Geoscience Reference
In-Depth Information
Chapter 8
The Greenwich and Paris Meridians
in the Space Age
As the viewing conditions worsened in London, meridian observations at the observatory
in Greenwich ceased and the instruments relocated to the country village of
Herstmonceux in Sussex after the Second World War. The Greenwich meridian did
not pass through Herstmonceux and observations made there of stars passing
overhead were corrected to the Greenwich Meridian as if made at Greenwich. Even
these telescopes were progressively shut down during the second half of the twentieth
century as the observing conditions deteriorated even in the countryside in the
United Kingdom and better conditions were found on distant, foreign mountains.
The Greenwich Meridian was maintained only as a fictitious artifact deduced from
telescopes all over the world.
The calculations by which the telescopic observations were combined were carried
out at the Bureau International de l'Heure (BIH), established in 1920 at the Paris
Observatory as the result of an agreement reached by an international conference
in 1912. The Bureau also has the responsibility to combine radio transmissions of
time signals into an agreed “exact time” (
l'heure definitive
) which now goes under
the name of Universal Time (Coordinated), or UTC. It is an approximation to the
time that would be kept by a clock stationed at the Greenwich Meridian. The rivalry
in this topic between France and England has persisted into the twentieth century
and some English people complained that Greenwich Time and the Greenwich
Meridian had ceased to be a fundamental reality located in Britain and had become
an abstraction maintained by the French.
Although these developments reduced the reality of the Greenwich Meridian as
the prime meridian of the longitude system, the science associated with measure-
ments of terrestrial position improved. The measurements that the large number of
telescopes made, and their accuracy when they were combined, increasingly
revealed irregularities in the motion of the Earth as it spun on its axis. Both the
equator and the North Pole have shifted relative to the continents and the latitude
and longitude system has altered accordingly. There are progressive changes in the
length of the day caused by the friction of the tides on the land and the post-glacial
melting of the icecaps, which slow the rotation of the Earth. There are also seasonal
and irregular changes in the rotation of the Earth caused by slippage between its
molten core and its mantle as well as by climatic conditions such as the friction of
high winds at mountain ranges and the transfer of moisture between the northern
