What Do Parallels on a Map Measure?
When a ship is lost at sea, it might send its coordinates to people on shore so that they can locate it. These coordinates are actually the lines of latitude and longitude that intersect at the boat's location. Lines of latitude and longitude are not physical lines; rather, they are imaginary lines that represent the angular distance of a point on Earth's surface from the equator.
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Lines of Latitude and Longitude
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If you look at a globe, you will see a grid of lines---some of the lines running vertically from the North Pole to the South Pole, and others running horizontally parallel to the equator. The lines running vertically are called lines of longitude, and the lines running horizontally are called lines of latitude. These lines are imaginary, but they are used to identify locations on Earth.
Significance
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Latitude and longitude enable sailors and other navigators to pinpoint their location and describe that location to somebody else. There is only one point on the globe with a given latitude and longitude, so, if a traveler were to give her latitudinal and longitudinal coordinates to someone else, her location would be easily identifiable. Also, although little on Earth remains the same over thousands of years, the coordinates of the Earth should always remain constant.
What is a Parallel?
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This point is at the angular distance of 45 degrees from the equator
A parallel is an imaginary line connecting all of the points at the same angular distance from the equator. As you can see in the figure, the line of latitude 45 degrees north of the equator connects all of the locations on Earth's surface that are at a 45 degree angular distance from the equator. These imaginary lines circle the Earth and run parallel to the equator.
Measuring Degrees of Parallel
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There are 90 degrees of parallel north of the equator and 90 degrees of parallel south of the equator. If you were on the 45 degree parallel shown below, you would say that your latitude is 45 degrees north. In some cases, the designations "north" and "south" are omitted, so northern latitudes are represented by positive values and southern latitudes by negative values.
What is the Distance Between Two Parallels?
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On average, the distance between two parallels is about 69 miles. However, the distance between lines of parallel near the equator can be as high as 69.4 miles, and the distance between lines of parallel near the poles can be as low as 68.7 miles. This is due to the flattening of the Earth near its poles.
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