“IT is one small step for man, but a giant leap for mankind.”

— Neil Armstrong, as he took his first step on the moon.

HAVING hurtled across the inner Solar System — numbering two planets, and the outer Solar System, numbering six planets — we have now come to rest on our own: the shimmering green-blue crown of the Solar System, Earth.

Earth, the third planet out from the Sun is a rocky, hard-surface body of average size and dimensions. Its hardness goes deep down, towards the centre called core, but mainly in the crust and then the mantle. But wait for a moment. We cannot say for sure that it is rock-hard all the way down. It has chasms, crevices and long, winding, meandering caves running thousands of miles in all directions and across.

Much of the inner material is plastic — real hot and somewhat fluid, as evidenced by the spewing lava we confront every now and again… destructive at the time it spews out, but very productive (fertile) as time passes on and when it cools down.

In fact, the caves criss-cross and intertwine aimlessly for a million miles like the veins and arteries in our bodies. Not in the same regular and orderly manner though, they were caused during the course of the hardening of the inner as well as the outer parts of the planet — a process which began slowly but ominously soon after its formation 4.6 billion years ago and continued for some two billion years.

Until the crust, the uppermost part of the Earth became hard too and began to hold the weight and immense pressure of the mountains and seas most of which formed in the last one billion years.

Earth is 12,756km across equatorially. All planets (in fact all spherical bodies) are slightly larger at the Equator than at the Poles. Due to the irrepressible centrifugal forces at work, they are not perfect balls or round in all directions. That is, they carry a bulge round the middle rather than the top. That happens before a body becomes solid from the fluid form in the first billion years, later, from the fluid/soft state, to plastic, to solid state in due course of time running into hundreds of millions of years. The axial rotation of the body keeps pushing, or accumulating it outward following the laws of centrifugal motion until it becomes oblate and pot-bellied, slightly larger without causing any imbalance or difference in quantity in the aggregate (total) matter. (Equatorial diameter, as such is 50 km more than the polar). The same driving centrifugal is at work even today but has no influence on the hard area it is trying to change.

Earth is the only planet known so far to harbour life, the only place anywhere in the big, wide universe known to us thus far. In fact, it has the widest possible variety of life one can possibly imagine. Complete life forms, weird ones, strange shapes, more than complete forms (those having gone full circle in stages of evolution, such as snakes), and what not.

No other place in the known universe has any life — whatever kind. Not to our knowledge. Not even the most basic and rudimentary, such as the amoeba, or other single-cell life unable to replicate itself.

Earth spins on its axis once in 23 hours 56 minutes 04.1 seconds (not exactly 24 hours as is generally believed for convenience). Also of prime importance is the speed at which Earth moves in its orbit around the master star, the Sun. It covers the distance 149,597,890km multiplied by two, in 365.3 days. Its inclination of Equator to its orbit is just about 23 1/2 degrees, which means that as the globe goes around in its orbit, it is tilted by that much.

You must have noted that in the shape a globe is made, it is never straight but bent by 23 1/2 degrees. We all are accustomed to seeing the globe tilted. Ecliptic is the path, or celestial highway, on which all planets (and our moon) move in their orbit. This path is 23 1/2 degrees in keeping with the tilt of the Earth. This ominous tilt of the Earth also determines the weather on our blessed planet.

Another factor which has an abiding bearing on weather, climate and various weather patterns of the entire planet is the almost continually changing distance of the planet from the Sun which keeps changing, if slightly.

As you know, the nearer a planet is to the Sun, the faster it must move in its orbit to save itself from falling into the Sun. As such, Mercury moves the fastest; but as we keep drawing away their speeds keep decreasing. After Jupiter, the planets merely chug along at leisurely pace. Those after Uranus simply worm their way across the cold, frigid space. The last one, Pluto just groans at a miserly pace, slower than anyone else. The one unique factor that led to many others is the amount of water we have on the planet: water occupies 70.8 per cent, the land just 29pc or so.

As planet Earth rolls on steadily, uniformly for an average observer, it makes another weird motion in the sky. Driven by gravitational forces, the Earth, as it spins like a top, draws a small circle in the sky. Imagine a boy’s top. We have all played with it at one time in our lives. Watch carefully as it spins. As the top spins, its top (top of the top!) wobbles and draws yet another circle, howsoever small as it struggles to maintain its balance, fighting the force of gravity.

Expanded into the sky, the circle follows a few stars. This is the all-important Precession Circle. In the present age, the star that meets the imaginary south-north axis in Northern hemisphere, when that line is extended into the sky, is the Pole Star (Polaris in the constellation, Small Bear or Ursa Minor, to the Americans, the Small Dipper). But 4,000 years ago when the largest of pyramids were being built in ancient Egypt, or when Mohenjo-Daro in our part of the world was in full bloom, it was the star Thuban in the constellation Draco (Dragon).

The Earth completes one full Precision Circle in about 26,000 years (actually 25,772). It is apparently a small circle for the boy’s top, but like” a small step...” it is “a giant leap....”

Next we come to various layers of Earth which hold it together. First comes the crust. For all its pains it is woefully thin. No more than 70 kilometres thick. It is thicker where it bears the weight of mountains but very thin at other places. At places it is only 10km thick.

Below the crust lies the mantle. Remember that as we keep going down heat keeps increasing, because there is more and more thickness of matter (those rocks, which increase with the distance) that apply unbearable weight on surrounding layers. However, a small portion of the resultant pressure is converted into heat.

More about various layers of mantle and core in the next session!