Science of a Second

The old definition of a second was based on the rotation of the Earth. As it takes the Sun one day to rise in the east, set in the west and rise again, a day was almost arbitrarily divided into 24 hours, the hour into 60 minutes, and the minute into 60 seconds. However, the Earth doesn’t rotate uniformly. In fact, it’s rotation decreases at a rate of about 20 millionths of a second every calendar year due to tidal friction caused by the Moon. Atomic time relies on the energy transition with ...