What is a meridian time?

The zero meridian used by satellite navigation systems (on the WGS84 datum) is 102.5 metres (336.3 feet) to the east of the line marked at Greenwich. The plane of this geodetic meridian passes through the centre of the Earth, unlike the plane of the astronomical meridian which contains the direction of gravity (indicated by a plumb line) which points opposite to the direction of the zenith, to which astronomical instruments are aligned. The angle between these two meridian planes at the Royal Observatory, the east-west component of vertical deflection, is 5.31". The WGS84 datum is an average of the various continental drifts. As a result, the astronomical meridian through Airy's transit instrument drifts toward the east as it is carried by the European portion of the Eurasian tectonic plate, closer to the geodetic meridian, by about one centimetre per year.