Take Your Leap Second to Think About the Weird Nature of Time

Summer is going to stretch on a little bit longer this year thanks to the addition of a leap second. But it’ll be a fleeting add-on. On June 30, 2015, clocks will move from 11:59:59 p.m. to 11:59:60 before continuing normally into midnight on July 1. Why?

“Earth’s rotation is gradually slowing down a bit, so leap seconds are a way to account for that,” NASA scientist Daniel MacMillan said in a statement.

Adding this leap second will bring the time standard set by precise measurements of Earth’s rotation (UT1) more into line with the time standard set by atomic clocks (UTC). If the two time standards aren’t coordinated, the gap between the two could widen over years until an atomic clock is reading 6 p.m. when the sun is directly overhead.

The whole necessity of the leap second is another small reminder that time as we compartmentalize it is a wholly human construction—not rooted in anything fundamental about the nature of time itself. Seconds started out as a fraction of an average day as experienced by humans in the 11th century. But the same factors that necessitate a leap second have necessitated a more precise definition. Since 1967, a second is officially defined as “the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom”—hence our ability to keep ultra-precise time with atomic clocks.

But the ultimate reality of time is even more mind-blowing, as these short videos from World Science Festival co-founder and Columbia University physicist Brian Greene reveal.