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I found that..midnight Mars's position was three and a half degrees to the east.

Finally, it required a stable viewing platform, rendering the technique useless on the rolling deck of a ship at sea. 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.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.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.The vertical north–south plane still intersects the plane of the Greenwich meridian at some angle; that angle is the astronomical longitude, calculated from star observations.The longitude shown on maps and GPS devices is the angle between the Greenwich plane and a not-quite-vertical plane through the point; the not-quite-vertical plane is perpendicular to the surface of the spheroid chosen to approximate the Earth's sea-level surface, rather than perpendicular to the sea-level surface itself.Each degree of longitude is sub-divided into 60 minutes, each of which is divided into 60 seconds.A longitude is thus specified in sexagesimal notation as 23° 27′ 30″ E.Unlike latitude, which has the equator as a natural starting position, there is no natural starting position for longitude. It was a popular practice to use a nation's capital as the starting point, but other locations were also used.