In order to understand about positions on the surface of earth, we need to first learn about Great Circles. Just as a remider, we had learnt about the Sexagesimal System of Direction Measurement in our earlier lesson. In case you have not seen the post, please go through the lessons on measurement of directions.

In order to understand Great Circles, take a globe and put an elastic band exactly in its mid-point. You can see that the rubber band would stay in place only at one particular position which is the exact mid-point of the sphere. In all the other places the elastic band would slip away. The path followed by the elastic band over the surface of earth represents the Great Circle. Equator and Meridians are perfect examples of Great Circles.

### PROPERTIES OF A GREAT CIRCLE

Great Circles have some unique properties which differenciate them from other lines drawn on the earth’s surface. The first point is that, Great Circles are circles on the surface of the earth whose Centre and Radius are the same as that of the earth. In fact, you cannot draw any line or circle on the earth’s surface that is larger than the Great Circle.

If you draw such a circle, whose centre and radius is same as that of earth, its plane would cut the earth in two equal halves. Since Great Circles have the same diameter and circumference as that of earthit would have the greatest possible area.

You can easily observe that only one Great Circle can be drawn between any two points on the surface of earth. The only exception to this rule is when the two selected points are exactly opposite to each other. These diametrically opposite points on the earth surface are called Antipodal Points.

In this lesson, we had learnt about Great Circles and their properties which differenciate them from other lines on the earth’s surface. In our next lesson, we shall understand about the Equator and Meridians.