Rotation and Revolution ICSE Class 9: Day, Night and Seasons

What Rotation and Revolution 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). Rotation and Revolution is topic 1(c), Our World, of the 2028 syllabus.Source: CISCE ICSE 2028 Geography syllabus, Class IX

Key concepts

Rotation: direction and speed

Rotation is the spinning of the earth on its axis.

• Direction: from west to east. Seen from above the North Pole, it turns anticlockwise.
• Time: one rotation relative to the sun takes 24 hours (a solar day); relative to the stars it takes about 23 hours 56 minutes.
• Speed: every point completes one circle in the same time, but points on larger circles travel farther. At the equator the circumference is about 40,000 km, so the speed is about 1,670 km per hour. The speed decreases towards the poles, where it is zero.
• Circle of illumination: the line separating the lit half of the earth from the dark half.

We do not feel the motion because it is smooth and steady, and the atmosphere moves with the earth.

Worked example

(2 marks) In which direction does the earth rotate? Why is its speed of rotation greatest at the equator?

Try it first, then show the solutionSolution

1. From west to east (anticlockwise seen from above the North Pole) - 1 mark.
2. The equator is the largest circle; points there cover the greatest distance in the same 24 hours - 1 mark.

Answer: The earth rotates from west to east. Its speed is greatest at the equator (about 1,670 km/h) because points there travel round the largest circle in the same 24 hours.

Effects of rotation: day and night, sunrise in the east

Day and night: at any moment only half the earth faces the sun. As the earth rotates, places move from the lit half into the dark half and back, so each place has day and night in turn. Without rotation, one side would have permanent day and the other permanent night.

The sun rises in the east and sets in the west: because the earth turns from west to east, the sun appears to move in the opposite direction, from east to west, across the sky. This is an apparent motion; the sun does not move round the earth.

Other effects: places to the east see the sun earlier, giving differences in local time; rotation also helps cause the daily rhythm of tides and keeps temperatures from becoming extreme on any one side.

Worked example

(3 marks) Explain why (i) we have day and night, (ii) the sun appears to rise in the east.

Try it first, then show the solutionSolution

1. Only half the earth faces the sun at a time - 1 mark.
2. Rotation brings each place into light and darkness in turn - 1 mark.
3. The earth turns west to east, so the sun appears to move east to west, rising in the east - 1 mark.

Answer: (i) Only the half of the earth facing the sun has day; as the earth rotates, each place passes in turn through the lit and dark halves. (ii) Because the earth turns from west to east, the sun appears to move the opposite way, rising in the east and setting in the west.

Coriolis effect

Because the earth rotates, and points at different latitudes move at different speeds, freely moving objects such as winds and ocean currents do not travel in straight lines relative to the earth's surface. They are deflected:

• to the right in the Northern Hemisphere;
• to the left in the Southern Hemisphere.

This is the Coriolis effect (or Coriolis force), and the rule is known as Ferrel's law.

The effect is zero at the equator and increases towards the poles.
Importance: it changes the direction of planetary winds (for example the trade winds blow from the north-east in the Northern Hemisphere, not from the north), makes cyclones spin anticlockwise in the Northern Hemisphere and clockwise in the Southern, and shapes the circulation of ocean currents.

Worked example

(3 marks) What is the Coriolis effect? How does it affect winds in the two hemispheres?

Try it first, then show the solutionSolution

1. Deflection of moving air or water caused by the earth's rotation - 1 mark.
2. Northern Hemisphere: deflected to the right - 1 mark.
3. Southern Hemisphere: deflected to the left; zero at the equator, maximum at the poles - 1 mark.

Answer: The Coriolis effect is the deflection of freely moving bodies such as winds and currents due to the earth's rotation. Winds are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere; the effect is zero at the equator and strongest near the poles.

Revolution and the inclined axis

Revolution is the movement of the earth around the sun in a slightly elliptical orbit.

• Time: about 365¼ days. The extra quarter day is adjusted by adding a day (29 February) every fourth year, the leap year.
• Speed: about 30 km per second.
• The earth is nearest the sun (perihelion) in early January and farthest (aphelion) in early July; this small change in distance is not the cause of seasons.

Inclined axis: the earth's axis is tilted at 23½° from the perpendicular to the plane of its orbit; in other words it makes an angle of 66½° with the orbital plane. The axis always points in the same direction in space (towards the Pole Star). This is called the parallelism of the axis.

Because of the tilt and its fixed direction, first the Northern and then the Southern Hemisphere leans towards the sun as the earth moves round its orbit.

Worked example

(2 marks) At what angle is the earth's axis inclined to the plane of its orbit? What is meant by the parallelism of the axis?

Try it first, then show the solutionSolution

1. 66½° to the orbital plane (23½° from the perpendicular) - 1 mark.
2. The axis points in the same direction (towards the Pole Star) throughout the year - 1 mark.

Answer: The axis is inclined at 66½° to the plane of the orbit (23½° from the vertical). Parallelism means the axis always points in the same direction in space, towards the Pole Star, at every position in the orbit.

Effect: variation in the length of day and night

Because of the tilted axis, the circle of illumination does not usually pass through the poles. So, except at the equinoxes, the hemisphere tilted towards the sun has more than half of each parallel in light: its days are longer than its nights. The other hemisphere has shorter days.

• At the equator, day and night are always about 12 hours each, because the circle of illumination always divides the equator equally.
• The farther a place is from the equator, the greater the variation through the year.
• Inside the polar circles, there is at least one day of 24-hour daylight in summer and one of 24-hour darkness in winter.
• At the poles, there are about six months of continuous day and six months of continuous night.

Worked example

(3 marks) Why are days and nights always nearly equal at the equator but very unequal near the poles?

Try it first, then show the solutionSolution

1. Because of the tilt, the circle of illumination usually does not pass through the poles - 1 mark.
2. It always divides the equator into two equal halves, so day ≈ night ≈ 12 h there - 1 mark.
3. Near the poles most or all of a parallel may be in light or in darkness, so days or nights become very long - 1 mark.

Answer: The tilted axis means the circle of illumination usually does not pass through the poles. It still divides the equator equally, so days and nights there stay about 12 hours. Near the poles, most or all of each parallel can be in light (summer) or darkness (winter), giving very long days or nights, even six months at the poles.

Effect: seasons, equinoxes and solstices

As the earth revolves with its axis tilted, the vertical (overhead) rays of the midday sun shift between the Tropic of Cancer and the Tropic of Capricorn. The hemisphere tilted towards the sun receives more direct rays and longer days, so it has summer; the other has winter.

• 21 June: summer solstice in the Northern Hemisphere. The North Pole tilts towards the sun; the sun is overhead at the Tropic of Cancer; the Northern Hemisphere has its longest day; areas north of the Arctic Circle have 24-hour daylight. It is winter in the Southern Hemisphere.
• 22 December: winter solstice in the Northern Hemisphere. The sun is overhead at the Tropic of Capricorn; the Southern Hemisphere has summer and its longest day; areas south of the Antarctic Circle have 24-hour daylight.
• 21 March (vernal or spring equinox) and 23 September (autumnal equinox): neither pole tilts towards the sun; the sun is overhead at the equator; the circle of illumination passes through both poles; day and night are equal (12 hours each) everywhere. Spring and autumn follow.

'Solstice' means 'sun standing still': the overhead sun seems to pause at a tropic before turning back. 'Equinox' means 'equal night'.

Worked example

(3 marks) What happens on 21 June? Mention the position of the overhead sun, the length of day in the Northern Hemisphere and the season in the Southern Hemisphere.

Try it first, then show the solutionSolution

1. The sun is overhead at the Tropic of Cancer (23½°N) - 1 mark.
2. Longest day in the Northern Hemisphere; 24-hour day north of the Arctic Circle - 1 mark.
3. Winter in the Southern Hemisphere, with its shortest day - 1 mark.

Answer: On 21 June (summer solstice) the midday sun is overhead at the Tropic of Cancer; the Northern Hemisphere has its longest day and areas north of the Arctic Circle have continuous daylight; the Southern Hemisphere has winter and its shortest day.

Rotation and Revolution 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

• Rotation, Revolution, Day and Night and the Seasons

Rotation and Revolution: key facts

Rotation

West to east; 24 h (solar day); about 1,670 km/h at the equator

Zero at the poles.

Revolution

$About365¼days;about30km/s$

Leap year every 4 years.

Axis

$66½°toorbitalplane=23½°fromperpendicular$

Parallelism: always points to the Pole Star.

Solstices

21 June: sun over Tropic of Cancer; 22 December: over Tropic of Capricorn

Equinoxes

$21Marchand23September:sunovertheequator;day=nighteverywhere$

Coriolis effect

Right in N. Hemisphere, left in S. Hemisphere

Ferrel's law; zero at the equator.

How the school exam asks Rotation and Revolution

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

Tricks to remember Rotation and Revolution

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

• J-C, D-C

  June → Cancer; December → Capricorn.

  Use it when: Overhead sun on solstices.

• Equinox = equal night

  21 March and 23 September: 12 hours each everywhere.

  Use it when: Equinox questions.

• North turns Right

  Coriolis: Northern Hemisphere deflects Right; Southern deflects Left.

  Use it when: Wind and current directions.

Rotation and Revolution: 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 Rotation and Revolution

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

• ✗Mistake: Saying seasons are caused by changing distance from the sun

  A common misconception.

  Wrong working

  It is summer when the earth is nearest the sun.

  Right working

  Seasons are caused by the tilt and parallelism of the axis during revolution.

  ✓Do this: Remember: aphelion is in July, northern summer.

  Usually costs: 2-3 marks.

• ✗Mistake: Wrong Coriolis direction

  Hemispheres swapped.

  ✓Do this: North: right; South: left.

  Usually costs: 1 mark.

• ✗Mistake: Mixing up the solstice dates and tropics

  Similar names.

  ✓Do this: June-Cancer, December-Capricorn.

  Usually costs: 1 mark.

• ✗Mistake: Giving 23½° as the angle with the orbital plane

  Two ways of stating the tilt.

  ✓Do this: 66½° with the orbital plane = 23½° from the perpendicular.

  Usually costs: 1 mark.

Practice questions on Rotation and Revolution

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 · MCQExam level

   The earth rotates from:
   (a) east to west
   (b) west to east
   (c) north to south
   (d) south to north

   Hint

   The sun rises in the east.

   Answer

   (b) west to east.

2. Q2 · 1 mark · MCQExam level

   On which date are day and night equal all over the world?
   (a) 21 June
   (b) 22 December
   (c) 23 September
   (d) 4 July

   Hint

   Equinox.

   Answer

   (c) 23 September.

3. Q3 · 2 marks · Very short answerExam level

   What is a leap year and why is it needed?

   Hint

   365¼ days.

   Answer

   A year of 366 days, with 29 February added every fourth year, because revolution takes about 365¼ days and the four quarter days add up to one day.

4. Q4 · 2 marks · Very short answerHarder exam level

   Define the circle of illumination.

   Hint

   Boundary of light.

   Answer

   The imaginary circle that divides the half of the earth lit by the sun from the half in darkness.

5. Q5 · 2 marks · Very short answerHOTS / competency

   Why do cyclones in the Northern Hemisphere rotate anticlockwise?

   Hint

   Air flows inward and is deflected.

   Answer

   Air flows inward towards the low-pressure centre and the Coriolis effect deflects it to the right, setting up an anticlockwise spin around the centre.

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

   Why do we not feel the earth's rotation?

   Hint

   Smooth and shared.

   Answer

   The motion is smooth and steady, and everything around us, including the atmosphere, moves with the earth, so there is no relative motion to feel.

7. Q7 · 2 marks · Very short answerHarder exam level

   Describe the conditions at the North Pole on 22 December.

   Hint

   Tilted away.

   Answer

   The North Pole is tilted away from the sun, so it is in continuous darkness (polar night); areas north of the Arctic Circle have 24-hour night.

8. Q8 · 3 marks · Short answerExam level

   Distinguish between rotation and revolution on three points.

   Hint

   Motion, time, effect.

   Answer

   (1) Rotation is spinning on the axis; revolution is moving round the sun. (2) Rotation takes about 24 hours; revolution about 365¼ days. (3) Rotation causes day and night and the Coriolis effect; revolution with the tilted axis causes seasons and varying day length.

9. Q9 · 3 marks · Short answerHarder exam level

   Explain why it is summer in the Southern Hemisphere in December.

   Hint

   Tilt and overhead sun.

   Answer

   In December the South Pole is tilted towards the sun; the overhead sun is at the Tropic of Capricorn, so the Southern Hemisphere receives more direct rays and longer days, giving summer.

10. Q10 · 3 marks · Short answerHOTS / competency

    A place has 14 hours of daylight on 21 June. Will it have more or less than 12 hours of daylight on 22 December? Is it in the Northern or Southern Hemisphere? Explain.

    Hint

    Longer day in June.

    Answer

    It has a long day on 21 June, when the Northern Hemisphere leans towards the sun, so it is in the Northern Hemisphere. On 22 December the Northern Hemisphere leans away, so it will have less than 12 hours of daylight (about 10 hours).

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.Time for one rotation (solar day)?Show answer

   24 hours.

2. 2.Time for one revolution?Show answer

   About 365¼ days.

3. 3.Tilt of the axis from the perpendicular?Show answer

   23½°.

4. 4.Where is the sun overhead on 21 June?Show answer

   Tropic of Cancer.

5. 5.Dates of the equinoxes?Show answer

   21 March and 23 September.

6. 6.Coriolis deflection in the Southern Hemisphere?Show answer

   To the left.

Quick revision checklist

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

• ☐Rotation: west to east; 24 h; about 1,670 km/h at equator, 0 at poles.
• ☐Effects: day and night, sun rises east, Coriolis (right N, left S), time difference.
• ☐Revolution: about 365¼ days; elliptical orbit; leap year.
• ☐Axis: 66½° to orbit, 23½° from perpendicular; parallelism (towards the Pole Star).
• ☐Day length: constant 12 h at equator; varies more towards the poles; 6 months day/night at poles.
• ☐21 June: sun over Cancer; 22 December: over Capricorn; 21 March and 23 September: over equator, day = night everywhere.

Rotation and Revolution: frequently asked questions