Wednesday, January 4, 2017 [Tweets] [Favorites]

The Leap Second

Jack Forster:

UTC used to be based on the rotation of the Earth around its axis, as observed at Greenwich. Once upon a time – a simpler, happier time – the second was exactly 1/86,400 of a day. By the mid-1950s, however, clocks had gotten accurate enough that we’d figured out that the Earth’s rotation on its own axis was irregular, so in 1952, the International Union Of Astronomers decided to define the second as a fraction of one orbit of the Earth around the Sun: a second would now be 1/31,556,925.9747 of a tropical year.

However, the year turned out to have the same basic problem as the day; it’s irregular, changing slightly in length from one year to the next. (This is different, by the way, from the problem that requires the insertion of an extra day in a Leap Year; the Leap Year is inserted to keep the Gregorian Calendar in sync with the seasons, but the reason for the Leap Year, is that there isn’t a whole number of days in a year, not that an astronomical year varies slightly in length from one year to the next.)


As it turns out, atomic clocks are much more stable than the Earth’s rotation around its axis, or its orbit around the Sun, and it soon became clear that while an atomic clock-based time standard (UTC) was great to have, it meant that there was going to be a cumulative difference between UTC, and observed mean solar time. While both the astronomical day, and year, are irregular, the day overall has been getting slightly longer for at least the last few centuries. To keep UTC and mean solar time in sync, a Leap Second is occasionally added to UTC.

Via Nick Heer:

Accurate time is also essential for things like HTTPS certificates and, apparently, Cloudfare’s CDN services.

Previously: Intercalation.

1 Comment

This post brought back some fond memories of my first job after finishing university. I was hired as a communications engineer at the European Space Agency, and my first assignment was participation in a deep-space tracking campaign of the Ulysses satellite, as it made a sling-shot trip around the sun. At the time, ESA had never had the opportunity to operationally test their deep-space tracking system, and the idea arose to track Ulysses jointly with NASA to compare our results to theirs.

Also at the time, my department was doing work in the area of timing systems, and were working the Swiss observatory in Neuchatel in the development of an alternative atomic clock standard, and so we decided to transport their experimental clock to the French Guyana, where the tracking campaign would happen, to compare its performance against the cesium-based clocks the ESA ground stations were using at the time.

As the fresh-out-of-university guy who knew little, one of the tasks I was assigned was physically getting the clock (which is quite large) from Switzerland to South America. And I made the terrible mistake, when contacting AirFrance, to say we were trying to transport an "atomic clock". They thought it was a nuclear device! Things went smoother when I called back, and just said we needed to transport a "clock".

The major challenge of transporting a device like that, is that it must remained powered on at all times. Had the device lost power in the trip, we couldn't have used it, because it requires a full month to stabilize after start-up.

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