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    Re: Dropping leap seconds and the impact on celestial navigation
    From: UNK
    Date: 2011 Sep 12, 13:58 +0000
    On 2011-09-10 16:24, Fred Hebard wrote:
    ... I wonder what will happen 200 years from now.  Will noon be 11:30 am?  That would be unacceptable to me.


    If all of a sudden the earth started to speed up so much that 1800 negative (!) leap seconds would be required within the next 200 years to keep UTC in step with earth rotation, mankind would face more serious problems than the philosophical question of the "proper" alignment of a measurement scale with natural phenomena. In reality, the drift between uniform time and earth rotation will not be nearly as drastic (and in all likelihood in the opposite direction).

    Nevertheless, the question that I believe you meant to ask expresses a valid concern: The "day" will officially be dis-coupled from the course of the sun even more than it already is. Dictionaries will have to add a new meaning to the entry for this word. For astronomers, that has long happened: As far as duration is concerned, a "day" just stands for 86400 SI seconds. The connection with earth rotation is merely historical.
    The only thing that's new is that the civil day will no longer supposedly be centered approximately on the meridian passage of the mean sun. (The ephemeris day never was.) That's a good thing, because it removes a small inconsistency. An absolutely exact alignment is strictly incompatible with the postulated duration. Does the absence of alignment with the solar day pose a practical problem beyond the linguistic one? I don't think so. That the beginning and length of a "day" should depend on an atomic clock is no more unnatural than that a "foot" should be defined as exactly 0.3048 m. You can still use your own foot as a make-shift measuring device for common purposes that don't require high accuracy. Similarly, a popular usage of the words "day", "morning", "noon", "midnight", etc. will evolve that best reflects common needs and can be translated to some more formal concept where and when required.

    Looking closely at what you actually wrote, I am not sure whether that was with tongue in cheek or whether your finger slipped. I find it amusing either way. The answer to your question is: No, noon will never occur ante meridiem, not even in the new system! Clearly, the antiquated a.m. / p.m. terminology has no place in a timescale that does not track the sun. It would immediately lead to confusion in the first year that would have required a leap second under the old system. I consider this a bonus: Regardless of leap seconds or not, i
    t is high time that formal communication (such as railway schedules, airplane tickets or posted observatory operating hours (!) ) gets rid of the clumsy 2x12 hour notation. Europe has done so a long time ago.

    Herbert Prinz

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