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What is the prime meridian?

A meridian is an arbitrary north-south line used by an astronomer as a zero point from where to take measurements. By comparing thousands of observations taken from the same meridian it is possible to build up an accurate map of the night sky.

The meridian line in Greenwich represents the Prime Meridian of the world, Longitude Zero (0° 0' 0"). Every place on the Earth is measured in terms of its angle east or west from this line. To stand astride the line is to have one foot in the eastern and one foot in the western hemisphere of the earth – just as the Equator divides the northern and southern hemispheres.

The Prime Meridian at Greenwich passes through a massive special telescope called a transit circle. The transit circle was built by Sir George Airy, the seventh Astronomer Royal, in 1850. The cross-hairs seen in the eyepiece of this transit circle define Longitude 0° for the world.

The 'universal day' is measured from the Prime Meridian. It is the average of a year's worth of 'natural' days and is a scientific time scale used irrespective of time zones.