The Time Lords and the Leap Second

images-1My previous partner used to call me “The Time Lord” (taken from Dr. Who). Because I was a stickler for punctuality. When I was an administrative law judge, and a hearing was scheduled for 10:00, it started at 10:00 – not 10:01. (Except once or twice when I overslept.)

As you may know, a “day” is from one sunrise to another; the year has 365 days, the time it takes for the Earth to circle the Sun. Except that it actually takes 365-1/4 days. unknown-2So we have leap years. Except that it doesn’t take exactly 365-1/4 days either. So we omit the extra leap day once every 100 years. Except for every fourth century, when we don’t. This keeps things just about right.

Our “hour” is based on dividing by 24 the planet’s rotation time. The hour, minute, and second, are as long as they are simply so that 60x60x24 equals one day, with no need for any fudge factor, like with leap years. However, here too there’s a wee problem. The rotation is slowing! It actually now takes a teensy bit more than 24 hours. The discrepancy wasn’t noticed until we started measuring time with super-accurate atomic clocks.

The world actually does have Time Lords. You’ve heard of “Greenwich mean time?” That refers to all clocks being set by reference to a master clock in Greenwich, England. images-2This system’s superintendents are the Time Lords (so to speak). It’s one o’clock when they say it’s one o’clock. And to keep time absolutely accurate, since 1972 they’ve inserted, every other year or so, an extra second into the year, based on their calibration of the Earth’s current rotation time.*

A one-second adjustment might seem like no big deal. But whereas, in past epochs, people were content merely to tell time roughly by hours, lacking timepieces capable of greater accuracy, today’s world runs on global time synchronicity down to the millisecond. And it’s actually important that the exact time in New York matches the exact time in Tokyo.

For example (as Michael Lewis’s book Flash Boys, about high speed trading, illuminated), it’s crucial for financial transactions that the sequence of events – purchase orders and their execution – occur unambiguously. The extra leap second throws a monkey wrench into this. It might be no problem if, when the Greenwich Time Lords insert the leap second, all clocks and computers and time-incorporating mechanisms throughout the world automatically adjust. But of course they don’t.

There have been global gabfests trying to straighten this out. A lot of people don’t like it that some self-important British nerds get to decide what time it is, and to change it on whim. But not surprisingly the Brits are extremely reluctant to let go of this vestige of the epoch when they really did rule the world.

images-4So far, no resolution has been achieved. For a Time Lord like me, it’s terrifying to think that when my watch says it’s 10:00, it may actually be 10:00:01.


* Without such adjustment, the discrepancy would cumulate, and in around 20,000 years, noon and midnight would be switched.

2 Responses to “The Time Lords and the Leap Second”

  1. Jon Radel Says:

    “Our “hour” is based on dividing by 24 the planet’s rotation time.” Well, only if the time lords confuse the planet’s rotation time, sidereal time, with mean solar time, which would be the average noon to noon time (though that definition isn’t precise enough so you actually measure the location of distant radio sources, etc., in the sky and convert). The discrepancy is roughly 3 minutes 57 seconds, give or take, and dwarfs those pesky little leap seconds. The mean solar time is the one that is 24 hours long, give or take a smidgen and which second we’re talking about today.

    Also it’s best not to use the term Greenwich Mean Time in the fashion you do, as that’s the descendent of mean solar time and the leap seconds are applied to UTC (Coordinated Universal Time) in order to keep it from drifting too far from GMT, or more properly UT1. In other words, and these words are frequently used loosely, GMT is based on where the sun is in the sky, and UTC is based on the SI definition of a second maintained on a whole bunch of atomic clocks. Except when somebody uses GMT to mean civil time at a time zone offset of zero (which is a proper usage in Britain), which is why you’re better off talking about UT1 once you start worrying about the leap seconds…

    UTC is the one largely used for civil time, though I believe there are still exceptions. Civil time is, of course, the only one that normal people really care about.

    International Earth Rotation and Reference Systems Service, which is the governing body for leap seconds, is not a British organization. Though since there is a pretty precise process for deciding when leap seconds should be applied as part of the definition of UTC, I doubt there’s much suspense about the matter among those who keep track of time.

    And if you really want your head to hurt, worry about TAI (International Atomic Time) and GPS time, which are synchronized with UTC but both are offset by a different number of leap seconds from UTC.

    As for the nice people at Greenwich? Their biggest contribution these days to time is that they’re time zone 0 and that by this point everyone defines their local civil time as some reasonable offset from the time at Greenwich. Even the North Koreans, when they changed their time zone by 30 minutes in 2015, kept the difference to a neat fraction of an hour.

  2. rationaloptimist Says:

    Thanks for your clarifications. I based my post on an article in The Economist’s magazine “1843.”

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