We take it for granted that a day is twenty-four hours. Indeed, our body’s circadian rhythm is finally tuned to cope with a 24-hour-day. However, a day on Earth was not always 24 hours long.
In the early days of the Earth, a day was incredibly short – just five hours long, but by the time of the Jurassic period, when dinosaurs roamed the Earth, a day had lengthened to about 22.5 hours.
Of course now, a day is 24-hours and has been since humans evolved, but what has caused this gradual lengthening. The answer lies with the Moon.
The moon used to be a lot closer to the Earth and the effect of its gravity was therefore, a lot stronger. As the moon drives tidal systems, these were a lot stronger in the early days of the Earth, and the consequence was that the Earth’s spin slowed, the tugging of the moon’s gravity and tidal forces on the Earth, acing like a brake on the rotation of the planet.
Now the moon is farther away, and is continuing to move away even farther, however the effect of the moon is still felt on Earth, with a consequence that Earth’s day is still slowing down, albeit minutely.
With modern atomic clocks, it is now possible to account for this slowing and the global timescale used by most technologies to ensure time synchronisation, UTC (Coordinated Universal Time), has to account for this gradual slowing, otherwise, because of the extreme accuracy of atomic clocks, eventually day would slip into night as the Earth slowed and we didn’t adjust our clocks.
Because of this, once or twice a year, an extra second is added to the global timescale. These leap-seconds, as they are known, have been added since the 1970’s when UTC was first developed.
For many modern technologies where millisecond accuracy is required, this can cause problems. Fortunately, with NTP time servers (Network Time Protocol) these leap seconds are accounted for automatically, so any technologies hooked up to an NTP server need not worry about this discrepancy.
NTP servers are used by time sensitive technology and computer networks worldwide to ensure precise and accurate time, all the time, regardless of what the heavenly bodies are doing.