# Rotation and Revolution

## Class 9 - Total Geography Morning Star

#### Question 1

What is meant by the inclination of earth's axis? At what angle is the earth's axis inclined?

The earth is always tilted in the same direction as it moves round the sun. The angle at which the earth is tilted is known as the inclination of the earth's axis.

The axis of the earth makes an angle of 23 1/2° with the vertical or 66 1/2° with the plane of the earth's orbit.

#### Question 2

Name the two chief motions of the earth.

The two chief motions of the earth are rotation and revolution.

#### Question 3

Describe two characteristics of rotation.

Two characteristics of rotation are-

1. Direction- The earth completes one rotation from west to east on its axis in 23 hours, 56 minutes, 4.09 seconds.

2. Inclination of axis- The earth is always tilted in the same direction as it moves round the sun. The angle at which the earth is tilted is known as the inclination of the earth's axis.

#### Question 4

State two effects of the rotation of the earth.

Two effects of the rotation of the earth are-

1. Day and night are caused by the rotation of the earth on its axis.
2. Rotation causes difference in time over various places on the earth.

#### Question 5

Mention any two possible consequences if the axis of the earth was vertical instead of inclined.

Two possible consequences if the axis of the earth was vertical instead of inclined are-

1. The length of day and night would have been equal at all places on the earth.
2. All the places on the earth would have had the same temperature throughout the year. There would be no seasons.

#### Question 6

State one reason why we do not feel the motions of the earth.

We do not feel the motions of the earth because the force of gravity keeps everything attached to the earth. This keeps all objects at rest with reference to the motions of the earth.

#### Question 7

Define 'revolution'.

The motion of the earth around the sun in an elliptical orbit is called revolution.

#### Question 8

State two chief characteristics of the revolution of the earth.

Two chief characteristics of the revolution of the earth are-

1. The earth moves round the sun in an elliptical orbit at an average speed of 29.8 km per second.
2. When the earth is closer to the sun, the gravitational pull makes the earth move faster than when it is away from the sun. Thus, the speed of revolution is not uniform.

#### Question 9

Mention two effects of revolution.

Two effects of revolution are-

1. Revolution causes different seasons on earth due to differences in the heating of the earth.
2. The spherical shape of the earth along with revolution of the earth causes difference in the distribution of heat on the earth's surface and results in the creation of heat zones.

#### Question 10

On which two days are the days and nights equal all over the world and why? What name do you give to these days?

The days and nights are equal all over the world on March 21st and September 23rd because the sun crosses the plane of the earth's equator making day and night of equal length.

March 21st is known as Spring or Vernal Equinox and September 23rd is known as Autumnal equinox.

#### Question 11

Which is the longest and which is the shortest day in the Northern Hemisphere and why?

The longest day in the Northern Hemisphere is June 21st as the sun's rays fall vertically at the Tropic of Cancer and the North Pole is tilted towards the sun.

The shortest day in the Northern Hemisphere is December 22nd as the sun's rays fall vertically at the Tropic of Capricorn and the North Pole is tilted away the sun.

#### Question 12

On which dates does the sun shine vertically overhead at:

(a) Equator
(b) Tropic of Cancer
(c) Tropic of Capricorn

The sun shines vertically overhead at the:

(a) Equator on 21st March and 23rd September
(b) Tropic of Cancer on 21st June
(c) Tropic of Capricorn on 22nd December

#### Question 13

Why is noon hotter than morning?

Noon is hotter than morning because at noon, the sun is overhead. So, vertical rays of the sun fall at noon making the place hotter than the morning when slanting rays of the sun fall at a place.

#### Question 14

Why are days and nights equal at all places on earth on September 23? What are the seasons in the Northern and the Southern Hemispheres on this date?

Days and nights are equal at all places on earth on September 23 because on this date, the sun shines vertically over the equator, making day and night of equal length.

It is autumn season in the northern hemisphere and spring season in the southern hemisphere.

#### Question 15

What is meant by Summer Solstice? When do we have Summer Solstice in the Northern Hemisphere?

On June 21, the northern hemisphere is tilted towards the sun and the sun is overhead at the Tropic of Cancer. So, the northern hemisphere enjoys the longest day. This day is called Summer Solstice.

We have Summer Solstice in the Northern Hemisphere on June 21st.

#### Question 16

What is meant by Equinox?

Equinox refers to the time when the sun shines vertically over the equator. At the equinoxes, the days and nights are of equal length throughout the world.

#### Question 17

What will be the duration of daylight on March 21st at 23°30'N and 66°30'S latitudes? Give reason.

The duration of daylight on March 21st at 23°30'N and 66°30'S latitudes will be 12 hours because on March 21st, the sun shines vertically over the equator and so the length of day and night is equal all over the world.

#### Question 18

What is the relationship of seasons between the Northern and Southern Hemispheres?

The northern and southern hemispheres always experience opposite seasons. This is because due to the tilted axis of the earth, the north pole remains inclined towards the sun for a part of the year while the south pole is tilted away. For the remaining part of the year, the situation is reversed.

#### Question 19

How has the phenomenon of the 'Midnight Sun' come about?

During the summer solstice in the Northern Hemisphere, the rays of the sun fall vertically on the Tropic of Cancer. The duration of sunlight increases from 12 hours at the equator to 13 hrs 27 minutes at 23 1/2° N and further to 24 hrs at 66 1/2° N. Beyond the Arctic Circle, the regions experience 24 hours daylight. This phenomena is called the ‘Midnight Sun’ and it only takes place at the poles.

#### Question 20

Mention one effect of seasons in low and high latitudes.

The effect of seasons in low and high latitudes is distinct through various phases of different seasons. Low latitude areas experience high temperature and get ample rainfall while the high latitude areas are cold and receive less rainfall.

Tropical regions are always warm with heavy rainfall season, the temperate regions are cold with scanty rainfall and the polar regions are covered with snow throughout the year due to extremely low temperature.

#### Question 21

Draw a well labelled diagram to show changes in the altitude of the midday sun at different times of the year.

## Distinguish between the following

#### Question 1

Rotation and Revolution

RotationRevolution
It is the spinning of the earth on its axis.It is the motion of the earth around the sun in an elliptical orbit.
The time taken for one rotation is 23 hrs 56 min.The time taken for one revolution is 365 days, 5 hrs and 48 min.
It causes day and night.It causes seasonal changes.

#### Question 2

Vertical and Slanting Rays

Vertical RaysSlanting Rays
Vertical rays travel over a shorter distance and thus loose less heat.Slanting rays travel over a larger distance and loose more heat.
Vertical Rays of the sun fall within its migration belt i.e. between the Tropics of Cancer and Capricorn.Slanting Rays of the sun fall in temperate and Polar regions.

#### Question 3

Equinox and Solstice

EquinoxSolstice
It refers to the time when the sun crosses the plane of the earth's equator, making night and day of equal length.It refers to the time when the sun reaches its highest or lowest point in the sky at noon, resulting in the shortest and longest days of the year.
It happens on March 21st and September 23rd.It happens on June 21st and December 22nd.
It happens at the start of spring and autumn.It happens during summer and winter.

## Structured Questions

#### Question 1(a)

What is meant by rotation of the earth? State three effects of the rotation of the earth.

The spinning of the earth on its axis from west to east once in 24 hours is called rotation of the earth.

Three effects of the rotation of the earth are:

1. Day and night are caused by the rotation of the earth on its axis.
2. Rotation causes difference in time over various places on the earth.
3. Sunrise and sunset are caused by the rotation of the earth from west to east.

#### Question 1(b)

What is the effect of the inclined axis of the earth on day and night?

Due to the inclined axis of the earth, the duration of day and night is different at different places of the earth. The time of day at any place is determined by the height of the sun in the sky.

#### Question 1(c)

Give a geographical reason for each of the following:

(i) Kuala Lumpur rotates faster on the earth's axis than London.
(ii) We always see the sun rising in the east.
(iii) Norway is called the Land of the Midnight Sun.

(i) Kuala Lumpur rotates faster on the earth's axis than London because Kuala Lumpur is closer to the equator while London is away from the equator and the speed of rotation is greater at the equator and decreases towards the poles.

(ii) We always see the sun rising in the east because the earth rotates from west to east.

(iii) Norway is called the Land of the Midnight Sun as Norway lies beyond the Arctic circle and hence, the sun is visible just above the horizon continuously for almost three months.

#### Question 1(d)

Draw a well labelled diagram to show inclination of the earth on its axis.

#### Question 2(a)

Describe how the duration of sunlight changes from the Equator to the Poles with respect to the angle of incidence during the solstices.

As the angle of incidence changes from 0° to 90° from Poles towards the Equator, the duration of sunlight also changes.

On June 21st, the equator receives 12 hours of sunlight which increases to 24 hours at the North pole and decreases to 0 hours at the South pole.

Similarly, on December 22nd, the equator receives 12 hours of sunlight which decreases to 0 hours at the North pole and increases to 24 hours at the South pole.

#### Question 2(b)

Describe how seasons are made and reversed between the Northern and Southern Hemispheres.

Due to the tilted axis of the earth, the Northern Hemisphere gets maximum sunlight from March to September and Southern Hemisphere from September to March. So, the seasons are also reversed e.g. the summer season in Northern Hemisphere correspond by winter in Southern Hemisphere and the spring is exchanged with Autumn season.

#### Question 2(c)

Give a geographical reason for each of the following:

(i) The length of day and night is not equal at all places on the earth.
(ii) The period of twilight and dawn increases polewards.
(iii) 25th of December in New Zealand may be one of the hottest days of the year.

(i) The length of day and night is not equal at all places on the earth due to the tilted axis of the earth and the migration belt of the sun between Tropics of Cancer and. Capricorn.

(ii) The period of twilight and dawn increases polewards because of the oblique path of the sun rays towards the poles than the vertical path along the equator. Thus, the length of dawn and twilight is greater, as it is nearly one hour near equator and 50 days at the poles.

(iii) On December 22, the southern hemisphere is tilted towards the Sun. The sun shines directly overhead the Tropic of Capricorn. The places in the southern hemisphere experience the summer season. Therefore, 25th of December in New Zealand may be one of the hottest days of the year.

#### Question 2(d)

Draw a well labelled diagram to show the tilt of the earth's axis and how the revolution causes seasons and variation in the length of day and night.

## Project/Activity

Draw a diagram to show the winter solstice and answer the following questions.

#### Question 1

Name the correct date depicted in your diagram.

December 22nd

#### Question 2

Name the latitude where the sun is shining vertically overhead.

23 1/2° S

#### Question 2

Name the part of the globe where there is night on this date.

The places lying beyond the arctic circle have night on this date.

#### Question 4

Name the latitude on which length of the day will be 24 hours.

66 1/2° S

#### Question 5

Give the date when the earth will be directly opposite to the above position.

June 21st

#### Question 6

What will be the vertical position of the sun on that date?

Sun will shine vertically on Tropic of Cancer (23 1/2° N).

#### Question 7

What is the effect of the inclination of the earth upon the length of day and night?