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    Re: Leap seconds
    From: Frank Reed
    Date: 2009 Jan 11, 18:31 -0800

    Greg, you wrote:
    "Are you reading this? Because this is EXACTLY what I was referring to earlier 
    (i.e. that whatever time scale we use as navigators is totally irrelevant - 
    as long as we can correlate it to the time in an almanac, or whatever is used 
    to obtain the date/time for a celestial event)."
    
    You and George were talking at cross-purposes here. I don't think George 
    objected to the point you're making above. He was picking at one small issue: 
    namely your comment that we no longer keep time astronomically. I think 
    you're saying this because UTC is allowed to fluctuate from mean solar time 
    by up to 0.9 seconds. Myself, I would have to say, "yes and no" on this 
    point. Clearly since it does fluctuate by that amount, and we know that it is 
    "wrong" by that amount, then we are not exactly keeping astronomical time 
    anymore. It's those banks of atomic clocks casting their ballots that 
    determine the exact time to the fraction of a second. But at the same time, 
    it's the deviation from astronomical time that is used to correct UTC so in 
    that sense the Earth's rotation casts the final ballot. 
    
    But the point which you considered your primary point, noted again above, is 
    certainly correct. Any time scale that we can calculate by a known algorithm 
    from UTC will serve our needs. Indeed, it would be easy today to construct 
    watches that keep, for example, Greenwich Apparent Time. In the days of 
    mechanical clocks, this was considered very difficult, and rather pointless, 
    but it's trivial in software, so why not? Then we could go back to publishing 
    the almanacs as they were before 1834. You would read the GAT off your watch 
    and enter the almanac for that time to take out your ephemeris data. 
    Functionally the same as today. This would let us drop one column of data 
    since the GHA of the Sun is then equal to the GAT (converted from hours and 
    minutes of time to degrees and minutes of arc... in other words, at 0300 GAT, 
    the GHA of the Sun is 45 degrees exactly). So should we do this? Almost 
    certainly not because of the "old book effect". We want to keep the vast 
    storehouse of information on navigation published within the past fifty years 
    relevant and useful, especially since very little is being published new 
    today. So if they DO eventually drop leap seconds, rather than using UTC 
    (which would differ from GMT at a rate of about 0.5 seconds per year), it 
    would probably make good sense to publish an annual "DT" correction to add on 
    to UTC as broadcast. Then enter the tables published for GMT as usual. The 
    alternative would be to do the calculations in UTC without leap seconds, 
    which is easier in some way, and then you would have to adjust GHA values by 
    DT (converted to arc) and also some of the other planning data like sunrise 
    times would eventually have to be adjusted.
    
    -FER
    PS: In your second paragraph, you wrote: "I still don't know if you were being troll-ish earlier"
    
    I think it's best to avoid net-isms like "trolling" (I never liked that one 
    anyway). This community is very much Internet-lite. Many people here would 
    not have the slightest idea that trolling refers to a particular type of 
    posting game, and many probably don't even know that such a thing exists. 
    Additionally, since there are many non-native English speakers, and many 
    others who speak English natively but from somewhat different cultures, like 
    those crazy upside-down Australians (kidding!), it's always a good idea to 
    use language that is culturally neutral, even to the point of being dull. 
    That's the best way to avoid accidentally offending people.
    
    
    
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