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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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