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    D.H. Sadler on Ephemeris Time, 1954
    From: Paul Hirose
    Date: 2018 Mar 16, 11:23 -0700

    The message below was recently posted to the LEAPSECS list. D.H. Sadler
    headed the Nautical Almanac Office 1936 - 1970. In the summer of 2016 I
    wrote about the accuracy improvement possible with his alternate
    algorithm for the HO 211 sight reduction tables.
    
    Sadler's "A Personal History of H. M. Nautical Almanac Office" is online
    at the HMNAO site:
    
    http://astro.ukho.gov.uk/nao/history/dhs_gaw/home_copyright.html
    
    In his 1954 monograph Sadler wrote about the approaching adoption of
    Ephemeris Time (in 1960) as the basis for astronomical almanacs:
    
    "A separate ephemeris of the Moon, in terms of U.T., is essential for
    navigational purposes; the accuracy required, 0′.1, is such that it can
    be calculated, with reasonable certainty, several years in advance.
    Times of moonrise and moonset (in U.T.) will doubtless be based on this
    ephemeris, though the precision required required is not so high. The
    phases of the Moon, although strictly in E.T., can be regarded as in
    U.T. Other practical requirements for navigation and surveying, for
    example the ephemerides of the Sun and planets, are less sensitive and
    no difficulties arise in providing for them in terms of U.T."
    
    "Values of ΔT will in future be available, to sufficient accuracy for
    most purposes, not more than one year in arrear; these will be based
    upon the observations of the Moon's position with the special dual-rate
    Moon cameras at the U.S. Naval Observatory and elsewhere. It is not
    possible to collect, reduce and discuss the occultation observations
    within less than two years; but these observations will still be of value."
    
    "The second may now be defined as the 86 400th part of the ephemeris
    mean day, which is 1/ 365.25636042 of the sidereal year at epoch 1900.0,
    or 1/ 365.24219878 of the tropical year at epoch 1900.0. As can be seen
    from the table, it is possible, in arrear, to connect the ephemeris mean
    day directly with the mean solar day, and thus to connect the ephemeris
    second with the mean solar second (corrected for short-period
    variations). This standard may be used to increase the absolute
    precision of physical measurements; the period of the Earth's rotation
    can vary by nearly one part in 10^7, while frequencies may be compared
    to about 100 times this precision. Standards of frequency of great
    precision are being developed, but it is unlikely that they can compete
    with the long-term permanence of the standard set by ephemeris time."
    
    
    -------- Forwarded Message --------
    Subject: [LEAPSECS] D.H. Sadler in 1954
    Date: Tue, 13 Mar 2018 21:16:51 -0700
    From: Steve Allen 
    Reply-To: Leap Second Discussion List 
    To: Leap Second Discussion List 
    
    In 1954 D.H. Sadler produced a monograph on the changes in time
    that had been resolved at the 1952 IAU General Assembly.
    His writeup is clearer than almost anything else for the next 60 years.
    It was published in Occasional Notices of the RAS, and it has been hard
    to find until now.
    https://www.ucolick.org/~sla/leapsecs/twokindsoftime.html
    This is one of the series of documents produced starting in 1948 and
    proceeding through the next 20 years where astronomers explained that
    two kinds of time would be needed to satisfy all applications.
    

       
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