The earth


Origin of the earth

Our earth was one of the bodies which separated away from the vapory filament (a slender thread-like body) drawn out from the sun by the gravitational pull of an unknown star. Probably, it occurred some two or three thousand million years ago. This is gathered from geological consideration.

As the time passed the vapory mass of our earth cooled into liquid. The liquefaction was facilitated by the vastly smaller size of its mass as compared to that of the sun. It is not certain how long it took to liquefy .

The next stage was solidification of this liquid mass. Further cooling, encrustation, buckling and scrapping went on. At last, the surface of the earth formed an irregular appearance of hollows and hills.

There was no human eye to see these initial stages of the formation of our planet. The whole process might have taken thousands of years. “It was a momentous smoking desert, cindery underfoot with no more scenery than sand-dunes.

Here and there, out of a crack, comes a crawl of molten rock, like very course gained tar, hardening and blistering on its surface as it cools and creeping out in form beneath its crust in an ugly sort of way.

No sun by day, or moon by night, nor any stars, but a thick curtain of cloud over everything and hiding the heavens. And beneath the cloud there was a dense and dusty, un-breathable air with carbon dioxide gas and water vapor and much nitrogen, but only a trace of oxygen.”

The size and shape of the earth

To our eyes, it appears that the earth is flat, but actually that is not the case. It is spherical in shape, that is, not a flat plain, but a curved surface. It has been found that the earth can be represented with sufficient accuracy for most purposes as an oblate spheroid, i.e., a figure formed by the revolution of an ellipse about its shorter axis.

The earth is flattened at the poles, the diameter along the axis being shorter than a diameter in the equatorial place. The diameter of our globe has been computed to be 8,000 miles approximately along the equatorial plane and about seventeen miles less along the lines of poles.

Discoveries about the earth

The orbit of an artificial satellite vehicle is determined by the gravitational pull of the earth together with the effect of air resistance. As the latter effect may be separated out, it is possible from precise observations, to obtain information about the earth’s gravitation. This in turns depends on the shape of the earth, which may therefore be studied in a new and effective way.

More detailed information has become available about the shape of the earth from precise observations of the orbits of the first satellites. This is because the force exerted by the earth on satellite depends on the figure of the earth.

Although it has been known for a long time that the earth is not exactly spherical, but is flattened at the poles, satellite studies have shown that the amount of flattening is not exactly that was previously presumed.

It is usually given in terms of the ratio, of the difference between equatorial and polar radius to the equatorial radius. The value of this ratio was taken to be 1/297.1, but analysis of the data obtained from satellite orbit observations show that it should be 1/298.24.

It has been found that the earth is slightly pear-shaped, with the stalk towards the North Pole. The scale of this effect is such that the South Pole is 50 feet nearer the center of the earth than the North Pole.

A determination of the ellipticity of the equator has also been made.

Additionally, much more precise information can be obtained of map distances by simultaneous observations from three or more ground sections of the position of the satellite. In order to make such observations practicable, the satellite Anna B was launched by the Americans at the end of 1962. The satellite was equipped with a discharge lamp, which give light flashes of short duration but very high intensity.

The interior of the earth

It is not possible to penetrate very deep into the earth. Our knowledge of its interior is gathered by indirect methods. The temperature of the earth increases rapidly from the crust inwards. Hot springs and volcanoes testify to this fact. All the same, there is no direct evidence to the continuous gradual rise in temperature at great depths.

It is believed that the earth has liquid core with a diameter of more than 4,000 miles. The core (the inner part) is probably, mainly metallic. The crust (the hard rind or outside coating of anything) outside this core is composed of heavy basic rocks.

The rotation of the earth

To the ancient, the earth was the center of the universe. They believed that it was fixed and everything else the sun, the moon, the planets and the stars moved around it. It was not until the sixteenth century A.D that Copernicus (1473-1543), a Polish-German astronomer, told the world that the earth and other planets revolve round the sun.

For fear of punishment, perhaps torture to death, he did not disclose his discovery to anybody except his friends. The first printed pages of his great work were put into his hands as he lay dying. He could feel them but he could not see what he had written. He died the same day. Bruno tried to propagate this fact, was burnt to death in punishment.

The earth rotates round its own axis, once in a day. This fact causes the succession of days and nights on the surface of the earth. It also revolves round the sun in an orbit which is more or less elliptical. The change in seasons occurs on this account.

Seasons on the earth

The path of the earth around the sun, although elliptical, is not for removed from a circle, and consequently the cold of winter and the warmth of summer as experienced in the temperature zones, are not due to the earth being farther from the sun in winter than in summer.

The explanation is to be found in the fact that the amount of heat received by a surface from source of radiant heat depends on the inclination of the surface. As the axis about which the earth rotates is not perpendicular to the plane in which revolves around the sun, at one portion of the earth’s orbit, and six months by an atmospheric which is a mixture chiefly of nitrogen and later it is titled (slanted) away from the sun.

The earth’s atmosphere

The earth is surrounded by an atmosphere which is a mixture chiefly of nitrogen and oxygen. Water vapor is great present at the surface to the extent of 1.2 percent of the total. At great height, the lighter gases predominate.

Why the sky is blue?

The sky is blue. It is so because the earth posses an atmosphere. When light passes through a medium containing numerous small particles, a part of it is scattered side-ways on account of these particles. The shorter the wave-length of the light, the greater will be the scattering.

The blue light is, therefore scattered to a much greater extent than the red light. The light, as it travels, is then gradually robbed of its blue proportion and appears red. The molecules of the atmosphere scatter the blue light towards us and produce the blue appearance of the sky. The freer the air is from the comparatively larger dust particles, the purer and deeper will be the blue light.

The golden tints of the sunset are also due to the same cause. When the sun is nearer the horizon, the light from it, which reaches an observer, passes through a much greater length of atmosphere than when it is higher in the heavens. A greater portion of the blue light is lost.

Therefore, the light that reaches the observer is tinted red. The beautiful colors, which frequently accompany the setting of the sun, are mainly due to dust particles, and depend very largely upon the amount of dust in the atmosphere.

Atmospheric structure

The pressure, density, temperature and wind distribution in the atmosphere up to altitudes of 100 km. or so was investigated at white sands, New Mexico, at Fort-Churchill, Canada at Wood-mere, Australia and at several locations in Russia, using vertical sounding rockets.

Pressure and density were measured using pressure gauges located at suitable places on the shell of the rocket. Russian investigators made direct measurements using resistance thermometers, but in other countries the temperature distribution was obtained indirectly by deriving the speed of sound as a speed of height.

This was done by a sound ranging method in which grenades were ejected at regular intervals from rocket, to explode at a distance of 50 feet or so from its trajectory. The grenade explosions are located in space by photography or radio in time by recording the times of arrivals of the light from each flash at photo-electric detectors on the ground.

Finally the time of arrival of the sound pulses from each explosion are recorded by an array of microphones. From this information it is possible to derive not only the mean speed of sound through the atmosphere between successive grenade explosions but also the mean east-west and north sound wind speeds.

The grenade method in this form can not be expanded above 80 km. because of the weakness of the sound signal from greater heights. From 100 to 150 km explosion of a grenade produces a glow through reaction with atmospheric atomic oxygen.

This lasts for twenty minutes or so. From observations of the rate of growth of the glowing region the air density may be obtained and from the changes in shape the wind speeds may also be derived. The grenade glow may be replaced by a fluorescing cloud of sodium vapor, the sodium having been ejected during twilight from a rocket.

Results obtained have conformed about the general nature of the variation of the temperature to height which had been deduced from ground based or balloon assisted studies. Evidence of small diurnal and seasonal effects has been obtained.

Direct measurements above 50 km are much fewer in number but above 200 km a great deal of information is available from analysis of satellite orbit data.

Air resistance causes the period of revolution of a satellite to decrease gradually at a rate which depends on the air density at the perigee (the closest point of approach to the earth’s surface). It the dimensions of the satellite are known it is possible to derive the air density from the observed rate of change of period at any time.

So many results have been obtained in this way that the Committee on Space Research (C.O.S.P.A.R) prepares tables of mean atmospheric pressure, density and temperature out to altitudes of 100 km where the density is about 4×10-14 of that at ground and the temperature is about 1400oK.

As the altitudes increases the air density becomes more and more variable. The atmosphere above 300 km bulges in the polar cycle from the disturbance maximum to the minimum. It is also strongly sensitive to solar disturbances.

See also: The future of life on earth


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