The shape of the earth
The globe is a model of the earth. It is the nearest likeness that we have. The axis is an imaginary line, about 12,713 km long, which passes through the middle of the earth. The top or northern end is called the North Pole and the lower or southern end is the South Pole.
From the moon the astronauts could see the whole earth as beautiful round sphere. The pictures they took from the moon show us the spheroid they saw.
The size of earth
About 2000 years ago learned Greek named Eratosthenes who lived in Egypt, was able to make a rough estimate of the distance round the earth. He did this observing the different altitudes of the sun in the sky, as seen on a given date from two places (Aswan and Alexandria) that were 800 km apart on a North-South line. He calculated from these observations that the circumference of the earth must be 40,000 km which is about 25,000 miles.
Some important lines on the earth
All shapes have a bulge at the centre. The Equator is an imaginary line that passes through the centre of the earth. This divides the earth into equal halves as the Northern and southern hemispheres.
The position of each place on the crust of the earth is measured by the angle that place makes with the centre of the earth. For example, Calcutta is 231/2 degree north. This means that an imaginary line drawn from Calcutta to the centre of the earth will make an angle of 23 ½ degrees with the Equator which is `0’ degree. The last line at the North Pole will make an angle of 90 degrees with the equator. Thus we have lines that circle the globe in an east –west direction from the equator to the North Pole. These lines run parallels to the equator. These imaginary lines are called the parallels of latitude. Each one gets smaller and smaller as get nearer the pole. This is repeated in the southern hemisphere. For example, if a place is 80 degrees North or 80 degrees south it means tha6t this place is never the pole than the equator.
There are another set of imaginary lines that pass from north to south and meet at the North and South Poles. The space between them is widest at the equator and it continues to lessen till the lines touch the poles.
On a globe these lines look like semicircles which meet at the poles. These are called the meridians of longitude because all places on the same meridian have noon at the same time.
The prime (or first) meridian passes through Greenwich, London. All the other meridians of longitude are calculated either east or west of Greenwich, e.g., Chennai is 82 degrees. But remember that the earth is circular which means that there are only 360 degrees. So we have 179 degrees meridians to the east and 179 degrees meridians to the west of Greenwich. Every meridian has its corresponding partner on the other side. So meridian `0’ degrees continues the other side as 180 degrees these two lines are common to both east and west. Just as the equator divides the earth into Northern and Southern Hemispheres, so the two meridians, 180 degrees and `0’ degrees divide the earth into two halves-east and west.
When we want to locate a place we must have its latitude and its longitude. To get the location of a place we have to find the lines of latitude and longitude that cut across each other at that point. This is called a grid, e.g., Delhi lies at 77 degrees east and 28 degrees north.
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