Latitude and longitude are imaginary lines on the surface of the Earth which demarcate, respectively, the north/south and east/west parts of the World. They are imaginary to the extent that there are no such lines to be seen, physically, on the surface of the Earth. Their reality is a mathematical concept which allows the position of any place in the World to be calculated and defined. By reference to its latitude and longitude the location of any place can be stated absolutely and not relative to any other place.
Parallels of latitude are so called that because they circle the Earth and are parallel to each other. The defining circle is the longest, which has as its center the center of the Earth itself. It is called the "Equator". The parallels of latitude diminish in length north and south of the equator until they "disappear" at the points of the North and South Poles.
Lines of longitude are called meridians and they all meet at the North and South Poles. Meridians of longitude do not circle the Earth. They extend from one pole to the other and cross the parallels of latitude always at right angles.
Although the precise position of anywhere on Earth can be defined by reference to its latitude and longitude it was only recently that this became possible. Using latitude was easy. Longitude presented more difficulties. Ptolemy, the Greek mathematician, solved the problem of locating places by reference to parallels of latitude centuries ago. There have been refinements and re-definitions since but the use of latitude in the location of places was simple compared with the finding of where one meridian of longitude was relative to another. This became possible only after modern engineering permitted the manufacture of metal gears and cogs with which to make accurate clocks.
Parallels of latitude north or south of the Equator are measured in degrees. This takes advantage of the fact that a circle has been determined to contain 360 degrees. A degree of latitude is about 70 miles or 110 kilometres from the one next to it. Between two parallels a subdivision can be made into minutes. There are 60 minutes to a degree. If further accuracy is required the minutes are divided into 60 seconds. A parallel of latitude can be determined by reference to the Sun or any fixed star. It is a matter of simple geometry to calculate the distance between two points whether or not either is precisely on a parallel. Thus the "level" on one place relative to another can be established.
The same basic distances, 70 miles or 110 kilometres, are true for meridians of longitude. The problem in establishing the relative positions of two places with respect to meridians of longitude arises from the fact that the earth rotates on its axis from west to east. There is no fixed point on which calculations can be based as is the case with latitude. Effectively this means that the position of a starting point changes over time with respect to any other point. Calculating position for longitude can be done by reference to time. It is necessary to know at the second point the time at the starting point and the time, now, at the second point. The time now at the second point can be established by reference to the Sun at noon. An accurate clock will record the time elapsed since leaving the starting point. Until there was the engineering capacity to manufacture accurate clocks establishing position by reference to longitude was all but impossible. It was important for navigation purposes to know a ship's longitude. At sea a pendulum clock was not adequate for recording time. Now, again, it is a matter of basic geometry to gauge the distance between the two points. Being able to pinpoint the position of a place in terms of parallels of latitude and meridians of longitude means that any place can be located.
Although the location of places is important, latitude and longitude also have a bearing on time and seasons. The differences in these aspects relative to a particular place have to be taken into consideration by anyone moving from one country to another. This is very important for retirees who may have commitments and ties to their places of origin for family or business reasons.
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