 Fri Sep 22, 2006 11:48 am GMT |
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Hi,
"Greenwich Mean Time" (GMT) is a term originally referring to mean solar time at the Royal Observatory, Greenwich in England. 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.
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). The fictitious mean sun is the annual average of this nonuniform motion of the true Sun, necessitating the inclusion of mean in Greenwich Mean Time.
Historically the term "GMT" has been used with two different conventions for numbering hours. The old astronomical convention (before 1925) was to refer to noon as zero hours, whereas the civil and more modern convention is to refer to midnight as zero hours. The more specific terms "UT" and "UTC" do not suffer this ambiguity, always referring to midnight as zero hours.
Regards
Jan
by courtesy of Wikipedia
If you use UTC you make Englishmen mad and boiling
I use UTC |
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Jan
I'm here quite often ;-)
Joined: 01 Apr 2006
Posts: 285
Location: at sea
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