Leap Second Time Comes Again

Add a second at midnight, because the Earth is slowing down.

Leap seconds are added at the end of the day on June 30 or December 31, depending on necessity.  The reason the occasional leap second is needed is that the earth’s rotation is slowing down.  In 1820, the length of the day was defined as 86400 seconds.  Since then, the earth’s rotation has slowed by two milliseconds a day, so, for example, the last leap second was added in 2012.  Those two milliseconds a day do add up; five hundred days and you’ve lost a whole second.

So, as expected, the earth’s rotation is slowing down, and has slowed down, since the earth was created, some four or four and a half billion years ago.  To be precise, the United States Geologic Survey says that the Earth and the solar system were created 4.54 billion years ago.

Here is an official-looking website that tells what will happen at midnight tonight.