On 06.08.2015 02:06, Alexander Belopolsky wrote: > On Wed, Aug 5, 2015 at 7:18 PM, M.-A. Lemburg <[email protected]> wrote: >> and their direct influence on the UTC-TAI difference: >> >> http://hpiers.obspm.fr/eop-pc/index.php?index=leapsecond&lang=en > > Cool plot, but "due to the initial choice of the value of the second > (1/86400 mean solar day of the year 1820)" sounds like nonsense. How > did they measure the "mean solar day of the year 1820" in caesium-133 > radiation periods without sending someone back in time with an atomic > clock? I thought only Guido had a time machine!
I guess in this case, it's more a coincident than Guido lending someone his time machine :-) In the 18th and 19th century, a second was defined as 1/86400 of a average mean solar day (fraction of a tropical year). At the time, people apparently believed this to be mostly constant. Early in the 1900s, it was found that Earth's rotation is not constant enough to base a standard on it. So the definition was adapted to mean a certain fraction of a specific tropical year (rather than an average over many years), in this case 1900: the ephemeris second. However, not to the effect of making one ephemeris second a 1/86400 fraction of a mean solar day in 1900. In the late 1960s, the definition was again refined to be based on the atomic cesium clocks: the SI second was born. Comparing this definition to the definition used in the 18th and 19th century, it was then determined that one SI second corresponds to one second (using the old definition of the mean solar day, but now for specific years, rather than averages) in the year 1820. This is how we ended up with one SI second = 1/86400 mean solar day of the year 1820. Our time keepers are doing a pretty good job there, I must say :-) More details on all this are available at: http://tycho.usno.navy.mil/leapsec.html (provided their server is up) This also has a nice chart showing how the length of a day varies with time. Hmm, I wonder what traders would make of such a chart - I guess it's time to buy some LOD stocks now :-) Now, if we could only get Earth to behave and speed up it's rotation again... -- Marc-Andre Lemburg eGenix.com Professional Python Services directly from the Source (#1, Aug 06 2015) >>> Python Projects, Coaching and Consulting ... http://www.egenix.com/ >>> mxODBC Plone/Zope Database Adapter ... http://zope.egenix.com/ >>> mxODBC, mxDateTime, mxTextTools ... http://python.egenix.com/ ________________________________________________________________________ ::::: Try our mxODBC.Connect Python Database Interface for free ! :::::: eGenix.com Software, Skills and Services GmbH Pastor-Loeh-Str.48 D-40764 Langenfeld, Germany. CEO Dipl.-Math. Marc-Andre Lemburg Registered at Amtsgericht Duesseldorf: HRB 46611 http://www.egenix.com/company/contact/ _______________________________________________ Datetime-SIG mailing list [email protected] https://mail.python.org/mailman/listinfo/datetime-sig The PSF Code of Conduct applies to this mailing list: https://www.python.org/psf/codeofconduct/
