<P> In 1612 Galileo Galilei demonstrated that with sufficiently accurate knowledge of the orbits of the moons of Jupiter one could use their positions as a universal clock and this would make possible the determination of longitude, but the method he devised was impracticable for navigators on ships because of their instability . In 1714 the British government passed the Longitude Act which offered large financial rewards to the first person to demonstrate a practical method for determining the longitude of a ship at sea . These rewards motivated many to search for a solution . </P> <P> John Harrison, a self - educated English clockmaker, invented the marine chronometer, the key piece in solving the problem of accurately establishing longitude at sea, thus revolutionising and extending the possibility of safe long distance sea travel . Though the Board of Longitude rewarded John Harrison for his marine chronometer in 1773, chronometers remained very expensive and the lunar distance method continued to be used for decades . Finally, the combination of the availability of marine chronometers and wireless telegraph time signals put an end to the use of lunars in the 20th century . </P> <P> Unlike latitude, which has the equator as a natural starting position, there is no natural starting position for longitude . Therefore, a reference meridian had to be chosen . It was a popular practice to use a nation's capital as the starting point, but other locations were also used . While British cartographers had long used the Greenwich meridian in London, other references were used elsewhere, including El Hierro, Rome, Copenhagen, Jerusalem, Saint Petersburg, Pisa, Paris, Philadelphia, and Washington D.C. In 1884 the International Meridian Conference adopted the Greenwich meridian as the universal Prime Meridian or zero point of longitude . </P> <P> Longitude is given as an angular measurement ranging from 0 ° at the Prime Meridian to + 180 ° eastward and − 180 ° westward . The Greek letter λ (lambda), is used to denote the location of a place on Earth east or west of the Prime Meridian . </P>

Mean solar time varies with your east-west location (longitude) because