Leap Second on 30 June 2015

Network Time Foundation exists “to fix and prevent your headaches regarding Network Time”. One such headache that is sure to generate a lot of noise this year is the upcoming leap second to be introduced world-wide as the last second of June 2015. With this blog post and with software updates (see article by Harlan Stenn) and through its other activities, NTF plans to introduce a bit of signal into the noise.

Leap seconds are the mechanism that Coordinated Universal Time (UTC) currently uses to adjust the freakishly steady frequency of ticks resulting from the international network of atomic clocks to match the quirky motions of Mother Earth. The Earth is a spinning top and while its sidereal rotation rate (that is, with respect to the fixed stars) is very constant, our atomic clocks are now much more precise timepieces than our spinning planet. In short the Earth spins like a top so it has the wobblies just like a top as well.

It is important to distinguish accuracy from precision. Atomic clocks are indeed very precise, but to accurately track the rotation of the Earth, UTC requires the introduction of an occasional leap second. The length of Earth’s actual solar day currently differs by just a very few milliseconds from 86,400 SI-seconds* (86,400 = 24 hours x 60 minutes x 60 seconds). These milliseconds accumulate every year or two into a whole leap second. It is the responsibility of the International Earth Rotation and Reference Systems Service to monitor the accumulating clock differences and to announce upcoming leap seconds.

Over the long term the Earth’s rotation is also slowing as its angular momentum is transferred to the ever growing orbit of the Moon. This effect is measurable using the retro-reflectors left behind by the Apollo astronauts, amounting to about +4 cm per year. The rough equivalence is that the Earth’s day will become one SI-second longer as the Moon recedes two kilometers over the next 50,000 years. That is, in 500 centuries we would need one leap second per day (rather than one per year or two) since the day would become 86,401 SI-seconds long.

This tidal coupling between the Earth and its Moon has been going on for aeons. During the Devonian Period 350 million years ago – the “Age of the Fishes” – each day was only 22 (modern) hours long and there were 400 days in a year as confirmed by coral measurements. While the Network Time Protocol (NTP) and other precision timekeeping technologies have proven very durable, we must admit this was even before NTF opened its doors!

For some purposes the tiny modern discrepancies are negligible. For other purposes like steering spacecraft, pointing telescopes, or for applications where knowing and keeping the legally-mandated correct time is important, these corrections are quite significant. There has been an ongoing debate for several years in the International Telecommunication Union (ITU), an agency of the United Nations, about finding some other mechanism than leap seconds to accommodate the differences between solar time and atomic time. This is coming up again for a vote at the World Radiocommunication Conference (WRC-15) in Geneva, Switzerland in November 2015. NTF takes no position in this debate. Interested readers can find additional information at http://futureofutc.org.

What the NTP community needs to know is that Network Time Foundation’s NTP Project remains committed to providing reliable infrastructure and support for the network distribution of standard time signals in general, and in particular, Coordinated Universal Time — however it may be defined in the future. NTF recommends that all users of NTP and other NTF-supported software maintain updated installations of these critical packages.

Help NTF help you – join or donate now!

* “SI” is the “Système International d’Unités” or “metric system”

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