[GreenKeys] Leap Second

[Craig Sawyers]

> Leap Second inserted at
> 23:59:59 UTC on the 31st December, due to the earth slowly slowing down.

Well, no - the earth is not slowing down. On long term average the rotation speed is remarkably
constant (slowing by less than 2 milliseconds per century). But depending on the detailed mechanics
of how the earth's core behaves (the core-mantle boundary), and the relative positions of the moon
and planets, leap seconds are either added *or subtracted* from time to time, every few years, in
order to align a celestial measurement of rotation speed with UTC.

On shorter averaging time, the earth's speed varies by +/-4 milliseconds.