Glossary - Pierre DeRoche PDR 4018 Royal Retro Mode D'emploi

Glossary

Retrograde
A retrograde display is distinguished by a hand moving across
the arc of a circle (instead of pivoting around an axis) and which
returns instantly to its initial position upon reaching the end of its
trajectory in order to begin a new cycle.
GMT
The abbreviation "GMT" stands for Greenwich Mean Time and
refers to mean solar time at the Greenwich meridian crossing
the royal observatory near London and which serves as the basis
for calculating longitudes. "Greenwich Mean Time" served as a
temporal reference until 1972, the year when it was replaced by
Universal Time Coordinated (UTC). Despite this new reference, the
term GMT is still widely used.
Historically speaking, the surface of the globe had been divided
into 24 equal-sized time zones, in which the hour was identical
throughout each. The dateline had initially been fixed at the 180°
meridian. This subdivision was subsequently modified to take
account of the needs of certain countries wishing for example to
have only one legal time, despite the fact that their territory extends
over several time zones (such as China) or that their legal time
should be different from that theoretically allocated (e.g. Spain).
Moreover, some countries (such as Iran) have adopted a legal time
corresponding to 30 and 45-minute fractions of a time zone.
By now there are actually 41 time zones, including some that exist
only temporarily (Daylight Saving Time or "summer" and/or winter
time, southern and/or northern hemisphere); among them 36 are
valid year-round.
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