 23:59:59 on June 30, UTC will move to 23:59:60, and then to 00:00:00 on July 1. In practice, this means that clocks in many systems will be turned off for one second,” a NASA press release concludes.