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Greenwich Mean Time
Greenwich Mean Time (GMT) is a term originally
referring to mean solar time at the Royal Observatory,
Greenwich in London.
It is now often used to refer to
Coordinated Universal Time (UTC)
when this is viewed as a time zone,
although strictly UTC is an atomic time scale
which only approximates GMT in the old sense.
It is also used to refer to Universal Time (UT),
which is the astronomical concept
that directly replaced the original GMT.
In the community of Greenwich,
GMT (in the form of UTC) is the official time
only during winter
(during summer the time in Greenwich
is British Summer Time rather than GMT).
Noon Greenwich Mean Time
is not necessarily the moment
when the Sun crosses the Greenwich meridian
(and reaches its highest point in the sky in Greenwich)
because of Earth's uneven speed in its elliptic orbit
and its axial tilt.
This event may be up to 16 minutes away
from noon GMT
(this discrepancy is known as the equation of time).