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    Re: Dropping leap seconds and the impact on celestial navigation
    From: Frank Reed
    Date: 2011 Sep 10, 22:45 -0700

    John Huth, you asked:
    "I always think of the time standard as 'mean solar time' - placing the sun, on average, over the prime meridian at noon. Does this shift imply a slow drift away from this? I think you say this in your message, but I wanted to double check."


    Of course, we already have some HUGE differences. For most of the year, in the US everyone on Eastern Time, which is observed from Maine to Indiana, is actually keeping a time that is correct (in the "mean noon" sense) at 60 degrees west longitude. Grab a map and trace that out. It's a long way from Boston and it's a really long way from Indianapolis! On a day, for example close to April 15, when the equation of time is close to zero (so the true Sun has caught up with the mean Sun), just as the Sun is reaching the meridian way out in Louisburg, Nova Scotia (close to the eastern tip of Nova Scotia), and before it has reached the meridian in Bermuda, and also before it has reached the meridian in Martinique and Grenada, all the clocks in the Eastern Time Zone in the US strike noon. If leap seconds are dropped, these huge differences will slowly increase by a second every year or two and a little faster in future decades. If that additional difference ever becomes problematic (and it surely would at some point down the line), all we have to do is drop one "Spring Ahead" date. By skipping one jump to DST while keeping the corresponding "Fall Back" later in the year, we move our time zones one step westward. It's quite possible that by the time this would become necessary, no one would care anymore.

    And you wrote:
    "That being the case, what becomes the standard? A clock somewhere?"

    Yes, the global ensemble of atomic clocks. In other words, physics time! Even before atomic clocks, there was "ephemeris time" which was also "physics time". Basically, you apply the equations of Newtonian gravitation to the Solar System (supplemented by ralativistic corrections), and you find that the whole thing is consistent only if a certain time variable is used which does NOT match mean solar time. This time variable drifts and meanders away from mean solar time due to the non-uniform rotation of the Earth. It is the source of the "delta-T" in all ephemeris calculations. More importantly for leap seconds, the modern SI definition of the second in terms of atomic phenomena was based on a 19th century mean. Leap seconds primarily correct for the difference between the current rate of rotation of the Earth and its rate as established by Newcomb and other astronomers about 120 years ago.

    You asked:
    "What happens to the definition of right ascension - does it remain the same?"

    Good question. Paul Hirose in a recent post mentioned the ICRS. This is a non-earth-based coordinate standard that has been adopted recently. Catalogues of stellar right ascensions and declinations used to be referred to a particular epoch: 1900.0, 1925.0, 1950.0, 2000.0 (1975 was skipped). But these should now fade into history. It implies somewhat more computation to get an apparent position, but computation is now dirt cheap. Of course, there will still be apparent right ascension and apparent declination.

    You added:
    "I guess I have a fairly deep philosophical issue - if the historical basis of 'time' was a synchronization to celestial events, then I have very serious reservations about severing this bond."

    That bond is long gone! :) As soon as mechanical clocks reached a level of quality where they did not need to be reset every day or every week due to their own internal mechanical foibles, we have been living on a time that is no longer tied to the Sun. At first clocks were "corrected" by tables of the equation of time so that they would yield proper Sun time. As recently as the 19th century, local apparent time was still called "true time" to contrast it with the time of machines --mean time. As the decades passed, machine time (mean time) became the preferred time and sundials spouted equation of time tables to "correct" them. That difference between mean and apparent time amounted to less than twenty minutes. With the introduction of mean time zones in the late 19th century, even the average time during the year was no longer matched with the Sun's position, and this added up to thirty minutes more separation from the celestial bond. In the past century, time zones have pushed east globally (that is, regions would tend to legislate themselves onto a time zone east of where they "should be") and DST/Summer Time have moved whole countries east for most of the year. And the discrepancy between celestial time and clock time has now reached several hours in many parts of the world. As I noted above, in the US we keep our clocks from Maine to Indiana set as if we all lived far out in the middle of the Atlantic Ocean. And most people hardly notice at all.

    You concluded:
    "What's the motivation for the shift?"

    The primary issue if the synchronization of the computer networks that control and influence almost every facet of our lives. UTC is critical to the global economy. It's intriguing that GPS signals are more important economically, not for latitude and longitude, but because they provide exceedingly accurate time which is used to synchronize everything from cell phone towers to the Internet generally? Every GPS receiver is an "atomic clock repeater". But in order for all this to work, these systems need to be informed of the schedule (and history) of leap seconds added to the calendar. These are arbitrary and unpredictable because the rotating Earth is, quite simply, not a good clock. While nothing has yet gone wrong due to a leap second error (that I am aware of), the risk is real. Imagine a computer trading system (where pricing data are compared at the millisecond level) getting information from one exchange tagged one or two seconds out of synchronization with another exchange. Smart software would probably catch the error. Buggy software would generate billions of dollars of trades trying to catch the "obvious" arbitrage opportunity. If we have a "flash crash" that costs hundreds of billions of dollars caused by leap second issues, can we excuse it by saying "we like to keep noon where it is for the sake of astronomical tradition"? There are other solutions to these issues. Dropping leap seconds is not the only way forward. But it may be a good, cheap piece of the puzzle.


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