Prime Meridian

Anne is shown standing on the rail that marks the Prime Meridian in Greenwich, England. By convention this rail lies at 0º 0′ 0″ W longitude, but my iPhone told a different story. According to it the Prime Meridian is actually located 5″ west or about 100 meters west of where it is supposed to be. This reading frustrated me and I was already set for a full on iPhone rant, when Anne pointed out to me the explanation on a nearby plaque. As it turns out my iPhone was correct and the Greenwich meridian is misplaced. The coordinate system used for satellite navigation in phones takes into account later data on the Earth’s slightly irregular shape. Because of this there is a small difference between the Prime Meridian and the satellite meridian: the latter is about 100 meters further east. Wait though, if the Prime Meridian was decided to be 0º longitude, then isn’t it the satellite system that is wrong? Also on a wall at the Royal Observatory are various units of measure: a foot, yard and gallon. All of these old standards are obsolete, replaced now with newer, more accurate and permanent standards. 

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