NASA add leap second to master clock

NASA missions, including the Solar Dynamics Observatory (SDO) which watches the Sun, will join timekeepers around the world to add a leap second to its clocks.

Clocks will add the extra second to keep in sync with Earth's rotation, which gradually slows down over time.

When the dinosaurs roamed Earth our globe took only 23 hours to make a complete rotation. In space, millisecond accuracy is crucial to understanding how satellites orbit.

"SDO moves about 3 kilometres every second. So does every other object in orbit near SDO," said Dean Pesnell, project scientist for SDO at NASA's Goddard Space Flight Centre in the US.

"We all have to use the same time to make sure our collision avoidance programs are accurate. So we all add a leap second to the end of 2016, delaying 2017 by one second," said Pesnell.

The leap second is also key to making sure that SDO is in sync with the Coordinated Universal Time (UTC) used to label each of its images.

SDO has a clock that counts the number of seconds since the beginning of the mission. To convert that count to UTC requires knowing just how many leap seconds have been added to Earth-bound clocks since the mission started.

When the spacecraft wants to provide a time in UTC, it calls a software module that takes into consideration both the mission's second count and the number of leap seconds - and then returns a time in UTC.