How do we sync “solar time” with “true atomic time” and why does it need adjusting? 1. Leap seconds
2. The earth’s rotation is slowing.
From the BBC:
Tidal friction is causing the Earth’s rotation to slow down. This means that everyday timekeeping, based on the rotation of the Earth (solar time), tends to drift out of sync with respect to the time kept by atomic clocks.
For this reason, the standard for everyday timekeeping, known as Coordinated Universal Time, or UTC, must be adjusted every so often.
This takes the form of individual seconds being added or subtracted from the length of a day, either a 30 June or a 31 December. The first leap second in seven years will be added at the end of 2005.
This is a fascinating article. Enjoy