Eclipses

An eclipse is partial or total obstruction of light from a celestial body as it passes through the shadow of another celestial body. On the earth we are familiar with two types of eclipses i.e., solar eclipse and lunar eclipse. Apparently, eclipses should occur every month because of revolution of the earth around the sun and that of moon around the earth, but the plane of moon’s orbit around the earth is inclined at an angle of 5°9’ to the plane of the ecliptic.

Solar Eclipse

When the moon comes exactly between the earth and the sun it obstructs a part or whole of the sun and a partial or total solar eclipse occurs. The moon is between the earth and the sun and it obstructs the sun causing solar eclipse. A solar eclipse will occur only if the sun and the moon are on one side of the earth. Usually we have partial solar eclipse but total or full solar eclipse also occurs, although at varying intervals of time depending on the position of the sun, the earth and the moon. Usually a solar eclipse occurs between sun rise and sun set at new moon. Since the size of the moon is very small as compared to that of the sun, total solar eclipse is of very short duration and lasts only for a few minutes. On an average, it lasts for 2½ minutes but in certain cases it may last upto 7½ minutes. e path of totality does not exceed 275 km.

The Atmosphere

What is Atmosphere?

Atmosphere is a vast expanse of air which envelops the earth and has become an integral part of the earth due to the earth’s gravitational force. e air of the atmosphere is colourless, odourless and tasteless. It is mobile, compressible and expandable. We can not feel the presence of the air unless it blows as wind. About 99 per cent of the total mass of the atmosphere is within the height of 32 km from the earth’s surface.

There are dierences in opinion regarding the thickness of the atmosphere. Previously it was considered to be only 800 km but latest discoveries hint at 32,000 km thick atmosphere surrounding the earth. It has no clear-cut upper limit and it gradually merges with the outer space.

Composition of the Atmosphere

The atmosphere is a mixture of many gases. In addition, there are water vapours and dust particles also.

Gases

Following Cable shows that there are several gases in the atmosphere of which nitrogen constitutes the major portion. is accounts for 78.03% of the total volume of the atmosphere. Next is oxygen which is 20.99 of the atmospheric volume. Thus, nitrogen and oxygen together make up over 99% of the atmospheric air by volume.

Lunar Eclipse

Lunar eclipse occurs when the earth comes between the sun and the moon i.e. lunar eclipse will occur only if the sun and the moon are located on the opposite sides of the earth. It usually occurs at full moon. The earth is between the sun and the moon and it casts its shadow on the moon. Depending upon the position of the moon during its revolution around the earth, we can have partial or total lunar eclipse.

Name of the gas Percentage by volume
Nitrogen 78.03%
Oxygen 20.99%
Argon 0.94%
Carbon dioxide 0.03%
Hydrogen 0.01%
Neon 0.0018%
Helium 0.0005%
Krypton 0.0001%
Zenon 0.000009%
Ozone 0.000001%

Some important gases are briey described below:

Nitrogen constitutes the largest proportion of the atmosphere. The atmospheric pressure, force of winds and rection of light is largely due to nitrogen. This gas has no colour, odour or taste. It does not easily enter into chemical union with other substances, but gets red into soil. Nitrogen regulates combustion and avoids quick burning. It would be dicult to control there in the absence of nitrogen. Nitrogen generates protein in the plants which is the main source of food. s gas extends to a height of 128 km.

Oxygen is called life-giver because it is very essential for breathing. It combines with all the elements and is most combustible. Therefore, it helps in burning the fuel. This is the main source of energy and provides solid base to industries. Although it extends to 64 km, its main concentration is coned to 16 km only. It is in negligible quantity at the height of 120 km. 

Carbon dioxide

This is the heaviest gas and is coned to lower layers only, although it can be found upto a height of 32 km in small quantity. In spite of the fact that it forms just 0.03% of the atmosphere, it is the most essential gas for the growth of vegetation. It is important meteorologically also because it is transparent to the incoming solar radiation but opaque to the terrestrial radiation reected from earth’s surface. It keeps the air warmer near the ground and along with water vapour is largely responsible for greenhouse effect of the atmosphere. The carbon dioxide content of the atmosphere has been alarmingly rising in the past few decades due to increase in the burning of fossil fuels. There has been considerable increase in the content of carbon dioxide in the atmosphere since the beginnings of the Industrial Revolution in the 18th century. This has increased the temperature of the atmosphere. This is a light gas and extends to 1100 km over the heavier gases. It accounts for 0.01% of our atmosphere. This is a special variety of oxygen gas and is found in very small quantity in the upper atmosphere. It saves us from the ultraviolet radiations from the sun. It is mainly concentrated from 20 to 25 km above the earth’s surface.

Water Vapour

In addition to the above mentioned gases, water vapours are also present in the atmosphere. They enter the atmosphere through evaporation which takes place mainly from the surface of water bodies and from an important component of the atmosphere. In fact water vapour is sometimes more important than other gases of the atmosphere. The count of water vapour varies from 4% to 1% with an average of 2%. The amount of water vapour decreases with altitude. It is estimated that half the water vapour in the air lies below an altitude of about 2,000 metres. Nearly 90 per cent of it lies below 6 km of the atmosphere. It also decreases from the equator towards the poles due to lower rate of evaporation there. Water vapour absorbs parts of the insulation from the sun and reduces its amount reaching the earth’s surface. It also preserves the earth’s radiated heat. It, thus acts like a blanket allowing the earth neither to become too cold nor too hot.

Dust Particles

Dust particles which are present in air are derived by strong winds from different sources on the surface of the earth. The dust particles which keep on suspended in the air play significant role from the metrological point of view. Many of them act as hydroscopic nuclei around which water vapour condenses to produce cloud. They also intercept and rect insulation. Dust in the air produces marvellous optical phenomenon of red and orange hues in the sky at the sunrise and the sunset which are known as ‘dawn’ and ‘dusk’ respectively. Besides, dense haze and smog (smoke + fog) are also caused due to the presence of dust particles. Sky looks blue due to the presence of dust particles in the atmosphere.

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