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    Re: Keep checking your GPS
    From: Frank Reed
    Date: 2015 Feb 10, 12:12 -0800

    John Howard, you wrote:
    "Let us keep time somewhat tied to the sun."

    But it's centuries too late for that, as you note, in part, in your message. Normal time has not been "tied to the sun" since the era when every town set its clock by a sundial. Eliminating leap seconds would be an exceedingly minor change compared to what has come before. Consider the list of steps that has removed our time-keeping from solar time:
    1) Accurate mechanical clocks. The invention of accurate mechanical clocks capable of telling time reliably for many days immediately set up mean time as something to be preferred over solar time. The difference is the equation of time and amounts to as much as 16 minutes during the course of the year. Our "normal" time has been disconnected from "Sun time" by as much as a quarter of an hour for roughly 200 years (in some places and contexts, much sooner than others). It varies through the year. The mean (absolute) offset is on the order of 8 minutes.
    2) Standard zone times. Beginning in the 1880s, large regions began to adjust their clocks to keep the mean time of the nearest (within reason) standard meridian on the well-known 15° grid of standard longitude meridians. This disconnected "normal" time another 15-30 minutes from "Sun time" for average locations in addition to the equation of time above.
    3) Eastward zone shifts. Regions observing standard time began attaching themselves to neighboring standard time zones (almost always eastward), jumping a whole hour, for business reasons and for other social reasons (thus, for example, moving Ohio and Michigan into the Eastern Time zone). This disconnected "normal" time in some regions by as much as one hour more away from "Sun time" but more typically half an hour. Given that only some regions made this change, average it to an additional 10 minutes for any given location.
    4) Daylight time and similar "decree" or national statute time. Beginning around 1918, nations began shifting all of their time zones one zone to the east (adding an hour for DST) for a large fraction of the year of in some cases year-round (e.g. decree time in the USSR). This added yet another hour of separation from Sun time, which we can count as half an hour since it was originally used only for about half of the year and few countries adopted it year-round.
    5) Extensions of DST. Daylight time is now observed in the USA and many other countries for more than 60% of the year achieving a nearly year-round eastward movement of time zones. This extends the DST separation increasing that net offset added by DST to about 40 minutes (ten minutes on addition to the earlier amount in step 4, above).
    6) The definition of the SI (metric unit) second completely broke the connection between the most fundamental unit of our calendar and the rotation of the Earth. The second was defined slightly "wrong" compared to the Earth's current rotation requiring the steady, approximately biennial addition of leap seconds to maintain some approximate match between Earth orientation and the count of seconds. The separation between mean "Sun time" and the calendrical count of SI seconds is allowed to grow up to as much as 0.7 seconds (UTC-UT1). This defines modern UTC.
    7) If leap second insertions were terminated, the difference between UTC and UT1 would be allowed to grow over decades and would no longer be limited to 0.7 seconds. In forty years, in the year 2055, the difference would be approximately 20 to 30 seconds. The amount cannot be predicted far in advance but it is easily observed one year ahead and can be included easily in nautical almanacs.

    Let's summarize where we are now. The approximate, average numbers from above:

    • 8 minutes offset from Sun beginning time 200 years ago,
    • 15 minutes more 120 years ago from time zones,
    • 10 minutes from time zone adjustments (averaging, actual adjustments can be much more),
    • 40 minutes average from so-called "daylight time",
    • a fraction of one second from UT1 under current UTC rules,
    • if leap seconds are dropped, less than about one minute before the year 2100.

    All of these steps have long since separated us from the Sun. We do not keep time by the Sun except in a long-term average sense. It is not at at all unusual for clocks reading "normal time" to differ today by 90 minutes from local apparent solar time. We are a long, long, long way from Sun time. Dropping leap seconds would change this only by about two seconds per year in coming decades. Surely we recognize that this is nothing compared to 90 minutes, right?? This rate would increase in later centuries which is a problem that has to be considered whether we keep leap seconds or eliminate them.

    The negative navigational consequences of dropping leap seconds are trivial. Dropping leap seconds would add one small task to a celestial navigator's work: adding the adjustment (UTC-UT1) to the time before entering a nautical almanac (technically we should do this even now but very few celestial navigators have any requirement for that additional accuracy). This is nothing more than a simple "watch error" adjustment. And celestial navigators are a rare group! Meanwhile, innumerable systems in many fields of human endeavor with software governing synchronization must be designed with uncertain rules for future time. There is no way today to predict how many seconds will elapse between midnight March 1, 2015 and midnight March 1, 2025. This is an arbitrary and uncertain component of our calendar, and it is a potential source of bugs and errors that will come back to bite us if we do not fix it in advance. Dropping leap seconds is a simple and efficient solution to this problem. It does entail some expense, and there are risks in any implementation, but those risks are finite and contained. We should not exaggerate the supposed merits of a fanciful and entirely historical connection between normal time-keeping and apparent solar time. 

    As a reminder, you can read my paper on the subject of "Traditional Celestial Navigation and UTC" here (published in the Journal of the American Astronautical Society as part of the proceedings of the Exton, PA conference on leap seconds held in October, 2011; also attached). And for entertainment, you can listen to me talking around this topic generally as interviewed by "astronomy superstar" Neil deGrasse Tyson here.

    Frank Reed
    Conanicut Island USA
    PS: David Broughton has suggested that anyone who favors dropping leap seconds from our calendar system is effectively in league with "high-frequency traders". And we know that they are a sordid group! Nasty, naughty HF traders! I'll answer his post separately, covering some of this same ground, but I want to say here and now that this is, first of all, an example of a logical debate fallacy known as the "association fallacy" or "guilt by association". Second, I strongly dispute the idea itself: high-frequency traders may be one constituency who would like to see leap seconds eliminated, but they are a minor one.



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