Letni a zimni cas

Jan Houstek housj0am na alma.karlov.mff.cuni.cz
Úterý Říjen 29 14:38:20 CET 2002


> 	Dle toho, co o UTC vs. GMT vim se domnivam, ze Vas popis je spravny, 
> ale zaver je presne opacny:
> 
> 1. pripad 1) je prave cas GMT
> 2. pripad 2) je prave cas UTC (a UTC se skutecne tak meri... BTW jak je 
> na tom GPS?), ktery se vsak pri prepoctech upravuje prave z pripadu 1); 
> jinymi slovy se da rici, ze UTC bez uprav je de-facto to, co kolega 
> Kankovsky oznacuje za TAI...

Tak jsem se na to podival a zjistuji, ze to co jsem psal, je spravne:

Coordinated Universal Time (UTC) is the basis for legal time worldwide and
follows TAI (see below) exactly except for an integral number of seconds,
presently 32. These leap seconds are inserted on the advice of the
International Earth Rotation Service (IERS) (http://hpiers.obspm.fr) to
ensure that, on average over the years, the Sun is overhead within 0.9
seconds of 12:00:00 UTC on the meridian of Greenwich. UTC is thus the
modern successor of Greenwich Mean Time, GMT, which was used when the unit
of time was the mean solar day.

International Atomic Time (TAI) is calculated by the BIPM from the
readings of more than 200 atomic clocks located in metrology institutes
and observatories in more than 30 countries around the world. TAI is made
available every month in the BIPM Circular T
(ftp://62.161.69.5/pub/tai/publication). We estimate that TAI does not
lose or gain with respect to an imaginary perfect clock by more than about
one tenth of a microsecond (0.0000001 second) per year.

Tedy jako dodatek k memu predchozimu prispevku lze napsat jen to, ze GMT
je skutecne stredni slunecni cas greenwichskeho poledniku (tedy zakladni
jednotkou je 1/86400 stredniho slunecniho dne), kdezto UTC je kalibrovany
cas podle TAI tak, aby se od GMT lisil maximalne o 0.9s.

GPS s nejvyssi pravdepodobnosti pouziva interne UTC nebo TAI, rozhodne ne
cas zalozeny na delce periody rotace Zeme.

-- Honza Houstek



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