Chapter 3 Motions of the Earth

Earth practices two types of motion: Rotation and Revolution.
Axis is an imaginary line which makes an angle of 66 ½◦.
Days and nights are caused due to rotation.
Seasons are caused due to revolution.
Circle of illumination is the circle which divides day from night on globe.
Leap year comes after every 4 years and at that time, February has 29 days. It means, earth usually have 365 days but in the leap year earth will have 366 days.
The longest day and shortest night is observed on 21^st June.
21^st March and 23^rd September experiences equal day and night which is known as equinox.

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Exercise 1 ( Page No. : 21 )

Answer the following questions briefly. (a) What is the angle of inclination of the earth’s axis with its orbital plane? (b) Define rotation and revolution. (c) What is a leap year? (d) Differentiate between the Summer and Winter Solstice. (e) What is an equinox? (f) Why does the Southern Hemisphere experience Winter and Summer Solstice in different times than that of the Northern Hemisphere? (g) Why do the poles experience about six months day and six months night?

Ans:

(a) The angle of inclination of the Earth’s axis with its orbital plane is 66 ½◦.

(b) Rotation:

The movement of the Earth on its axis is called rotation.
The Earth takes 24 hours to complete one rotation.
Days and nights are caused due to rotation of the Earth.

Revolution:

The movement of the Earth around the sun in a fixed path or the orbit is called revolution.
The Earth takes 365 days six hours to complete one revolution.
Seasons are caused due to revolution of the Earth.

(c) A year with 366 days is called leap year. It occurs once in four years on the month of February and it is 29^th day of that month. It is also called intercalary year. The Earth do not take exact 365 days in an year and 24 hours in a day, so we add them as one extra day once in four year.

(d) Differences between summer and winter solstice are as follows:

Summer Solstice	Winter Solstice
Summer solstice occurs on 21^st June in Northern Hemisphere.	Winter Solstice occurs on 22^nd December in Southern Hemisphere.
The Northern Hemisphere experiences more heat and light.	Southern Hemisphere experiences less heat and light.
It is summer in Northern Hemisphere.	It is winter in Southern Hemisphere.
Days are longer and nights are shorter.	Nights are longer and days are shorter.

(e) On 23^rd September and 21^st March, the rays of sun directly falls on the equator. So, at that position neither of the poles are tilted. So at that specific or we can say a particular day the earth experiences equal days and nights and this is called as equinox.

(f) The Southern Hemisphere experience winter and summers solstice in different times than that of the Northern Hemisphere because:

Southern Hemisphere is tilted towards the Sun on 22^nd December, so summer Solstice is there in Southern Hemisphere. At that time Winter Solstice is in Northern Hemisphere.
In 21^st June the Northern Hemisphere is tilted towards the Sun, so Summer Solstice is there and Winter Solstice is in the Southern Hemisphere.

(g) The poles experiences about six months day and six months night because:

When Northern Hemisphere is tilted towards the Sun, The North Pole remains in the light for the whole day. This position remains for six months from 21^st March- 23^rd September.
On the same time Southern Hemisphere is away from the Sun, the South Pole does not get light for six months. So it experiences night for six months and vice versa.

Tick the correct answers. (a) The movement of the earth around the sun is known as (i) Rotation (ii) Revolution (iii) Inclination (b) Direct rays of the sun fall on the equator on (i) 21 March (ii) 21 June (iii) 22 December (c) Christmas is celebrated in summer in (i) Japan (ii) India (iii) Australia (d) Cycle of the seasons is caused due to (i) Rotation (ii) Revolution (iii) Gravitation

Ans:

(a) (ii) Revolution

Seasons are caused due to revolution of the earth around the sun. Revolution is a movement of earth near the sun on a fixed path (orbit) in an elliptical way.

(b) (i) 21 March

On 21^st March, the rays of sun directly fall on the equator. So at that particular day, the earth is not tilted towards any pole and experiences an equinox (equal days and nights).

As Australia is present in the Southern Hemisphere so during December Australia experience summer due to revolution. And, in same manner at that time the Northern hemisphere experiences winter.

(d) (ii) Revolution

As earth revolves around the sun in an elliptical path, so it cause change in season.

Fill in the blanks. (a) A leap year has _______________ number of days. (b) The daily motion of the earth is _______________. (c) The earth travels around the sun in ______________ orbit. (d) The sun’s rays fall vertically on the Tropic of ___________ on 21st June. (e) Days are shorter during ___________ season.

Ans:

(a) 29

There are 29 days in February once in every four year which is an additional day of the year, also called the leap year.

(b) Rotation

Rotation is the daily motion of the earth. The earth completes one rotation in 24 hours. Days and nights are cause due to rotation of the earth.

Elliptical shape is something like oval. The orbit of the planets is an elliptical path. It is not a complete circle.

(d) Cancer

On 21^st June, the Northern Hemisphere gets tilted towards the sun. So, the rays of sun directly fall on the Tropic of Cancer (23½◦N).

(e) Winter

Due to summer solstice, the days get shorter and nights get longer than the days.

Chapter 3 Motions of the Earth

Exercise 1 ( Page No. : 21 )

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