Rotation and Revolution | ICSE Class 9 Geography Notes

This note covers the Earth's rotation and revolution, the direction and speed of rotation, day and night, the Sun's apparent daily movement, the Coriolis effect, axial inclination, changing daylight, seasons, equinoxes and solstices.

What are rotation and revolution, and how do they differ?

Rotation is the spinning of an object around its own axis. An axis is an imaginary line about which it turns. The Earth's axis passes through its geographic North Pole and South Pole, the two points where this line meets its surface.

Revolution is the movement of one object around another. The Earth revolves around the Sun while continuing to rotate on its axis. Its orbit is the path it follows around the Sun. These movements occur together, but describe different motions.

A season is a recurring part of the year with characteristic weather conditions. The annual sequence of seasons is linked to revolution and the tilted axis, whereas the repeated alternation of daylight and darkness is linked to rotation.

Which movement takes a day, and which takes a year?

The Earth completes one rotation in about 24 hours. It completes one revolution in about 365 days and 6 hours, or nearly one year. Keep the word “about” with these periods: they are useful rounded descriptions.

A spinning top helps illustrate rotation: it turns about its spindle. To represent revolution, follow the whole Earth along its orbit. A globe, a model of the Earth, can represent rotation when it turns on its stand. It does not by itself show revolution.

The distinction also helps separate two questions. “Why does daylight give way to darkness?” concerns rotation. “Why does the length of daylight change through the year?” requires revolution and the tilted axis. The same Earth participates in both processes.

Definition: A hemisphere is one half of a sphere. The Equator, an imaginary circle midway between the poles, divides the Earth into the Northern Hemisphere and Southern Hemisphere.

In which direction and at what speed does the Earth rotate?

The Earth rotates from west to east. Viewed from above the North Pole, this is anticlockwise. The viewpoint matters whenever a drawing uses a clockwise or anticlockwise arrow. West to east gives the geographical direction without leaving the viewing position unstated.

How is angular speed different from linear speed?

Angular speed describes the angle turned through in a given time. The symbol ° means degrees, the units used to measure angles. One complete turn is 360°. Using the rounded rotation period of 24 hours gives an average angular speed of about 15° per hour.

The calculation is 360° divided by 24 hours. This gives the angle of rotation per hour, not a distance travelled over the surface. The approximation comes from using the rounded daily period. No particular surface distance is needed for this calculation.

Linear speed describes the distance travelled in a given time. Surface points away from the axis move along circles as the Earth turns. These circles become smaller towards the poles, so the distance travelled during one rotation also becomes smaller.

The linear speed due to rotation is greatest at the Equator and decreases towards the poles. At the geographic poles it is zero because these points lie on the axis. This does not mean that the rest of the Earth stops rotating.

How should a rotation diagram be read?

The globe shows the axis, North Pole, South Pole, Equator and both hemispheres. A curved arrow beside the Equator points from west to east. The abbreviations NP and SP mean North Pole and South Pole.

See Fig. 12.3 in your NCERT textbook

Keep the two measures separate: all places share the Earth's turning motion, but places at different distances from the axis trace circles of different sizes. An answer about speed should say whether it means an angle per hour or distance per hour.

How does rotation produce day and night?

Sunlight illuminates the half of the Earth facing the Sun. This half experiences daytime. The half facing away is dark and experiences night. Rotation continually changes which parts of the surface face towards the Sun.

The circle of illumination is the boundary separating the sunlit half from the dark half in the simplified Earth-Sun model. It marks a lighting boundary. It is not a line drawn on the ground, and it should not be confused with the Equator.

What happens to a place during the daily cycle?

1. The Earth rotates from west to east while sunlight falls on the side facing the Sun.
2. A place moving from the dark side into the illuminated side experiences sunrise.
3. While the place remains on the illuminated side, it experiences daytime.
4. As the place moves from the illuminated side into darkness, it experiences sunset and then night.

This sequence explains why different parts of the Earth can have different lighting conditions at the same time. A place facing the Sun can be in daylight while another place on the opposite side experiences darkness. The Sun does not need to circle the Earth each day.

Parallel yellow sunrays approach the globe from the left. The left side is illuminated and the right side is dark. The tilted axis and the North Pole are marked above the globe.

See Fig. 12.5 in your NCERT textbook

Does every place receive twelve hours of daylight every day?

A rotation of about 24 hours includes both daytime and night-time. It does not require each part to last twelve hours at every place. The proportion spent in sunlight depends on position on the Earth and the seasonal arrangement of its tilted axis.

Polar conditions make this distinction especially clear. The North Pole receives sunlight for all 24 hours in June and is in darkness for all 24 hours in December. The Earth continues rotating during both conditions. Continuous daylight does not mean rotation has stopped.

Why does the Sun appear to rise in the east and set in the west?

Apparent motion means movement as it seems to an observer. The Sun appears to move across the sky from east to west because we observe it from the rotating Earth. The Earth's rotation is in the opposite direction, from west to east.

How does a merry-go-round explain apparent motion?

Imagine sitting on a merry-go-round and facing outwards while it turns anticlockwise. A stationary tree appears to move in the opposite direction. The tree has not started circling the rider; the changing view results from the rider's own turning motion.

Likewise, a person on the Earth changes viewing direction as the Earth rotates. Sunrise occurs as the person's location enters the illuminated part of the Earth. Sunset occurs as it enters the dark part. Both observations belong to the same daily rotation.

Use eastward direction and westward direction when describing sunrise and sunset generally. These expressions state the observed broad directions. They do not require the claim that sunrise and sunset occur at exactly the same points throughout the year.

Case study: What does the globe activity show about sunrise in India?

In India, sunrise first occurs in the eastern part and then in other parts. To investigate this, illuminate a globe with a torch and examine India while turning the globe. The eastern part enters the light first when the globe turns from west to east.

This activity connects the observed order of sunrise with the Earth's direction of rotation. Keep the torch in place and change the globe's orientation. Reversing the globe's turning direction changes which part enters the light first, showing why the correct rotation arrow matters.

The explanation needs no claim that the Sun travels around the Earth during a day. Describe the observer's changing position first, and then the Sun's apparent movement. This keeps the physical cause and the visible result clearly linked.

What is the Coriolis effect, and how is it related to rotation?

The Coriolis effect is the apparent deflection, or change in direction, of motion observed on the rotating Earth. The atmosphere is the envelope of gases surrounding the Earth. Rotation affects the direction of moving air, called wind.

Winds are deflected to their right in the Northern Hemisphere and to their left in the Southern Hemisphere. Here, right and left refer to the direction in which the wind is travelling. They do not simply mean the right and left edges of a page.

How does the effect vary?

The Coriolis effect on horizontal wind is absent at the Equator and greatest at the poles. Deflection is greater when wind velocity is high. Velocity describes both the speed and direction of motion; it therefore tells more than speed alone.

Latitude is angular distance north or south of the Equator. It provides the geographical reference for comparing the Equator, the hemispheres and the poles. A location's latitude and a wind's direction of movement answer different parts of a deflection question.

Does the Coriolis effect start the wind?

Atmospheric pressure is the force exerted by the weight of air on a unit area. Differences in atmospheric pressure set air in motion, from higher pressure towards lower pressure. Rotation affects the direction of that movement; the two roles should be distinguished.

A full explanation should therefore connect the effect to rotation, identify the hemisphere and state the direction of deflection. Saying merely that “winds turn” leaves out the geographical rule. Saying that every wind is turned eastwards would also misread the right-and-left rule.

How do revolution and the inclined axis work together?

The Earth's orbit is nearly circular when viewed from above. An elongated drawing can represent a side view of the orbit rather than its actual appearance from above. Do not infer a very stretched orbit simply from the shape used in a perspective diagram.

The orbital plane is the imaginary flat surface containing the Earth's path around the Sun. The axis makes an angle of 66½° with this plane. Equivalently, it is tilted 23½° from a line perpendicular to the plane; perpendicular means at a right angle.

Does the tilt change direction during the annual circuit?

In the annual model, the Earth maintains its tilt as it revolves around the Sun. The axes drawn at successive orbital positions should point in the same direction. The changing position around the Sun then changes which hemisphere is tilted towards the Sun.

This arrangement is often described as parallelism of the axis: the axes shown at the different positions remain parallel, or run in the same direction. It explains why the Northern Hemisphere faces towards the Sun in June and away from it in December.

The Sun is surrounded by Earth positions labelled March, June, September and December. North Pole labels accompany the tilted globes. June is labelled summer in the Northern Hemisphere, and December winter. Sizes and distances are not to scale.

See Fig. 12.9 in your NCERT textbook

Why are both tilt and revolution needed in the explanation?

Revolution changes the Earth's position around the Sun during the year. The maintained axial tilt makes the two hemispheres receive sunlight differently at different positions. The spherical surface also affects the area over which incoming rays spread. Together these explain seasonal differences in heating.

An upright-axis model would lack this alternating tilt of one hemisphere towards the Sun and the other away from it. The globe demonstration therefore needs a tilted axis kept in the same direction as the globe moves around the lamp.

Why do the seasons change during the year?

There are four main seasons across most regions of the world: spring, summer, autumn and winter. Regional patterns can differ, so these names do not imply identical weather everywhere.

How does the angle of sunlight affect heating?

Insolation means incoming solar radiation, the energy received from the Sun. More nearly vertical rays concentrate a given amount of energy over a smaller surface area. Oblique rays, which arrive at a slant, spread the same energy across a larger area.

Energy per unit area is therefore lower under the more slanting rays. Slanting rays also pass through a greater depth of atmosphere. More of their energy is absorbed or scattered before reaching the surface.

How does the duration of daylight affect heating?

A hemisphere tilted towards the Sun experiences more intense sunlight and longer daylight. It receives sunlight for a greater part of the daily rotation. A hemisphere tilted away experiences less intense sunlight and a shorter period of daylight. Both differences contribute to the seasonal contrast.

1. The Earth revolves around the Sun while maintaining its tilted axis.
2. At one part of the orbit, one hemisphere is tilted towards the Sun while the other is tilted away.
3. The hemisphere tilted towards the Sun receives more concentrated sunlight and longer days, producing summer conditions.
4. At the opposite part of the orbit, the arrangement reverses, bringing winter to that hemisphere and summer to the other.

Note: Seasons are not caused by a hemisphere simply being closer to the Sun when tilted towards it. Differences in distance are very small and do not explain the seasonal pattern. The Earth is closest to the Sun in January.

The two hemispheres experience opposite seasons. That contrast is a useful check on any explanation based only on distance from the Sun: both hemispheres belong to the same Earth, yet one has summer while the other has winter.

What happens at the June and December solstices?

A solstice marks one of the two annual extremes of daytime length in a hemisphere. In the Northern Hemisphere, the summer solstice occurs around 21 June, with the longest day, and the winter solstice around 22 December, with the shortest day.

What is the arrangement in June?

In June, the Northern Hemisphere is tilted towards the Sun and the Southern Hemisphere away from it. Northern places experience longer daytime and more intense sunlight, while southern places experience the opposite. The North Pole receives sunlight throughout the 24-hour rotation.

The Tropic of Cancer, the parallel of latitude at 23½° north, receives the vertical midday rays around the June solstice. A parallel of latitude is an imaginary east-west circle joining places with the same latitude.

What is the arrangement in December?

In December, the Southern Hemisphere is tilted towards the Sun. The Tropic of Capricorn, at 23½° south, receives the vertical midday rays around the December solstice. The Northern Hemisphere experiences shorter daylight, and the North Pole remains dark throughout the 24-hour rotation.

Case study: Why are December conditions different in Australia?

Australia lies in the Southern Hemisphere. When the Northern Hemisphere has winter in December, Australia has summer. A journey from northern winter to Australia therefore crosses into the opposite seasonal situation; the month alone cannot identify the season without the hemisphere.

The explanation follows the same sequence as the general model: identify the hemisphere, decide which way it is tilted relative to the Sun, and connect that tilt to sunlight intensity and day length. Do not assume that December means winter across the whole world.

What happens at the equinoxes, and how do the four key positions compare?

An equinox is one of the two annual positions when daytime and night-time are equal in the simplified seasonal model. The equinoxes occur around 21 March and 23 September. Daytime lasts twelve hours, with twelve hours of darkness.

At an equinox, neither hemisphere is tilted towards the Sun. The vertical midday rays fall on the Equator. This differs from a solstice, when the direct rays reach one of the tropics and the contrast in day length between the hemispheres is greatest.

Which seasons begin in the Northern Hemisphere?

The March equinox is the spring equinox in the Northern Hemisphere. The September equinox is the autumn equinox there. The Southern Hemisphere has the reverse seasonal sequence. Always identify the hemisphere before attaching spring, summer, autumn or winter to a position.

How does day length change between these positions?

After the Northern Hemisphere's summer solstice, daytime becomes shorter and night-time longer. By the autumn equinox, day and night are equal. Daytime then continues towards its winter minimum. The named positions are stages within a continuing annual movement.

Keep the dates qualified as around the stated calendar days. The date table is an approximate seasonal guide, not a claim that each astronomical event occurs on an unchanging date every year. Its main purpose is to connect position, hemisphere and lighting.

When reading an orbital diagram, follow its arrows and month labels together. Do not identify a season merely because a globe appears on the left or right side of the page: the direction of the tilt relative to the Sun supplies the explanation.

How can a globe model explain changing day length?

A lamp or torch can represent the Sun in a globe demonstration. Together they show which part of the Earth receives light and how a place's exposure changes as the Earth rotates and revolves.

How should the demonstration be arranged?

1. Mark a location on a globe with a small sticker so that its movement can be followed.
2. Place a torch at some distance and shine it towards the globe in a relatively dark room.
3. Rotate the globe from west to east and observe the sticker entering and leaving the illuminated half.
4. Using a lamp, move the globe to different positions around it while keeping the tilted axis pointing in the same direction.
5. At each position, rotate the globe again and compare how long the marked location remains in light and darkness.

The first rotation demonstrates sunrise, daytime, sunset and night. Moving the tilted globe around the lamp adds the annual change. A model that spins in one fixed place demonstrates the daily cycle but cannot show the full sequence of seasonal positions.

Two tilted globes receive rays from opposite sides of the page. June shows the North Pole in light and longer northern daytime; December shows it in darkness and shorter northern daytime. Labels compare daylight with twelve and twenty-four hours.

See Fig. 12.11 in your NCERT textbook

What should be checked before explaining the result?

First locate the Sun, then the axis and poles, and finally the marked place. Follow the rotation arrow to see whether that place is entering sunlight or darkness. For seasons, check which hemisphere is tilted towards the Sun before naming the season.

Remember that diagrams simplify sizes and distances. The orbital diagram is not to scale, and a side view makes the orbit look elongated. Use the drawing to study the relationships among tilt, position and illumination rather than measuring distances from the picture.

Glossary

• Rotation — The spinning movement of an object around its own imaginary axis.
• Axis — The imaginary line about which the Earth turns, passing through its geographic poles.
• Revolution — The movement of one object around another, as the Earth moves around the Sun.
• Orbit — The path followed by an object as it revolves around another object.
• Hemisphere — One half of the Earth, such as the half north of the Equator.
• Latitude — The angular distance of a place north or south of the Equator.
• Angular speed — The angle through which a rotating object turns in a given time.
• Linear speed — The distance travelled by a moving point during a given amount of time.
• Circle of illumination — The boundary dividing the illuminated and dark halves of the Earth in a simplified model.
• Apparent motion — Movement as seen by an observer, which may result from the observer's own movement.
• Coriolis effect — The apparent deflection of motion observed on the rotating Earth, affecting wind direction.
• Orbital plane — The imaginary flat surface containing the Earth's path of revolution around the Sun.
• Insolation — Incoming solar radiation, meaning the energy that the Earth receives from the Sun.
• Solstice — Either annual position associated with the longest or shortest daytime in a hemisphere.
• Equinox — Either annual position with equal daytime and night-time in the simplified seasonal model.

Common errors and misconceptions

• Misconception: Rotation and revolution describe the same movement. Correct: Rotation is spinning on the axis; revolution is movement around the Sun. Both occur together.
• Misconception: The Earth rotates from east to west because the Sun appears to do so. Correct: Earth rotates west to east, producing the opposite apparent daily motion.
• Misconception: A 24-hour rotation means twelve hours of daylight everywhere every day. Correct: Daytime length changes with location and season; polar daylight can last throughout a rotation.
• Misconception: Angular speed and linear speed mean the same thing. Correct: Angular speed measures angle per time; linear speed measures distance per time and varies with distance from the axis.
• Misconception: Coriolis deflection turns all winds eastwards. Correct: It is rightward in the Northern Hemisphere and leftward in the Southern Hemisphere, relative to the wind's motion.
• Misconception: Summer occurs because the Earth is closer to the Sun. Correct: Seasonal changes follow axial tilt, changing sunlight intensity and day length during revolution.
• Misconception: The axis makes 23½° with the orbital plane. Correct: It makes 66½° with that plane and 23½° with a perpendicular to it.
• Misconception: June is summer and December winter everywhere. Correct: Those are Northern Hemisphere seasons; the Southern Hemisphere has the opposite pattern.

Exam-style questions with model answers

Q1. The Earth spins on its own axis in about 24 hours and travels around the Sun in about 365 days and 6 hours. Name and distinguish these two movements. [2 marks]

1. Rotation is the Earth's spinning on its own axis, completed in about 24 hours.
2. Revolution is its movement around the Sun along its orbit, completed in about 365 days and 6 hours.

Q2. A full turn is 360°. Use a rounded rotation period of 24 hours to calculate the Earth's average angular speed in degrees per hour, showing the calculation. [2 marks]

1. Average angular speed is the total angle turned through divided by the time taken: 360° divided by 24 hours.
2. The result is about 15° per hour. It is approximate because the question uses a rounded rotation period.

Q3. A stationary torch illuminates half a globe. A sticker starts in darkness, and the globe turns from west to east. Explain the sticker's passage through sunrise, daytime and sunset. [3 marks]

1. As rotation carries the sticker from darkness into the illuminated half, it represents a place experiencing sunrise. The place moves into the light; the torch remains stationary.
2. While the sticker is within the illuminated half, the represented place experiences daytime because it faces the light source.
3. When rotation carries the sticker out of the illuminated half, sunset occurs at the represented place. It then experiences night on the dark side.

Q4. Use this information: rotation deflects horizontal winds rightward in the Northern Hemisphere and leftward in the Southern Hemisphere; the effect is absent at the Equator. Identify the effect and explain the two directional rules and the equatorial condition. [4 marks]

1. This is the Coriolis effect, the apparent deflection of motion observed on the rotating Earth. Rotation affects the direction of moving air.
2. In the Northern Hemisphere, a wind is deflected to its right, judged relative to its own direction of travel.
3. In the Southern Hemisphere, a wind is deflected to its left, again relative to its own direction of travel.
4. At the Equator, horizontal wind has no Coriolis deflection. The right-and-left rules therefore do not imply deflection at the Equator.

Q5. In June the Northern Hemisphere is tilted towards the Sun and receives more concentrated sunlight for longer each day. In December it is tilted away, receives less concentrated sunlight and has shorter days. Explain its seasonal contrast in five points. [5 marks]

1. In June, the Northern Hemisphere is tilted towards the Sun. This orientation establishes the arrangement that favours its summer conditions.
2. June sunlight is more concentrated: a given amount of solar energy falls over a smaller area, producing greater heating of that area.
3. Daylight also lasts longer in June, so the hemisphere receives sunlight for a greater part of each daily rotation.
4. In December, the Northern Hemisphere is tilted away from the Sun. Less concentrated sunlight spreads the received energy across a larger area.
5. December also has shorter days, reducing the daily duration of sunlight. Together, weaker intensity and shorter exposure explain winter compared with June summer.

Q6. For the Northern Hemisphere, the summer solstice is around 21 June with the longest day, the winter solstice around 22 December with the shortest day, and equinoxes around 21 March and 23 September have twelve-hour daytime within a 24-hour day-night cycle. Compare these four events. [4 marks]

1. The summer solstice occurs around 21 June and marks the Northern Hemisphere's longest daytime, distinguishing it from the equal-day equinox positions.
2. The winter solstice occurs around 22 December and marks its shortest daytime, the opposite daylight extreme to the June solstice.
3. The March equinox occurs around 21 March, when daytime lasts twelve hours rather than reaching either solstice extreme.
4. The September equinox occurs around 23 September and also has twelve-hour daytime. The two equinoxes share equal day and night conditions.

Q7. Australia is in the Southern Hemisphere. In December that hemisphere is tilted towards the Sun, has more intense sunlight and longer days; the Northern Hemisphere has the reverse conditions. Explain Australia's season and the northern contrast in three points. [3 marks]

1. Australia experiences summer in December because it lies in the Southern Hemisphere, which is then tilted towards the Sun.
2. The more intense sunlight provides more concentrated energy, while longer days extend the duration of solar heating. Both features support summer conditions.
3. The Northern Hemisphere experiences winter at the same time because its opposite orientation gives it less intense sunlight and shorter days.

Q8. A globe is moved around a lamp with its tilted axis pointing in the same direction. At every position it rotates west to east. In June its North Pole is lit throughout one rotation; in December that pole remains dark. Explain what this model demonstrates in five points. [5 marks]

1. Turning the globe about its own axis represents rotation. The west-to-east direction reproduces the direction of the Earth's daily spinning motion.
2. Moving the globe around the lamp represents revolution. The globe changes position around the light source while it also continues rotating.
3. Keeping the tilted axis pointing in the same direction represents the maintained axial orientation in the annual model of the Earth.
4. The illuminated North Pole in June shows that a place can remain in sunlight throughout one rotation. Rotation does not require a dark interval at that pole.
5. The dark North Pole in December shows the opposite seasonal lighting condition. The comparison connects revolution and maintained tilt with changing daylight duration.

Key takeaways

• Rotation is spinning on the Earth's own axis; revolution is its simultaneous movement around the Sun.
• The Earth rotates west to east in about 24 hours, appearing anticlockwise when viewed from above the North Pole.
• Rotation produces day and night and explains the Sun's apparent movement from east to west across the sky.
• Coriolis deflection is rightward in the Northern Hemisphere and leftward in the Southern Hemisphere, relative to motion.
• The axis makes 66½° with the orbital plane and maintains its orientation through the annual seasonal model.
• Changing sunlight intensity and day length explain seasons as the tilted Earth revolves around the Sun.
• Solstices mark extremes of daytime length; equinoxes have equal daytime and night-time in the simplified model.
• The hemispheres experience opposite seasons: northern June summer corresponds to southern winter, and December reverses this arrangement.

Test yourself

Which viewpoint makes the Earth's rotation appear anticlockwise?

Viewed from above the North Pole, the Earth's west-to-east rotation appears anticlockwise.

What is the difference between an axis and an orbit?

An axis is the imaginary line about which an object spins. An orbit is its path around another object.

Why does a place experience sunrise?

Rotation carries the place from the dark side of the Earth into the sunlit side.

What does rightward Coriolis deflection mean?

It means deflection to the right of the moving wind's direction, as occurs in the Northern Hemisphere.

Why must a seasonal globe model keep its axis pointing in the same direction?

This represents the maintained axial orientation as the Earth revolves, allowing alternate hemispheres to tilt towards the Sun.

Which two changes make summer sunlight more effective at heating a hemisphere?

More concentrated sunlight supplies more energy per unit area, and longer days extend the duration of sunlight.

How does an equinox differ from a solstice?

An equinox has equal day and night in the simplified model. A solstice marks a hemisphere's longest or shortest daytime.

Does continuous June daylight at the North Pole mean the Earth has stopped rotating?

No. The Earth continues rotating, but the North Pole remains within the illuminated part throughout the rotation.