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Another cause of starting the day at noon among astronomers was probably the
unreliability of pendulum clocks at the end of 17th century and later.
Astronomers used to ascertain the error of their clock every day then, if
possible. The easiest way for it was the moment of Sun's upper meridian
transit. Having determined the clock error at noon, astronomers were ready
for night observations. And from 1679, when the first astronomical and
nautical yearbook,
"Connaissance des temps", emerged, the noon became the main point of the day
in ephemerids for a long time . Up to the 20th century, some data were given
for the apparent noon, not for the mean one or midnight in nautical and
astronomical almanacs.

When the change in beginning the astronomical day arrived in 1925, some mess
sprung up in the nautical astronomy. The German formed another term beside
the "hour angle" - so called "time angle" reckoned from the lower part of
meridian from 0 to 24 hours, so that they could cope with the new begin of
astronomical day. It was quite logical and quite abstruse. The British, in
their well-known reverence for men of sea, have then introduced two new
quantities into the Nautical Almanac instead of former "Right Ascension of
the Mean Sun at Noon" and "Equation of Time" - nameless values R and E, by
whose
help the seadogs could ignore the shift of the day's outset in their sight
reductions and continue as previously. That arrangement survived up to the
WW II, when nautical and air almanacs gradually began to tabulate the hour
angles of celestial bodies for each hour / ten minutes directly.

The last remnant of the old start of the astronomical day at noon is the era
of the "Julian days", used for counting long intervals into the past and
future in astronomy. These Julian days begin at noon up to now, because
trying to suppress or shift half a day in their reckoning would destroy the
order of heaven.


Jan Kalivoda

   

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