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    Re: Dropping leap seconds and the impact on celestial navigation
    From: John Huth
    Date: 2011 Sep 12, 08:05 -0400
    Frank - 

    Is there a 'ready, set, go...' moment planned for this proposal when the 'switch' is thrown?  

    Despite all the arbitrariness you've enumerated, I feel like we're switching to auto-pilot.    Now, my astronomy colleagues are quick to point out all the complexities when you push precision at the limits of our metrology, but I still have this image of the planet going internal and little atomic oscillations replace the cosmos as the great clock.  

    John

    On Sun, Sep 11, 2011 at 11:31 PM, Frank Reed <FrankReed{at}historicalatlas.com> wrote:

    Fred, you wrote:
    "The leap hour fix seems to be good enough; I could live with 29 minutes of
    discrepancy, and it might keep me politically involved trying to prod
    government to change it."

    Also, since time zones are determined by governments, these assignments can be changed whenever convenient. In other words, it's not necessary (or desirable?) to wait for some "international" leap hour. Those states and countries which are already ahead of where they should be by more than an hour might decide to move west at an earlier date. So if the total accumulated skipped leap seconds amount to as little as ten minutes, that might be enough to convince Indiana, e.g., to switch to Central Time (as parts of the state already are). When the accumulated difference amounts to 20 minutes, perhaps Michigan would decide to move to Central Time. And so on. The states in the US have to go through a process of federal approval, but that seems to be mostly a technicality. Most other countries apparently legislate this at the national level.

    You added:


    "I believe you're exaggerating with statistics comparing the suns' position at
    noon EDT to noon time in Philadelphia, PA."

    Where's the exaggeration? It's a simple fact: for 65% of the year, states from Maine to Indiana keep time that would correspond to mean solar time at 60 degrees longitude (the fraction of the year was 57.5% before 2007). On any of those 238 days out of the year, when a clock in Pennsylvania or Ohio or most of Michigan or Indiana reads 12:00 noon, the Sun is over the 60 degree meridian of longitude well out into the Atlantic (actually due to the equation of time for the period in question, it varies from about 58.5 to about 64.0 degrees longitude).

    Consider Columbus, Georgia. Most people think of Georgia as an eastern state because it's on the Atlantic coast. But as originally drawn, most of Georgia should be in the Central Time Zone. Instead it's all in the Eastern Time Zone. At 85 degrees west longitude, Columbus, GA is ten degrees of longitude or forty minutes in time away from the normal center-line of the Eastern time zone. Since it's on EDT (equivalent to AST) for 65% of the year, the Sun does not reach the meridian in Columbus, GA until 1:40 in the afternoon, on average. Clocks are "disconnected" from Sun time by 1h 40m (ranging from 1h 35m to 1h 45m during the spring and summer but decreasing to 1h 24m around November 1, and then of course for the four months when DST is not used it ranges from 24m to 39m). When the clocks in Columbus read 12:00, it is never local apparent noon on any day of the year.

    You added:


    "The proper comparison is noon EST, which goes to 75W, within a few miles of Philadelphia, PA."

    That's the proper comparison for less than 35% of the year as the rules stand today. For MOST of the year (238 days), the eastern states and eastern midwest states of the USA keep Atlantic Standard Time. We LABEL it Eastern Daylight Time for various reasons, but that changes nothing: EDT=AST.

    -FER

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