Composition of the earth

The Earth is a rocky planet which is round (but not entirely) because the gravitational attraction of the interior is pulling its material towards the centre. The distance from the centre to the surface is a whopping 3,960 miles.

Then there is solid core. Outer part is fluid, or almost, for over 70 per cent is water. Besides, many thousand tonnes of atmospheric gases as well as moisture in the air reminds us of their presence. We don’t even realise their presence! The iron of the core is allied with nickel, sulphur and oxygen. The outer core is rocky mantle, then lastly, the crust and, on its top, the oceans, covering the planet in a warm embrace. Like a mother holding an infant!

The interior is hot, very hot under pressure from the overlying weight of Earth’s monstrous layers. Forces within Earth cause changes on the surface on continuous basis. Many other changes occur too. Some 100 types of elements are involved in this play. Pretty hectic you’d say!

The Earth is up to 30 miles of crust, mantle is 1,800 miles thick, outer core is 2,100 thick, the rest is inner core. Rough percentage of the material is: 35 per cent iron; 30 per cent oxygen; 15 per cent silicon; 13 per cent magnesium and 7 per cent other sundry elements.

Together they work as a magnet to pull everything, whether iron, plastic or water and solid towards the centre of Earth. Magnet, for its omnipresent power, is highly enigmatic. The study of Earth is essential for the reason that it is not only one of the vital planets but the only one where life is possible and flourishes whimsically.

Orbit of the Earth

The Earth has been dutifully orbiting in the cold region around the Sun (the region is called space, or outer space) at an average distance of 93 million miles, or, 150 million kilometres ever since, sort of. It moves at a speed of 18.5 miles per second (30 km). But if it had been 10,000 km further away from the Sun, all liquid on Earth would be frozen as in a freezer and no life would be possible.

On the other hand, if it had been 10,000 miles closer, all the water would have boiled away due to intense and relentless heat. Either way, life as we know it, would not exist.

There are two kinds of motions that all of us are aware of. These are orbital and axial rotations. The former goes around the Sun, (called its orbital motion), while the latter revolves around its axis. One produces the year (annual), the other causes day and night so that all portions of the planet receive ample sunshine through the day without any disturbance or distraction (this is called diurnal).

However, there is yet another motion that not everybody is aware of. In the course of both revolutions, both poles, north and south, undergo a top-like wobbly motion. This is called precession.

We have all played with the toy called, top. The spinning is done due to the pin at its bottom. At the same time, we watch the pin make a tiny, but wobbly motion at the top. It keeps on doing so until the top comes to a familiar stop and stops gingerly. Similarly, the Earth has the same kind of motion with the difference that it never ever stops. In doing so, the top of the Earth completes a circle in the sky, in about 26,000 years. While this happens, the exact pole of the Earth faces one star or the other, called Celestial Pole. These days this star is Polaris, or Pole Star, the famous star of the constellation Small Dipper (you can watch it in the north on a cloudless night). It has been guiding sailors for thousands of years. It will remain in its place for a few thousand years more.

Then, the Earthly Pole will face another star: the star Vega, which will be the Pole Star for a few thousand years. Then, it will be Polaris again — a full 26,000 years later. The game will go on forever in the future, till eternity. This famous game that the Earth plays with stars is precession.

Let us remember that the gravity of the Sun and moon also play a part in this play but more about it later on. In the meantime, think hard, for astronomy has a lot to do with thinking. The more you read, the better you will think.

Published in Dawn, Young World, February 20th, 2015