Prime meridian
One of the earliest known descriptions of standard time in India appeared in the 4th century CE astronomical treatise Surya Siddhanta.
[4][better source needed] Ptolemy's Geographia was first printed with maps at Bologna in 1477, and many early globes in the 16th century followed his lead.
Christopher Columbus reported (1493) that the compass pointed due north somewhere in mid-Atlantic, and this fact was used in the important Treaty of Tordesillas of 1494, which settled the territorial dispute between Spain and Portugal over newly discovered lands.
[8] In 1541, Mercator produced his famous 41 cm terrestrial globe and drew his prime meridian precisely through Fuerteventura (14°1'W) in the Canaries.
[10] In the early 18th century the battle was on to improve the determination of longitude at sea, leading to the development of the marine chronometer by John Harrison.
But it was the development of accurate star charts, principally by the first British Astronomer Royal, John Flamsteed between 1680 and 1719 and disseminated by his successor Edmund Halley, that enabled navigators to use the lunar method of determining longitude more accurately using the octant developed by Thomas Godfrey and John Hadley.
Even the French translations of the Nautical Almanac retained Maskelyne's calculations from Greenwich – in spite of the fact that every other table in the Connaissance des Temps considered the Paris meridian as the prime.
[13] The French argued for a neutral line, mentioning the Azores and the Bering Strait, but eventually abstained and continued to use the Paris meridian until 1911.
[16][b] The position of the historic prime meridian, based at the Royal Observatory, Greenwich, was established by Sir George Airy in 1851.
[18] Prior to that, it was defined by a succession of earlier transit instruments, the first of which was acquired by the second Astronomer Royal, Edmond Halley in 1721.
One remote longitude ultimately based on the Greenwich meridian using these methods was that of the North American Datum 1927 or NAD27, an ellipsoid whose surface best matches mean sea level under the United States.
Due to the movement of Earth's tectonic plates, the line of 0° longitude along the surface of the Earth has slowly moved toward the west from this shifted position by a few centimetres (inches); that is, towards the Airy Transit Circle (or the Airy Transit Circle has moved toward the east, depending on your point of view) since 1984 (or the 1960s).
With these advances there also arose the necessity to define a reference meridian that, whilst being derived from the Airy Transit Circle, would also take into account the effects of plate movement and variations in the way that the Earth was spinning.
Based on observations to satellites and celestial compact radio sources (quasars) from various coordinated stations around the globe, Airy's transit circle drifts northeast about 2.5 centimetres (1 inch) per year relative to this Earth-centred 0° longitude.
