If you have made all the things that I had suggested in the previous five articles of the series “Discover it” you are now ready to start measuring the universe. Let us see what we needed to start this science adventure :

A plastic ball, a pocket mirror, some string, some stones, a piece of stiff cardboard.

With the plastic ball we made a ball and ring mount. Fixing the mirror on this ball mount we made a solar telescope. With the string and stones we made an angle dangle meter to measure the angles of objects in the sky. We could fix the angle meter on the ball mount and take aim at the stars. The ball mount holds the angle meter steady, keeping our hands free.

Let us begin to measure the universe by measuring the radius of the earth. There are many ways of doing this. Here is one simple method. Measure your latitude by measuring the angle of Dhruva tara with your angle dangle meter. At Simla it will be around 31 degrees above the horizontal. (So the latitude of Simla is 31 degrees north of the equator.) At Delhi, the latitude will be measured as around 29 degrees . At Mumbai Dhruv tara is  around 19 degrees above the horizon. So the difference in latitude between Delhi and Mumbai is 10 degrees.

From my railway ticket I find that New Delhi is 1384 kilometres distant from Mumbai Central. But the railway track is not straight, and Mumbai is slightly to the west of Delhi. Let us make an approximation- the shortest distance between the 29 degrees latitude parallel and the 19 degrees latitude parallel is something less than 1384 km, say about three fourths of that distance i.e. around 1050 km. We are making a rough calculation, remember- what the above figures mean is that when you travel about 1050 km from north to south, your latitude changes by 10 degrees.

When you travel once around the earth from north to the south pole and then from south pole to north pole, and then again from north pole to south, till you reach the place where you started from, your latitude will change through 360 degrees, that is, 36 times 10 degrees. So the circumference of the earth is around 36 times 1050 km, i.e around 38,000 km, say 40,000 km. So the radius of the earth is this divided by two times pi, i.e 40,000 divided by 44 and multiplied by 7. This turns out to be around 6400 km.

Supposing you did not know the distance between Delhi and Mumbai. Could you still calculate the radius of the earth ? Yes. You can do it by doing a wonderful experiment on the west coast of India, by the sea, on a clear, cloudless day, as the sun sets. You can find out how to do this experiment on the website Or else, you could read my booklet “Measuring the universe with a string and a stone”.

You may ask, why do I need to measure the radius of the earth ? I can look up the answer in my school atlas. To that, I would answer – why do you need to travel to new places during your holidays ? You can read about these places in your school atlas, or in an encyclopaedia. But if you like to travel, then its great fun doing the experiment yourself and tracing the footsteps of the greatest scientists that humanity has known.

Who was the first person who measured the radius of the earth ? Everybody knows that it was the great Greek scientist Eratosthenes. But everybody who believes this is wrong. 200 years before Eratosthenes, a man called Anaxagorus first measured the radius of the earth. But he thought this was the distance of the sun from the earth. How did he make this amazing mistake ? We will discuss this in our next article.

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