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prime meridian,meridian that is designated zero degree (0°) longitude, from which all other longitudes are measured. By international convention, it passes through the original site of the Royal Observatory in Greenwich, England; for this reason, it is sometimes called the Greenwich meridian. Universal timeuniversal time
(UT), the international time standard common to every place in the world, it nominally reflects the mean solar time along the earth's prime meridian (renumbered to equate to civil time).
..... Click the link for more information. , the standard basis for determining time throughout the world, is civil timecivil time,
local time based on universal time. Civil time may be formally defined as mean solar time plus 12 hr; the civil day begins at midnight, while the mean solar day begins at noon.
..... Click the link for more information. measured at the prime meridian.
prime meridianThe arbitrary circle of zero longitude on the Earth or some other planet or mapped celestial body. The Earth's prime meridian is the Greenwich meridian.
(or zero meridian), the terrestrial meridian that passes through Greenwich, the site of the old Royal Observatory. The prime meridian is the starting point for the calculation of geographic longitude and is the central meridian of the zero time zone. Local mean solar time at the prime meridian is widely used in astronomy (so-called universal time).