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There are three types of information we can obtain from the observation of celestial bodies, namely, direction, latitude and time.

Direction, the easiest to find of the three, is defined using four cardinal points: North, South, East and West.  In the Northern Hemisphere, North can be found by locating the Polaris while in the Southern Hemisphere, South can be found similarly using the Southern Cross. Alternatively, direction could be told by means of a compass.

Latitude is slightly harder to find in the sense that it is more difficult to obtain accurate readings. The instruments employed in finding latitude share two common problems. Firstly, they are weather dependent and cannot be used when the sky is overcast because the view of the celestial objects is obscured. Secondly, they do not function properly under the unstable conditions of a ship.

Time is a quantity measured in hours, minutes and seconds. Many centuries ago, scientists discovered that time was the key to finding longitude on Earth. The counter-clockwise rotation of the Earth resulted in areas of different longitude having different sunrise and sunset timings, thus giving rise to different time zones. By finding the difference in time in different time zones, the separation in terms of longitude could be obtained. 

The longitude problem

Earth takes approximately 24 hours to make a full rotation (360) on its axis. Dividing 360 by 24, we get 15, which signify 15 difference in longitude of two places is equivalent to an hour difference between the local times of the two places. Local time could be found without much difficulty (for example, by using a Sundial), but keeping track of the time of a reference meridian (Prime Meridian) was a major problem. Before the Chronometer was invented, mathematicians and astronomers suggested a variety of ways to estimate the difference between the time of the reference meridian (Prime Meridian) and the local time, such as Dead Reckoning, Eclipse Timing Method and Lunar Distance Method. Christopher Columbus used the first two methods to determine his longitude. On his 1504 voyage to Jamaica, he claimed Jamaica to be 7 hours and 15 minutes later than Cadiz, when in actual fact, Cadiz was only 4 hours and 45 minutes earlier. This huge error of 2 hours and 30 minutes (equivalent to 37.5 longitude) showed how inaccurate the three abovementioned methods could be. Only a sufficiently accurate clock can solve the longitude problem.