DISCOVER IT – 4

We learn in school that the earth rotates around its axis once in a day , and that it also revolves around the sun once in a year. A geosynchron model helps us to understand this.

In my last article, Discover it 3, I promised to tell you how to make a geosynchron. Take a large spherical globe and place a tube through it as was described in my last article for doing the Dhruva tara experiment. You can use the plastic globe models of the earth which are available in most schools for this experiment, or else, you can use the same plastic ball which you can purchase in most toy shops. The metal tube through which you are going to look should pass through the centre of the sphere and come out at both ends at the north pole and at the south pole of the globe. (When you do this the metal tube will now be in the same position as the axis of the globe model of the earth)

As in the last experiment, set the globe on its ring mount outside, and fix it such that you can see Dhruva tara through the tube.

Without changing the position of the metal tube rotate the globe so that the map of India is on top. When you have completed this whole procedure, you have made a geosynchron.

What is a geosynchron (GS) ? It is a model of the earth which is exactly parallel to our real earth. India on our GS is parallel to real India. Australia on our GS will also be parallel to Australia in its real position in the southern hemisphere.

When it is noon in India, the sun will be right above our GS and it will be noon on our GS. When the sun is setting in real India, you will see that it is also setting for the small India map which is on the GS. In fact whatever time the sun is showing in real India, the same solar time will be seen on the India which is on our GS. Geosynchron means ‘same earth time’. It could also be called a Samantar Prithvi.

By the simple act of fixing the line of sight of the tube onto Dhruva tara, we have created a parallel earth which remains parallel to the real earth not only as the earth rotates around its own axis, but also as the earth goes around the sun in one year.

At noon, the shadow cast by an upright pole is shortest. As the earth rotates around the sun, the length of the shadow cast by an upright pole by the noonday sun changes from day to day and month to month. From the length of the noon day shadow, we could , if we wanted, construct a solar calendar. The noonday sun shadow of an upright pole would be longer in winter and shorter in summer. At the tropics it is zero on June 21st- the day of the summer solstice.

Since our geosynchron is exactly parallel to the real earth, we could place an upright pin on the map of India, and find out that the pin casts the same kind of shadow as an upright pole on the open ground. So we could construct a small sundial and place it on the GS India , which would tell the same time as a real full size sundial on real India.

Why does Dhruva tara always remain fixed ? This is because the earth’s axis, like the axis of a spinning top, always points in a fixed direction, and Dhruva tara happens to be in that direction. Secondly, because Dhruva tara is very very far away, so that even if the earth moves round the sun, that movement is very small compared to the distance of the sun and earth from Dhruva tara.

Dhruva tara is not the only celestial object which appears to remain fixed in the sky. There are some man made satellites which also appear to be fixed in the sky. These are the geosynchronous satellites on which are fixed the antennas, receivers and transmitters which are used for mobile phone and internet telecommunications. These satellites are all at a height which is tens of thousands of kilometres above the earth’s surface. At this height, the period of rotation of the satellite around the earth is equal to the period of rotation of the earth and therefore the satellite appears to be fixed above the earth.

Is it not wonderful that for two completely different reasons two kinds of celestial objects appear to be fixed in the sky ? Dhruva tara, because it is extremely far away and because our earth’s axis points at it . Geosynchronous satellites, because they are just so far away that they revolve around the earth at the correct speed.

Supposing you were sitting on a geosynchronous satellite which is just above India and looking down on real India from outer space. You would see the sun rising and setting on India. You would see the line of light and shadow which separates day from night slowly rotating around the globe once in 24 hours. You can have the same experience by standing next to your geosynchron on an open ground and looking at it at different times of the day.

You can imagine, that along that line which separates light from dark, millions of people are waking up, going to the bathroom, brushing their teeth, and getting ready to go to school or to work. Along another line on the other side of the globe, millions more are relaxing at home after a hard days work and watching TV, and getting ready for dinner. Enjoy your journey to the geosynchronous satellite thousands of kilometres above the earth in outer space.