Tuesday will be longer than Monday-- by one second. The world's timekeepers will be making up for infinitesimal variations in the length of the day but adding a second to clocks. Normally there are 86,400 seconds between one midnight and the next. That's the official length of a day — but the average length of a day is actually 2 milliseconds longer than that: 86,400.002 seconds. This slight inconsistency is a result of the way oceans move in response to the competing gravitational forces of Earth, the moon and the sun, continually slowing the Earth's spin. The round number, tracked by an atomic clock and available for everyone to synchronize with, is called Coordinated Universal Time, or UTC. The real-world measurement is called UT1. And while ignoring a couple extra milliseconds won't make you late for work, they do add up over time. Leap second is a way to account for that.
