On Jan 20, 2014, at 10:43 AM, Warner Losh <i...@bsdimp.com> wrote: > [*] An interesting side note about days: The ancient Egyptians regarded light > and night as two separate realms rather than as being two halves of the same > day. The notion of the synodic day thus dates only from the new kingdom,
While one could perhaps define a notion somewhat like a synodic period for the Ptolemaic system of deferent and epicycles (or perhaps one could not), the understanding of the wide variety of such periods in the solar system had to wait for Copernicus to put the Sun at its center. Ptolemy was 1500 years after the New Kingdom, and Copernicus about 1500 years after Ptolemy. That said, whether humans had any notion or not, day on Earth has always been the synodic day. The Earth rotates, and as it revolves around the Sun one rotation per year is unwrapped. > somewhere around 1400BC where the two different realms, each governed by 12 > starts was replaced by a system that unified them under 24 stars > (http://www.scientificamerican.com/article/experts-time-division-days-hours-minutes/). It would be reasonable to say that we still treat night and day as two different realms, hence the idiom “as different as night and day”. In that case the synodic cadence applies to each separately. > It wasn't until the Greeks around 100BC that we see the hours becoming fixed > length (although average people used seasonally varying hours for hundreds of > years after this: not until mechanical clocks of the 14th century did this > stop). From the 16th century until 1972, though, minutes and seconds were > uniform set by the fixed hour. So although sexagesimal notation for time was > invented thousands of years ago around the birth of Christ, it wasn't until > really good mechanical clocks of the 16th century that one could say that the > radix was truly fixed. Which leads me to why I equivocated. Hours can be fixed length or unequal, day can be deemed to start at sunrise, sunset, noon or midnight, the representation of time can be sexagesimal or a count since an epoch, static time zone offsets can be applied, periodic DST corrections made, eccentricity and obliquity can overlay the Sun’s apparent place in the sky with a charming analemma, and geophysics and Lunar tides can make it all wobble. And underneath it all the day is a synodic day. Deploying a civil time system that says otherwise would be a fundamental error of engineering. Leap seconds are a means to an end. Rob _______________________________________________ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
