Earth as a Planet ICSE Class 9: Shape and Conditions for Life

What Earth as a Planet is about

Weightage: CISCE publishes no chapter weightage and no question-paper layout for ICSE Class 9 Geography. Class IX is examined by your school: one 80-mark paper of 2 hours plus 20 marks of internal assessment (a record file and one project). Earth as a Planet is topic 1(a), Our World, of the 2028 syllabus.Source: CISCE ICSE 2028 Geography syllabus, Class IX

Key concepts

The earth in the solar system

The solar system consists of the sun and the bodies held by its gravity: eight planets, their moons, dwarf planets, asteroids, comets and meteoroids. In order from the sun the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The first four are small, rocky 'inner' planets; the outer four are large, mostly gaseous giants.

The earth:

• is the third planet from the sun, at an average distance of about 150 million km;
• is the fifth largest planet;
• has one natural satellite, the moon;
• rotates on its axis in about 24 hours and revolves around the sun in about 365¼ days;
• is called the 'blue planet' because of its large water surface.

Worked example

(2 marks) State the position of the earth in the solar system. Why is it called the blue planet?

Try it first, then show the solutionSolution

1. Third planet from the sun, between Venus and Mars - 1 mark.
2. About 71 per cent of its surface is water, which looks blue from space - 1 mark.

Answer: The earth is the third planet from the sun, between Venus and Mars. It is called the blue planet because water covers about 71 per cent of its surface and makes it appear blue from space.

Shape of the earth: a geoid

The earth is not a perfect sphere. Because of its rotation, it bulges slightly at the equator and is flattened at the poles. This shape is called an oblate spheroid, and the exact, slightly irregular shape of the earth is called a geoid ('earth-shaped').

The equatorial diameter (about 12,756 km) is greater than the polar diameter (about 12,714 km) by roughly 42 km. So a place on the equator is farther from the earth's centre than a place at the poles.

Worked example

(2 marks) What is meant by the term 'geoid'? Why is the earth not a perfect sphere?

Try it first, then show the solutionSolution

1. Geoid: the actual shape of the earth, bulging at the equator and flattened at the poles - 1 mark.
2. Rotation causes the bulge at the equator - 1 mark.

Answer: A geoid is the true 'earth-like' shape of the earth: slightly flattened at the poles and bulging at the equator. The bulge is caused by the earth's rotation, so it is not a perfect sphere.

Evidence that the earth is round

1. Ship's mast: when a ship approaches from far away, the top of the mast is seen first and the hull later; on a flat earth the whole ship would appear at once.
2. Lunar eclipse: the earth's shadow on the moon is always curved; only a sphere casts a round shadow from every direction.
3. Circumnavigation: travellers going steadily in one direction come back to the starting point, as in the voyage begun by Magellan (1519-1522).
4. Photographs from space show a round earth.
5. Widening horizon: the higher you climb, the farther the horizon, and it always appears circular.
6. Sunrise and sunset occur at different times at different places east and west.
7. The altitude of the Pole Star changes as one travels north or south.

Worked example

(3 marks) Give three proofs that the earth is spherical.

Try it first, then show the solutionSolution

1. Top of a ship's mast appears first on the horizon - 1 mark.
2. Curved shadow of the earth on the moon during a lunar eclipse - 1 mark.
3. Circumnavigation / photographs from space / sunrise at different times - 1 mark.

Answer: (1) An approaching ship's mast is seen before its hull. (2) During a lunar eclipse the earth casts a curved shadow on the moon. (3) Ships sailing continuously in one direction return to their starting point (also photographs from space).

Conditions that support life (1): temperature and water

Suitable distance from the sun: the earth lies in a zone where it is neither too hot (like Venus, average temperature over 400 °C) nor too cold (like Mars). Its average surface temperature is about 15 °C, which suits living organisms.

Presence of liquid water: because of this temperature range, water exists in all three states, mostly as liquid in oceans, rivers and lakes. Water is essential for all living things: it forms most of the body, carries nutrients and supports plant growth. Oceans also moderate climate.

Rotation: the 24-hour rotation spreads heating evenly, so no part becomes extremely hot or cold over long periods.

Worked example

(3 marks) Explain how the earth's distance from the sun helps to support life.

Try it first, then show the solutionSolution

1. The earth is neither too close nor too far from the sun - 1 mark.
2. This gives a moderate average temperature (about 15 °C) - 1 mark.
3. Water can therefore exist as a liquid, which life needs - 1 mark.

Answer: At about 150 million km from the sun, the earth receives the right amount of heat, giving a moderate average temperature of about 15 °C. This allows water to remain liquid, which is essential for life.

Conditions that support life (2): atmosphere and protection

Atmosphere: the earth's gravity holds a blanket of air made mostly of nitrogen (about 78 per cent) and oxygen (about 21 per cent), with small amounts of carbon dioxide and other gases.

• Oxygen is needed by humans and animals for breathing.
• Carbon dioxide is needed by plants for photosynthesis.
• Nitrogen is needed for the growth of plants.
• The atmosphere traps some heat (the natural greenhouse effect), keeping nights from becoming too cold.

Ozone layer: ozone in the stratosphere absorbs most of the harmful ultraviolet rays of the sun.

Magnetic field: deflects harmful charged particles from the sun.

Soil: weathered rock and organic matter form soil, which supports plants and so the food chain.

Worked example

(3 marks) State three ways in which the atmosphere makes life possible on earth.

Try it first, then show the solutionSolution

1. Provides oxygen for breathing and carbon dioxide for photosynthesis - 1 mark.
2. Ozone layer absorbs harmful ultraviolet rays - 1 mark.
3. Traps heat to keep temperatures moderate - 1 mark.

Answer: The atmosphere supplies oxygen for animals and carbon dioxide for plants; its ozone layer absorbs harmful ultraviolet rays; and it traps some heat, keeping temperatures moderate day and night.

The earth as the home of humankind

Because of these conditions, the earth supports millions of species, and human beings have spread to almost every part of it. The earth provides humankind with:

• land for settlement and agriculture;
• water for drinking, irrigation and industry;
• air to breathe;
• natural resources such as forests, minerals and energy sources;
• a variety of climates and environments to which people have adapted.

These conditions are delicate. Pollution, global warming and the depletion of the ozone layer threaten them, so the earth must be protected as our only known home.

Worked example

(2 marks) Why is the earth called a unique planet? State one threat to the conditions that support life.

Try it first, then show the solutionSolution

1. It is the only planet known to support life, because of suitable temperature, water and atmosphere - 1 mark.
2. Threat: pollution / global warming / ozone depletion - 1 mark.

Answer: The earth is unique because it is the only known planet with the combination of moderate temperature, liquid water and a life-supporting atmosphere. Global warming (or pollution, or ozone depletion) threatens these conditions.

Earth as a Planet in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Shape of the Earth and Conditions That Support Life

Earth as a Planet: key facts

Position

Third planet from the sun

Between Venus and Mars; fifth largest planet.

Distance from the sun

About 150 million km (average)

Diameters

$Equatorial\approx 12,756km;polar\approx 12,714km$

Difference ≈ 42 km.

Water cover

About 71% of the surface

Blue planet.

Atmosphere

$Nitrogen\approx 78$

Ozone in the stratosphere.

Average surface temperature

$About15°C$

How the school exam asks Earth as a Planet

The question patterns to expect, and the method that works for each.

Tricks to remember Earth as a Planet

Short memory aids. Say each one aloud once, then use it on a question the same day.

• My Very Educated Mother Just Served Us Noodles

  Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

  Use it when: Order of planets.

• Fat at the waist

  The spinning earth bulges at the equator and flattens at the poles.

  Use it when: Geoid shape.

• T-W-A-O

  Temperature, Water, Atmosphere, Ozone.

  Use it when: Conditions for life.

Earth as a Planet: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 8-mark question split into 2 + 3 + 3 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Earth as a Planet

The slips examiners see most often, and what to write instead.

• ✗Mistake: Calling the earth a perfect sphere

  Everyday language.

  ✓Do this: Write 'geoid' or 'oblate spheroid: flattened at the poles, bulging at the equator'.

  Usually costs: 1 mark.

• ✗Mistake: Listing proofs without explanation

  Students write 'ship's mast' only.

  ✓Do this: Explain what is seen and why it shows curvature.

  Usually costs: 1 mark per proof.

• ✗Mistake: Saying oxygen is the main gas in the atmosphere

  Oxygen is most familiar.

  ✓Do this: Nitrogen ≈ 78%, oxygen ≈ 21%.

  Usually costs: 1 mark.

• ✗Mistake: Giving conditions for life without linking them to life's needs

  Lists like 'water, air, sun'.

  ✓Do this: Say why: liquid water is needed by all organisms; oxygen for respiration; ozone blocks UV.

  Usually costs: 1 mark per point.

Practice questions on Earth as a Planet

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQHarder exam level

   The earth is the ______ largest planet of the solar system.
   (a) third
   (b) fourth
   (c) fifth
   (d) sixth

   Hint

   Four giants are bigger.

   Answer

   (c) fifth.

2. Q2 · 1 mark · MCQExam level

   The true shape of the earth is best described as:
   (a) a perfect sphere
   (b) a geoid
   (c) a flat disc
   (d) a cylinder

   Hint

   Earth-shaped.

   Answer

   (b) a geoid.

3. Q3 · 1 mark · Very short answerExam level

   What percentage of the earth's surface is covered by water?

   Hint

   Roughly seven-tenths.

   Answer

   About 71 per cent.

4. Q4 · 2 marks · Very short answerExam level

   Name the gas that forms the largest part of the atmosphere and the gas needed for breathing.

   Hint

   78 and 21.

   Answer

   Nitrogen (about 78 per cent) and oxygen (about 21 per cent).

5. Q5 · 2 marks · Very short answerHarder exam level

   Why does the horizon appear to widen as we climb higher?

   Hint

   Curvature.

   Answer

   Because the earth's surface is curved; from a greater height we can see farther over the curve, so the circular horizon becomes larger.

6. Q6 · 2 marks · Very short answerHarder exam level

   Why do sunrise times differ between Kolkata and Mumbai?

   Hint

   East and west on a round earth.

   Answer

   The earth is round and rotates from west to east, so places to the east, like Kolkata, face the sun earlier than places to the west, like Mumbai.

7. Q7 · 2 marks · Very short answerHOTS / competency

   How does the earth's gravity help support life?

   Hint

   Holding air.

   Answer

   Gravity holds the atmosphere and water to the earth; without enough gravity these would escape into space, and life could not exist.

8. Q8 · 3 marks · Short answerExam level

   Explain the importance of the ozone layer for life on earth.

   Hint

   UV rays.

   Answer

   Ozone in the stratosphere absorbs most of the sun's harmful ultraviolet radiation. Without it, more UV would reach the surface, causing skin cancer, eye damage and harm to plants and animals.

9. Q9 · 3 marks · Short answerHOTS / competency

   'The conditions that make the earth habitable are delicate.' Explain with two examples of human activity.

   Hint

   Greenhouse gases, CFCs, pollution.

   Answer

   Burning fossil fuels adds carbon dioxide and causes global warming, changing climates; chemicals such as CFCs have thinned the ozone layer, letting more UV through. Pollution of water and air also harms living things. So human activity can upset the balance that supports life.

10. Q10 · 3 marks · Short answerHarder exam level

    Describe two proofs of the earth's sphericity that a traveller can observe for himself.

    Hint

    Ship, Pole Star, horizon.

    Answer

    (1) Watching a ship approach: the mast appears before the hull. (2) Travelling north or south, the Pole Star appears higher or lower in the sky (also the horizon widens with height).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Earth's position from the sun?Show answer

   Third.

2. 2.Average distance from the sun?Show answer

   About 150 million km.

3. 3.Which diameter is longer, equatorial or polar?Show answer

   Equatorial (by about 42 km).

4. 4.Who led the first voyage round the world (1519-1522)?Show answer

   Ferdinand Magellan's expedition.

5. 5.What protects life from ultraviolet rays?Show answer

   The ozone layer.

6. 6.Average surface temperature of the earth?Show answer

   About 15 °C.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Earth: third planet, about 150 million km from the sun, fifth largest, one moon.
• ☐Blue planet: about 71% water.
• ☐Shape: geoid; equatorial ≈ 12,756 km, polar ≈ 12,714 km.
• ☐Proofs: ship's mast, curved eclipse shadow, circumnavigation, space photographs, widening horizon, sunrise times, Pole Star.
• ☐Life: moderate temperature (about 15 °C), liquid water, nitrogen and oxygen, ozone layer, gravity, magnetic field, soil.
• ☐Threats: pollution, global warming, ozone depletion.

Earth as a Planet: frequently asked questions