# [Datetime-SIG] Fwd: Calendar vs timespan calculations...

M.-A. Lemburg mal at egenix.com
Thu Aug 6 11:46:01 CEST 2015

```On 06.08.2015 02:06, Alexander Belopolsky wrote:
> On Wed, Aug 5, 2015 at 7:18 PM, M.-A. Lemburg <mal at egenix.com> 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

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