Earth as a Planet

3 poles, 5 km

circular

A is true but R is false.

Explanation — With increase in altitude the circular horizon also widens. Had the earth's surface been flat the horizon would have been the same irrespective of altitude.

90°, 0°

Both A and R are true and R is the correct explanation of A.

Explanation — The Pole star can be seen at 90° at the North pole and at 0° at the Equator. As the pole star lies in line with the axis of the earth, its angle decreases when one moves towards the Equator because of Earth's spherical shape.

Spain

12,756 km, 12,714 km

Oblate Spheroid

(c), (d), (e)

Carbon dioxide

Oceans

Both A and R are true and R is the correct explanation of A.

Explanation — Water can absorb enormous amount of heat without causing much change in its temperature. During the day, waterbodies rapidly absorb enormous amount of heat: thus, the earth remains fairly cool. At night the waterbodies release vast amounts of heat that they absorbed during the day, which along with other atmospheric effects, keep most of the surface from freezing at night.

Ozone

Both A and R are true but R does not explain A.

Explanation — Earth supports life mainly because of its liquid water, protective atmosphere, and moderate temperature range. Merely being a terrestrial (rocky) planet, like Mercury and Venus, is not enough, those planets lack the life-supporting conditions that Earth has.

Both A and R are true and R is the correct explanation of A.

Explanation — Earth is a watery planet with 70 per cent of the earth's surface being covered by water. Distribution of water moderates the climate and surface condition of the earth, creating ideal conditions for Earth to support life.

All of the above.

15 km

Ecosystem

Nutrition cycle

Carbon

Aryabhatta

Mast

England

The view of the earth's surface as seen from a height is circular. We can prove that the earth is a sphere by looking at the horizon as with increase in altitude, the circular horizon also widens. Had the earth's surface been flat, the horizon would have been the same irrespective of the altitude.

The earth is said to be spherical, but it is not a perfect sphere. The diameter at the equator is greater than the diameter at the poles. The bulge at the equator and compression at the poles gives earth the shape of an oblate spheroid or 'Geoid'.

The mean temperature of the earth is 17 degrees Celsius. This temperature is ideal for life to exist.

Earth is called a watery planet because 70% of the earth's surface is covered by water.

Biosphere is the life zone of the earth. It is that part of the earth's terrestrial system - including air, land and water - in which life develops and where life processes in turn get transformed.

The conditions necessary for life on earth are as follows:

1. Optimum distance from the sun
2. Ideal temperature of 17 degrees Celsius
3. Presence of Atmosphere
4. Presence of water
5. Presence of lithosphere and biosphere
6. Life-giving cycles operate in nature

Two features of the earth that make it a 'Unique Planet' are as follows:

1. Presence of Atmosphere
2. Presence of water

If the average temperature of the earth increased by half a degree, it would directly affect the climatic conditions and the ecosystem of the Earth. Species who cannot adapt to these changes would perish.

Cyclical Movement of Carbon in the three realms of the Earth

1. Carbon is present in the atmosphere as carbon dioxide (CO₂).

2. Plants absorb CO₂ from the atmosphere during photosynthesis, and convert it into organic matter.

3. When animals eat plants, carbon enters their bodies.

4. Both plants and animals release CO₂ back into the atmosphere during respiration.

5. When plants and animals die and decay, some of the carbon gets stored in the lithosphere as fossil fuels (like coal and petroleum).

6. When these fossil fuels are burnt, CO₂ is released back into the atmosphere.

7. Oceans in the hydrosphere absorb CO₂ from the air. Some marine organisms also use it to form shells.

8. When these organisms die, carbon settles to the ocean floor and becomes part of the marine sediments (lithosphere).

While standing at the seashore, watching an approaching ship, we first see only its mast. Then, as it comes closer, we can see the deck in the middle, then its funnel, and finally the hull comes into full view. This can only happen when the earth's surface is curved. Thus, it is proved that the earth is round in shape.

Fix three equal-length rods upright on the 1 m table, spacing them about 30 cm apart. Slip a 1 cm-thick folded foil under the centre rod so it stands a little higher, mimicking Earth's curvature. Now crouch at one end and line up your eye with the tops of the two outer rods. The middle rod rises above this line of sight, exactly what the Bedford Level experiment showed, demonstrating that Earth is spherical in shape.

(i) The sunrise and sunset occur at different times at different places because the earth rotates from west to east and therefore, people in the east can see the sun earlier than those in the west.

(ii) The earth is not a perfect sphere because the diameter of the earth varies at the equator and at the poles. This difference is due to the centrifugal force of the earth's rotation at a great speed which forms a bulge at the equator and a compression at the poles.

(iii) Venus is hotter than Mercury because Venus is mainly composed of carbon dioxide which produces Greenhouse effect on the surface of Venus. Thus, the temperature on its surface remains very high.

The atmosphere of the earth is composed of gases like nitrogen (78%), oxygen (21%), carbon dioxide, helium and argon (less than 1%). Atmosphere plays a major role in making earth a habitable planet as-

1. The atmosphere receives heat from the sun by solar radiation and loses heat by earth's radiation. In this way, a balance is maintained.

2. The earth's atmos[here is made up of life-supporting gases like nitrogen, oxygen and carbon dioxide.

3. Ozone present in the earth's atmosphere absorbs harmful ultraviolet rays of the sun.

4. The atmosphere also prevents loss of heat from the earth's surface and helps to keep earth warm.

Earth is a watery planet with 70% of the earth's surface being covered by water. Water plays a major role in making earth a habitable planet as-

1. Distribution of water is responsible for moderating the climate and surface condition of the earth.

2. Water bodies like lakes, ponds, rivers etc. are important parts of the ecosystems and biosphere.

3. Water moves in a continuous hydrological cycle. There is a proper balance between evaporation, condensation and precipitation without which life would not be possible.

4. Water bodies help to maintain the temperature of the earth's surface. They rapidly absorb enormous amount of heat during the day, and keep the earth fairly cool. At night, they release the heat absorbed during the day, which along with other atmospheric effects, keep most of the surface from freezing at night.

Temperature plays a major role in making earth a habitable planet as the earth is the third planet from the sun. It has an average temperature of 17 degrees Celsius which is suitable for life to exist. Other planets of the solar system are either too hot or too cold to support life.

Distance from the sun plays a major role in making earth a habitable planet as-

1. The earth is at an optimum distance from the sun. Hence, it is neither too hot nor too cold.

2. The distance between the earth and the sun makes life possible on earth. If it was closer to the sun, any life present would get burnt from too much heat of the sun. If it was farther from the sun, life would freeze because of lack of heat.

(i) Lunar Eclipse

The shadow of the earth on the surface of the moon is clearly visible from earth during lunar eclipse. It appears as an arc of a circle. Since the earth is bigger than the moon, it is rarely oriented in the same position as the moon but the shadow is always circular. This proves the circular shape of the earth.

(ii) Pole Star

The Pole Star can be seen at an angle of 90 degrees at the North Pole. It lies in line with the axis of the earth, and its angle decreases towards the equator. At the equator, the angle is 0 degrees. This can happen only in an arc of a circle. This proves the circular shape of the earth.

The self-regulating and self-sustaining structural and functional unit of the biosphere is called an ecosystem. This system depends upon the sun for its energy.

For example, Ecosystem of a forest include plants and trees which are eaten by herbivores (deer, rabbit). These animals are eaten by carnivores (lion, tiger). When these animals perish, they are decomposed by decomposers and return to the environment. Again they are consumed by producers and so on.

(i) Earth is a habitable planet as it is covered with green vegetation, enormous blue-green oceans containing over a million islands, a large number of streams and rivers, huge land masses called continents with mountains, ice caps and deserts. It also has ideal temperature for the existence of life.

(ii) From space Earth looks blue because 70% of the earth's surface is covered by water.

(iii) Water bodies reduce the day-night temperature variations as water can absorb enormous amounts of heat without causing much change in its temperature. Water bodies rapidly absorb enormous amount of heat during the day, and keep the earth fairly cool. At night, they release the heat absorbed during the day, which along with other atmospheric effects, keep most of the surface from freezing at night.

The sun rises from the east and sets in the west. Sunrise and sunset occurs at different times at different places because the earth rotates from west to east. Arunachal Pradesh is situated to the east of Delhi and hence, people in Arunachal Pradesh experience the sunrise and sunset earlier as compared to the people who live in Delhi.

This also proves that the earth is geoid in shape as had the earth been flat, the sunrise and sunset would have occurred at the same time all over the world.

If the sun did not rise for a day, it would have catastrophic consequences for the earth and its inhabitants:

1. The sun is the primary source of energy for all life on earth. The absence of sunlight would cause a rapid drop in temperature, leading to widespread damage to aquatic ecosystems.
2. Without sunlight, photosynthesis will not be possible. The lack of food for plants and herbivores would ultimately lead to the extinction of many species, causing a ripple effect on the food chain.
3. Plants regulate the oxygen and carbon dioxide in the atmosphere. The loss of plants will lead to decrease in oxygen production and affect all living organisms.
4. The sudden and prolonged absence of sunlight would have a profound impact on human society, with panic and chaos as people struggle to adapt to the situation.

Thus, it is clear that the Sun is essential for life on earth.