The recent question Why are leap seconds needed so often? pulled up some interesting details about the definition of the second, and I'd like to have some of them confirmed explicitly.
I'm specifically concerned with the definition of the second between 1956 and 1968, which was linked directly to the astronomical ephemeris time via the definition
the fraction 1⁄31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time.
(see also the Wikipedia section on the history of the second). More specifically, I'd like to have a clear record on this:
- why was the specific fraction 1⁄31,556,925.9747 chosen, and what does it represent?
Metrologically speaking, the reference year was chosen as 1900, but that does not by itself mean that that definition of the second is explicitly fixing the length of that year in seconds: it's definitely saying that some given year lasted exactly 31,556,925.9747 seconds, by definition of the latter, and my question is what year was that? Did the definition fix that to the year 1900? Or to the year 1956, when it was chosen? Or, as was claimed in the previous question to some ill-defined year in the early 19th century?
I would really like answers to provide suitable references, both to modern reviews and to contemporary documents, that support their claims.
And also, while we're here: why as the year 1900 chosen as the metrological golden standard? Why not 1950, say, or 1956? Which scientific and sociological factors went into that choice, and in what proportion?