[tz] 1911 France change from Paris to GMT

Steve Allen sla at ucolick.org
Sat Jun 13 21:37:00 UTC 2020

On Sat 2020-06-13T11:44:19-0700 Paul Eggert hath writ:
> “Art. 2. - Pendant la période s’étendant de la nuit du 10 mars au 11 mars
> (minuit), à la nuit du 30 juin au 1er juillet 1911 (minuit), l’indication de
> l’heure transmise aux navires en mer, par les stations côtiéres
> radiotélègraphiques ouveries au service public, sera suivie de la mention : «
> Heure de l’Europe occidentale ».”

This means that the clocks used at the coastal radio transmitters were
reset on the night of March 10/11, and that the signals were broadcast
using the new time starting that night, and that the Morse code
transmissions were made more complicated than before by keying an
alphabetic string after the time pulses.  Likely the transmitted
string was just the abbreviated initials.

At this point radio transmitters were spark gaps which emitted
broadband noise.  If more than one was in use then ionospheric
propagation could switch reception between one and another.  Compare
with the sinking of the Titanic in 1912 when there were so many ship
and land transmitters in operation that the ionosphere mixed strings
of characters from them to produce news reports that the Virginian was
towing Titanic.  See the size of the US Navy gear of that era at
which was used in 1914 to determine the longitude difference
between Paris and Washington.

> We have more detail to help us resolve this ambiguity, in the next page of the
> cited source, which says “Jusqu’a la nuit du 30 juin 1911 exclusivement, aucune
> modification ne sera faite à l’instant de l’envoi des signaux horaires par
> l’Observatoire de Paris. [Until the night of June 30, 1911 exclusively, no
> modification will be made at the time of the sending of time signals by the
> Paris Observatory.]” That is, through June radio time signals were still sent at
> the same time intervals as before, with the labels altered to be new time. In
> particular, signals were sent daily “La nuit à minuit 0 m. 0 s., minuit 2 m. 0
> s., minuit 4 m. 0 s. de temps moyen de Paris [At 00:00:00, 00:02:00, 00:04:00
> Paris Mean Time]” with labels “11 h. 50 m. 39 s., 11 h. 52 m. 39 s., 11 h. 54 m.
> 39 s. [23:50:59, 23:52:39, 23:54:39]”.

This means that the paper clock at Observatoire de Paris (which was
constructed by choosing one from all the physical clocks in the
basement and the astronomical observations of stars) was not reset
until July 1.  So during the interval from March 11 through June 30
the observatory continued to send Paris Mean Time and it was incumbent
on the staff at the radio transmitters to do math to verify that their
local clock was offset by 9:21 from the observatory clock.  Until the
WW2 German occupation in 1940 May the Observatoire de Paris clock was
conveyed to the transmitter at Tour Eiffel by a dedicated telephone
line.  So this likely means that the time signals from Tour Eiffel
stayed on Paris Mean Time through June 30, whereas the coastal
transmitters changed on March 11.

> We want to know what happened during the change around 1911-03-11 00:00. For
> example, were the radio signals sent at 1911-03-11 00:04 old time labeled
> “00:04:00 Paris Mean Time” or were they labeled “23:54:39 Western European
> Time”? The 1911-03-11 change was supposed to occur from “de la nuit du 10 mars
> au 11 mars (minuit)”, which suggests that it was intended to affect the
> broadcasts of 00:02 and 00:04 old time as they occurred after midnight old time.
> And it would have been odd for the broadcast of 00:00 old time to have been
> treated differently from the other two. So this suggests that for the
> radiotelegraph, the transition occurred at 00:00 old time.

Not really any of the above.  In that era the time broadcasts were not
continuous.  Each station broadcast time signals during a few brief
intervals each day.  Other intervals during the day either had no
broadcasts (powering a 5 kW transmitter was not always feasible) or
they were broadcasting other information such as commercial messages.
If everyone had transmitted at once with spark gaps they would
routinely be jamming each other, and more important messages, when the
ionosphere shifted.  Bulletin Horaire has a fun note saying that the
Saigon transmitter stopped sending time signals on 1941 December 8
because of more exigent radio traffic.  The intermittent nature of
time signals persisted into the 1960s where German DCF77 broadcast old
UTC (based on UT2) during some hours, SAT (stepped atomic time, based
on PTB cesium which was a contributor to what later became TAI) during
other hours, and nothing at all during other hours.

In any issue of Bulletin Horaire it can be seen which stations were
broadcasting at which hours.  As the earliest example from 1921
page 8 shows Tour Eiffel demi-automatic signals at 10:45 and 22:45.
page 11 shows rhythmic signals from Tour Eiffel a 10:00 and
from Lyon at 08:00

The "automatic" signal format was decided by the original International
Time Conference in 1912 which chartered the existence of BIH.
The "rhythmic" signal format was a different presentation also known as "vernier"
in common use from the 1920s until 1962.
Evolution of rhythmic is depicted on pages 320/321 of

So the answer is that because no station was broadcasting
continuously, there was no ambiguity about whether they were
broadcasting before or after the 1911 March 10/11 boundary.
Anyone listening to Tour Eiffel was getting old Paris mean time.
In any case, any of these radio broadcasts were irrelevant except to a
few trained radio operators.  The behavior of the railway clocks was
the most obvious official indicator, and a lot of the population would
probably have relied on whatever their local church bells did.

Steve Allen                    <sla at ucolick.org>              WGS-84 (GPS)
UCO/Lick Observatory--ISB 260  Natural Sciences II, Room 165  Lat  +36.99855
1156 High Street               Voice: +1 831 459 3046         Lng -122.06015
Santa Cruz, CA 95064           https://www.ucolick.org/~sla/  Hgt +250 m

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