Space Timekeeping: NASA's SDO Adds Leap Second to Master Clock

As The TeCake previously reported that time keepers will add a ‘leap second” on December 31, 2016 at 23 hours, 59 minutes and 59 seconds Coordinated Universal Time (UTC), which is calculated in Paris, France, at the International Bureau of Weights and Measures. Following the time keeper, the US space agency NASA has also an extra second in the Solar Dynamics Observatory (SDO) that constantly observes the Sun.

Clocks do this to keep in sync with Earth’s rotation, which gradually slows down over time. When the dinosaurs roamed Earth, for example, 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 1.9 miles every second,” said Dean Pesnell, the project scientist for SDO at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “So does every other object in orbit near SDO. 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.”

The leap second is also key to making sure that SDO is in sync with the Coordinated Universal Time, or 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.

Conventionally, time is considered as based on the mean rotation of Earth, which was defined in this reference frame and the second is related to celestial bodies. However, for a precise reporting, atomic clocks are defined that offers much more precise ‘atomic’ timescale, which is independent of earth rotation.

The computations and measurements show that Earth runs slow at about 1.5 to 2 milliseconds per day as compared to atomic time. It further suggests that the difference between atomic time and Earth rotation time would be about one second after around 500 to 750 days. The difference is calculated and observed by the International Earth Rotation and Reference Systems Service (IERS).

The organisation was established in 1972 and since then it has 27 seconds in last 44 years. It is not much of a difference for normal people but it is huge in scientific terms where very precise measurements are required.

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