Students will say that the lines across the map of the globe called latitudes, and those cutting these lines vertically called longitudes, help in locating a place in the globe.
It took a long time and a lot of work in drawing these lines.
First, the latitude lines were marked, by observing the position of the sun in the sky. On March 21st, the sun is over our head, at the middle of the earth called the equator. The next day it shifts slightly, to the north. The next day again, wait for the sun to come over the head at noon and this point is furthe north, by the same distance.
Latitudes are mentioned in degrees. Each degree is divided into 60 seconds. The shift in the position of the sun each day is 15 seconds. We have to travel north, so that the sun will come at the top of the sky. On the 21st of June, if we continuously move north, the sun can be seen at the top.
If the sun throws no shadow at all, on a stick planted vertically, we can say it is on top. The next day the sun starts moving in the reverse direction, southwards. So we can verify the correctness of our measurements, once more. If a number of schools and colleges tie up, we can say whether the lines shown in the maps are correct or not.
As the sun does not go beyond the lat. 23.5, the latitudes beyond this point can be calculated by measuring the angle of digression with referenc to the vertical line.
By measuring the angle of deflection, we can calculate the latitude of a place. The measuring instrument is called the sextant. I do not know how to manipulate it. I am sure school students these days, may not have heard the name.
The determination of the longitude was a real problem. For this we require clocks synchronized with one another. I am talking about those days, when even pendulum clocks were a rarety. There was a big ship wreck, because the captain did not have an idea how far away they were from London. A Royal Commission was appointed to find out a way to determine the longitude of a point. The Chairman of the Commission was Sir Isaac Newton. He failed.
One clock maker in Italy found the way out. He invented the gymbals. When suspended in this cage, the pendulum will always hang down, even when the ship is rocked in a storm. So they carried two clocks, one according to Greenwich mean time, and another according to local time. The difference in minutes, when divided by four minutes (time taken by the sun to travel from one longitude to the next), gives the number of the longitude.
Of course, we should know whether it is west or east of zero line.
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