source: How long is a day?⌚ | How long is a day?⌚ | By StarTalk | Facebook
Why
- Earth rotate faster than the Moon orbits Earth.
- The moon causing tides.
- ⇒ The moon pulls the tides against Earth rotation.
- ⇒ The sloshing tides on the ocean floor slow down the rotation of the Earth.
Hence
- We monitor how much time we lost.
- Then when needed, throw in a leap second on 30/6 or 31/12.
- Since early 70s, there has been 23-24 seconds added.