What is Greenwich Time?
As the earth spins round, the sun appears to rise sooner, the farther east we are, and to rise later, the farther west we are. So the apparent time, judged by the sun’s rising and setting, is different in different places, according as they are east or west of each other. Midday on one side of the earth is midnight on the other side.
So it is necessary to have some point from which to reckon our time, and the place on which many nations have agreed is Greenwich, near London, England. Countries have their own time for their own purposes; but for general purposes, as, for instance, events occurring in the sky, they refer to Greenwich time - that is to say, the time reckoned by what the sun seems to do at Greenwich. The lines on maps up and down the earth’s surface from north to south are called lines of longitude, or meridians. The spaces between them grow narrower as you go north and south of the Equator; and the lines meet at the poles. Places on the same line of longitude as Greenwich have Greenwich time exactly, and no other place can have it. That line is called the Greenwich meridian. Places east of that line are marked on maps so many degrees east longitude, and are sometimes spoken of as so many degrees east of Greenwich; and similarly for places west of the Greenwich meridian. Farthest east and farthest west of the Greenwich meridian is a single line that is both 180 degrees east and 180 degrees west of Greenwich. When it is noon at Greenwich, it is midnight along this line.
