WBBSE Class 6 Geography Solutions Chapter 4 Rotation of the Earth

Detailed explanations in West Bengal Board Class 6 Geography Book Solutions Chapter 4 Rotation of the Earth offer valuable context and analysis.

WBBSE Class 6 Geography Chapter 4 Question Answer – Rotation of the Earth

Objective Type Questions [1 mark each]

Question 1.
How much time does the Earth take for one rotation?
Answer:
23 hours 56 minutes 4 seconds.

Question 2.
State the direction of rotation.
Answer:
West to East.

Question 3.
Name the two ends of the axis of the earth.
Answer:
North pole and South pole.

Question 4.
At what angle is the axis of the earth inclined to the plane of the orbit?
Answer:
66 1/2°.

Question 5.
What is the speed of rotation at the equator ?
Answer:
1650 km / hour.

Question 6.
Which country is known as the land of the ‘midnight Sun’?
Answer:
Norway.

Question 7.
Which country is known as the land of the ‘rising sun’?
Answer:
Japan.

Question 8.
What is the average distance between the earth and the sun?
Answer:
150 million km.

Question 9.
What is the apparent movement of the sun and planets?
Answer:
iFrom east to west.

Question 10.
What causes flattering at the poles of the earth?
Answer:
Rotation.

Question 11.
What is meant by earth’s axis?
Answer:
Axis of the earth is an imaginary line on which the earth rotates.

Question 12.
We always see the sun rising in the east. Why ?
Answer:
Because the earth rotates from west to east.

Question 13.
Noon is hotter than morning. Why ?
Answer:
Because at noon sunrays are vertical.

Question 14.
Which movement of the earth causes day and night?
Answer:
Rotation.

Question 15.
How do you define day?
Answer:
The period taken for one rotation is called a day.

Question 16.
Which of the two places Cairo (30° E) or Chennai (80° E) will experience sunrise earlier?
Answer:
Chennai.

Question 17.
When it is noon along the Prime Meridian, which longitude will have mid-night ?
Answer:
180° longitude.

Question 18.
On which day the north pole has (i) 24 hours of daylight (ii) 24 hours of darkness ?
Answer:
21 June.

Question 19.
On which days all places on the earth have 12 hours of daylight and 12 hours of darkness ?
Answer:
21 March and 23 September.

Question 20.
What is known as the period of diffused light before sunrise ?
Answer:
Dawn.

Write short notes : [2/3 marks each]

Question 1.
Rotation of the earth.
Answer:
Rotation : The spinning of the earth on its axis from west to east is called rotation. The earth takes about 24 hours (23 hours 56 minutes 4 seconds) to complete one rotation. It is also known as daily motion of the earth.
Effects :

• Rotation causes days and nights.
• All heavenly bodies like sun, moon, planets appear to move from east to west.
• Tides occur regularly twice a day.
• Winds and ocean currents change their directions.
• The four main directions (north, south, east, west) can be determined with the help of rotation.
• We get a measure of time. A day consists of 24 hours-one complete rotation.
• The different times of a day like sunrise, noon, sunset, midnight can be determined due to rotation.

Question 2.
What are the effects of the inclination of the axis ?
Answer:

• There is apparent movement of the sun from the equator towards tropics.
• It causes opposite seasons in the two hemispheres.
• It results in the variation of length of day and night.
• When north pole is tilted towards the sun, the south pole turns away from the sun. So the two hemispheres are alternatively exposed to the sunrays.

Question 3.
What do you mean by Ferrel’s law ?
Answer:
At equator, the earth’s speed of rotation is about 1600 km per hour. The speed goes on decreasing poleward. At 60° latitude, it is 800 km per hour. This difference affects the general circulation of the atmosphere. Winds are deflectd to the right in the northern hemisphere and to the left in the southern hemisphere. Similarly, the ocean currents are also deflected. This is known as Ferrel’s law. Due to this the earth is flattened at the poles and bulged at the equator.

Question 4.
What is the effect of longitude on time?
Answer:
All the places located on the same meridian have the same local time. As the earth rotates from west to east, the places in the east will have sunrise earlier than places in the west. The places in the east gain time while the places in the west lose time. The Rule is (East – Gain – Add, West – Lose – Subtract).

Question 5.
What is a chronometer ?
Answer:
A chronometer is a highly accurate timepiece which is used to keep Greenwich Mean Time. The Greek word ‘chronos’ means time. It helps in determining the longitude of a place.

Question 6.
What do you mean by local time?
Answer:
Local time : The local time of a place is the time of its own meridian. The local time is calculated by the position of the sun at noon at a given place. When the sun at any place is highest in the sky, it is noon and it is 12 o’clock. All the watches of that place should be set according to that time.

Characteristics :

• Every meridian has a different local time.
• The places on the same meridian have the same local time.
• The eastern places will be ahead of western places for local time.
• Sundial was a simple and old method to determine local time.

Question 7.
What do you mean by standard time?
Answer:
Standard time : The standard time is the time of a central meridian of a country and is adopted by all the places. Standard time is a uniform time throughout a country. All the watches of the country are set according to this time. The Indian Standard Time (I.S.T.) is the local time of a place 82 1/2° E longitude which passes in between Allahabad and Mirzapur. It is 5 1/2° hours ahead of Greenwich Mean Time. When it is noon at Greenwich, it is 5.30 p.m. in India.

Short Answer-Type Questions : [3 marks each]

Question 1.
What are the results of the difference in the earth’s speed of rotation at various latitudes ?
Answer:
At equator, the earth’s speed of rotation is about 1600 km per hour. The speed goes on decreasing poleward. At 60° latitude, it is 800 km per hour. This difference affects the general circulation of the atmosphere. Winds are deflected to the right in the northern hemisphere and to the left in the southern hemisphere. Similarly, the ocean currents are also deflected. This is known as Ferrel’s Law. Due to this, the earth is flattened at the poles and bulged at the equator.

Question 2.
Why all the places south of the Antarctic Circle have 24 hours of darkness on June 21st ?
Answer:
On June 21st, it is winter solstice in southern hemisphere. The sun shines vertically on Tropic of Cancer. The South Pole-Antarctic Circle area is inclined away from the sun. So these places beyond Antarctic Circle have 24 hours of darkness.

Question 3.
What are the effects of the inclination of the axis?
Answer:

1. There is apparent movement of the sun from the equator towards tropics.
2. It causes opposite seasons in the two hemispheres.
3. It results in the variation of lengths of day and night.
4. When North Pole is tilted towards the sun, the South Pole turns away from the sun. So the two hemispheres are alternately exposed to the sunrays.

Question 4.
Why is dawn or twilight much shorter in duration at the equator than at polar areas?
Answer:
The brief period of refracted light between sunrise and full daylight is called dawn while that between sunset and complete darkness is called twilight. The sun rises and sets vertically at the equator. But the sun rises and sets obliquely in polar areas. So the period of dawn and twilight is longer in polar areas. As a result the sun takes longer time to pass through the refracted light.

Question 5.
On what dates is the sun overhead at the equator? What are these dates called?
Answer:
The sun is overhead at the equator on 23rd September and 21st March. These dates are called equinoxes meaning equal nights. 23rd September is known as autumnal equinox while 21st March is known as spring equinox.

Question 6.
Why is Norway called ‘the land of midnight sun’?
Answer:
Norway is located beyond Arctic Circle (66 1/2° N. latitude). In summer, on 21st June, the Sun never sets beyond the Arctic Circle. The polar areas have complete 24-hour continuous daylight. When there is midnight in other areas, the sun remains shining in Norway. So it is called the ‘land of midnight sun’.

Question 7.
Why is Japan called ‘the land of the rising Sun’?
Answer:
Japan is the easternmost country in the world. It is the first to receive sunrays as the earth moves from west to east. It has the sunrise earliest than other countries. So, it is known as the land of the rising sun.

Answer the following Questions : [5 marks each]

Question 1.
How are days and nights occurred ?
Answer:
Days and nights are occurred due to rotation of earth. The earth rotates on its axis from west to east. When it rotates, one half of the earth turns towards light and has, therefore, day. The other half turns away from the sun and remains in darkness and has night. During 24 hours, every part of the earth comes in turn before the sun and then goes into the darkness. Therefore, the position of day and night keeps on changing. When it is day in one hemisphere, it is night in the other.

Question 2.
Account for the unequal length of day and night.
Answer:
The length of day and night varies throughout the year. Sometimes the days are longer and nights are shorter or vice versa. It is due to two reasons :

• The inclination of the axis.
• The revolution of the earth.

Due to the inclined axis, one hemisphere leans towards the sun for the six months, the other hemisphere leans towards the sun for the next six months. In summer, on 21st June, days are longer in northern hemisphere and the nights are shorter. In winter, the conditions are reversed, the days are shorter and nights are longer in the northern hemisphere. If the axes were vertical there would have been equal days and nights everywhere.

Question 3.
(a) What is International Date Line?
(b) What is its necessity ?
(c) Explain its location.
Answer:
(a) The International Date Line is an imaginary line roughly corresponding to 180° meridian. Ships crossing this line change the date by one day.

(b) Necessity : A traveller going eastwards will be 12 hours ahead of G.M.T. (Greenwich Mean Time) till he reaches 180° east meridian. Similarly, he loses 12 hours when he reaches 180° west going westwards. Thus, there is total difference of 24 hours or a whole day at the 180° meridian. This causes a confusion of dates when travelling around the world. To avoid this confusion, nations have agreed upon the Internation Date Line to change dates. When Magellan returned in Spain in 1522 after travelling around the world, he thought he had arrived on the 5th of September. He was shocked to be told that the date was 6th september. As he travelled westward, he knew nothing about the day he had lost.

(c) Location of Date Line : The International Date Line passes through the Pacific Ocean forms North to South following 180° meridian. It avoids the land areas and deviates from 180°. This is done so that it does not divide group of islands under the same government. The date line goes zigzag to avoid the confusion of time between one island and another. It bends to the west of 180° to avoid the Aleutian islands. In the southern hemisphere it bends eastward to avoid Fiji and Tonga islands.

Question 4.
What is the rule of changing the date while crossing the International Date Line ?
(a) What is meant by ‘six-day week’ and ‘eight-day week’?
(b) Explain the above with the help of a map.
Answer:
Rule: On crossing the date line, the ships adjust the dates according to the following rules :
(a) When crossing westward towards Asia (from USA to Japan), a day will be added. When crossing the date line, ship misses a day from the calendar and have ‘six-day week’. If a ship crosses the date line on Monday, the next day will be Wednesday and not Tuesday.

(b) When crossing eastward towards America (from Japan to U.S.A) a day must be subtracted. When crossing the date line ships gain a day from the calendar and have an ‘eight-day week’. It repeats a day. If ship crosses the date line on Monday, the next day will be again Monday and not Tuesday.

Question 5.
Why do we have standard time?
Answer:
The local time of places which are on different meridians are bound to differ. For instance, it will be difficult to prepare a time table for trains which cross several longitudes. In India, for example, there will be a difference of about 1 hour and 45 minutes in the local times of Dwarka in Gujarat and Dibrugarh in Assam. It is, therefore, necessary to adopt the local time of some central meridian of a country as the standard time for the country. The longitude of 82 1/2° E (82° 30° E) is treated as the Standard Meridian in India. The local time at this meridian is taken as the standard time for the entire country. It is called the Indian Standard Time (IST).

Kabeer lives in Bhopal. He tells his friend Alok that they will not be able to sleep tonight. A day and night cricket match between India and England had started at 2 p.m. in London. This means that the match would begin at 7:30 p.m. in India and finish well in the night. Do you know what is the time difference between India and England ?

India located east of Greenwich at 82° 30′ E is 5 hours and 30 minutes ahead of GMT. So, when it is 2:00 p.m. noon in London, it will be 7:30 p.m. in India.

Some countries have adopted more than one standard time because they have a great longitudinal extent. For example, in Russia, there are as many as eleven standard times. The earth has been divided into twenty-four time zones of one hour each. Each zone thus covers 15° of longitude.

Identify the correct answers : 1 mark each

Question 1.
Deflection of wind on the Earth’s surface is caused by
(a) Revolution of the earth
(b) Rotation of the earth
(c) Attraction of the moon
Answer:
(b) Rotation of the earth

Question 2.
Rotation is the movement of the earth round the
(a) Sun
(b) on its axis
(c) round the universe
(d) round the earth
Answer:
(b) on its axis

Question 3.
The speed of rotation of the earth at the equator is
(a) 1550 km per hour
(b) 1650 km per hour
(c) 1639 km per hour
(d) 1570 km pr hour
Answer:
(b) 1650 km per hour

Question 4.
The sun rises in the east and sets in the west due to
(a) rotation of the earth
(b) rotation of the moon
(c) revolution of the earth
(d) revolution of the moon
Answer:
(a) rotation of the earth

Question 5.
The earth rotates on its axis once in
(a) 23 hours
(b) 24 hours 30 minutes
(c) 23 hours 56 minutes 4 seconds
(d) 22 hours
Answer:
(c) 23 hours 56 minutes 4 seconds

Question 6.
The time when the sun is at the highest position in the sky is called
(a) morning
(b) noon
(c) evening
(d) twilight
Answer:
(b) noon

Question 7.
The International Date Line follows the
(a) 0°
(b) 145°
(c) 180°
(d) 80 meridians of longitude
Answer:
(c) 180°

Question 8.
The spinning of the earth on its axis is called
(a) revolution
(b) travelling
(c) speed
(d) rotation
Answer:
(d) rotation

Question 9.
The speed of rotation of the earth at the poles is
(a) 0 km / hour
(b) 990 km / hour
(c) 1530 km / hour
(d) 1650 km / hour
Answer:
(a) 0 km / hour

Question 10.
The circular boundary between the sun-lit part of the sphere and the part in darkness is known as the
(a) latitude
(b) longitude
(c) shadow circle
(d) equator
Answer:
(c) shadow circle

Fill in the blanks with correct words [1 mark each]

Question 1.
The axis of the earth is inclined at an angle of ____ from the orbital plane.
Answer:
66 \(\frac{1}{2}\) °

Question 2.
The speed of rotation is negligible at the ____.
Answer:
poles

Question 3.
Everyday the earth makes a journey across the sky from _____ to _____.
Answer:
west, west

Question 4.
The movement of the earth on its axis is called _____.
Answer:
rotation

Question 5.
The earth’s axis always points to the ______.
Answer:
pole star

Question 6.
Our shadow is the at _____ the noon time or mid-day.
Answer:
minimum

Question 7.
The local time at P(40° N, 40° E) is 12 noon on Sunday. The local time at B (40° N .130° E) will be _____ on Sunday.
Answer:
6 p.m.

Question 8.
A (20° N . 60° E), B(60° S . 60° E) and C (20° N, 20° E) are places on the earth. ____ of these will have the same local time.
Answer:
A and B.

If the sentence is true, write ‘T’ and if false write ‘F’ against the following : 1 mark each

1. The Earth’s axis is an imaginary line joining poles and passing through the equator.
Answer:
False

2. The circular boundary between the sun-lit and dark part of the earth is known as shadow circle.
Answer:
True

3. Actually the sun does not move from east to west.
Answer:
True

4. Rotation is also known as the annual movement of the earth.
Answer:
True

5. The city of Kolkata at 88° 34′ E is considered as the Standard Meridian of India.
Answer:
False

6. The hours beginning from mid-night to noon is indicated by a.m.
Answer:
True

7. The axis of the earth makes an angle of 23 1/2° with the vertical.
Answer:
False

Write the differences: [2/3 marks each]

Question 1.
A.M. and P.M.
Answer:

A.M.	P.M.
1. The word ‘meridian’ means mid-day or noon. A.M. = Ante-meridian or before noon. (Ante = before)	1. P.M. = Post-meridian or afternoon (Post = After)
2. The period of time from 12 midnight to just before 12 noon is called ante-meridian (a.m.)	2. The period from 12 noon to just before midnight (12.00) is called postmeridian (p.m.)

Question 2.
Dawn and Dusk.
Answer:

Dawn	Dusk/Twilight
1. Just before sunrise, there is some diffused light. This period is called dawn.	1. Just before sunset, diffused light is left. This period is called dusk.
2. Dawn lasts till the sun rises and begins when the centre of he sun arrives below the apparent horizon 6° (for civil down) / 12° (for nautical dawn) / 18° (for astronomical dawn).	2. Dusk continues till the centre of the sun sinks below the horizon 6° (for civil twilight) / 12° (for nautical twilight) / 18° (for astrono-mical twilight).
3. Dawn at the equator is the shortest as the sun rises there in vertical path.	3. Dusk at the equator is the shortest as the sun sinks there in vertical path.

Question 3.
Local time and Standard time.
Answer:

Local time	Standard Time
1. Local time of a place is the time of its meridian.	1. Standard time is the time of a central meridian of an area.
2. Each place has its own local time.	2. All places have uniform or same time.
3. When it is noon, the local time is 12 o’clock.	3. It does not have any relation with noon or height of the sun.
4. All places on a meridian have same local time.	4. All places within a country have the same standard time.
5. The places in the east are ahead of west in time. For 1° longitude there is a difference of 4 minutes of time.	5. There is no change in time. So it is known as standard time.

Rotation of the Earth Class 6 WBBSE Notes

Axis of the Earth : The Earth rotates about its axis. It is inclined at an angle of 66 1/2° to the plane of the orbit.

Movements of the Earth : The Earth has two movements : (i) The Rotation and (ii) Revolution.

Rotation : Rotation is the spinning of Earth on its axis. Greek astronomer, Aristarchus of Samos (325-250 BC) stated that. It is the earth, and not the stars, that spins everyday.

Direction : Rotation is the turning around of the Earth on its axis, from west to east.

Effects : Day and night are caused due to daily motion of the earth. The earth depends upon the sun for its light. The sun can face only one half of the earth.

Everyday we see that the Sun rises in the east and sets in the west. It seems as if the Sun travels round the earth from east to west. This is known as ‘apparent movement of the Sun’. But the fact is otherwise. Actually the Sun does not rise or set. Like all other planets in the Solar System, the Earth also has two types of movements. The Earth rotates on an imaginary axis. It takes nearly 24 hours for the Earth to complete one rotation. The period taken for one rotation is called a day.

Movements of the Earth : In 1522 A.D., Nicolaus Copernicus, a Polish astronomer proved first in his book titled ‘On the Revolutions of the Heavenly Spheres’ that the earth spins daily on its axis and goes around the Sun every year along with the other planets of the Solar System. The Earth has two main motions or movements-
1. Rotation and 2. Revolution.

Rotation of the Earth:

Definition : The Earth spins on its own imaginary axis (66 1/2° inclined to its orbital plane) from west to east once in every 24 hours (23 hours 56 minutes and 4 seconds). This spinning of the Earth is termed as rotation. The rotation is called ‘diurnal motion’ of the Earth as it causes formation of day and night. This diurnal movement is also known as ‘the daily movement of the Earth’. According to A.N. Strahler, “The spinning of the Earth on its polar axis is termed as rotation”.

Salient features of the Earth’s Rotation:

Direction of Rotation : If we look the Earth (Globe) from outside, the Earth is found to rotate from west to east or in anti-clockwise direction. Since the Earth rotates in a west-east direction, we see the Sun rising everyday in the east and setting in the west.

Axis :

1. Rotation is the turning around of the Earth on its axis, from west to east.
2. The axis is the imaginary line passing through the centre of the Earth on which the Earth rotates.
3. The Earth spins, west to east, around its axis once in 24 hours.
4. The north pole and south pole lie at the ends of the axis.
5. The Earth’s axis is not vertical. It makes an angle of 23 1/2° with the vertical or 66 1/2° with the plane of the Earth’s orbit.
6. The Earth’s axis always remains pointed in the same direction (towards the Pole star) as the Earth goes around the Sun.
7. The tilt of the Earth axis is also known as the inclination of the Earth’s axis.

The Speed of Rotation : Being spherical in shape, the surface speed of rotation of the Earth is variable. It is highest at the equator, gradually diminishing towards north and south and becomes zero at the poles. The surface speed at the equator (0°) is 1630 km / hr.

Why do we not feel the Rotation of the Earth? We do not feel that the Earth on which we live because it is constantly rotating. The reasons are-

(i) There are no heavenly objects nearer to the Earth, either stationary or moving opposite of the Earth’s rotation by which we can compare the Earth’s rotation.

(ii) The rate of rotation is uniform and constant for each place on the Earth’s surface.
(iii) The atmosphere also rotates with the Earth uniformly.

What are the effects of the speed of rotation: The differences in the speed of rotation at the Equator and at the poles has important consequences.

1. It affects the shape of the Earth : The rotation of the Earth is considered responsible for the slight flattening of the Earth at the two poles and bulging at the equator. The equatorial diameter is 43 km longer than the polar diameter.

2. It affects the general circulation of the atmosphere and ocean currents :
(a) The moving air and ocean water in deflected at the right in the northern hemisphere and to the left in the southern hemisphere.
(b) Cyclones and anticylones are similarly deflected in both hemispheres.
(c) The movements of water in the ocean is affected in relation to rigid crust.

Effects of Earth’s Rotation –

The rotation of the Earth is responsible for the following major effects :

• Formation of days and nights
• Occurrence of sunrise, noon and sunset
• Determination of time
• Deflection of planetary wind
• Formation of tides
• Magnetization of the Earth
• Differentiation of temperature
• Determination of the direction
• Different local time
• Shape of the Earth
• Other.

Formation of Days and Nights : The Earth rotates around its axis from west to east once in 24 hours. So when a particular place comes in front of the Sun, it gets the sunlight and therefore experiences day. As the earth rotates, this particular place moves away from the Sun into darkness. The imaginary line that separates the lit-up half from the darkened half is called the circle of Illumination or Shadow Circle. In this way all places on the Earth’s surface continuously experience periods of daylight followed by periods of darkness called night.

Occurrence of Sunrise, Noon and Sunset : As the Earth rotates, the portion of the Earth which starts turning towards the Sun has sunrise. Slowly the inclination of the sunrays increases. When it is maximum, it is noon. When that portion gets away from the Sun, evening appears and soon the sun sets. The positions of the Earth’s various parts continue to pass through these conditions as the Earth continues to rotate.

Determination of time : The Earth takes a full day to make a complete rotation. We divide the day in 24 hours and then an hour into 60 minutes and a minute into 60 seconds. Thus we get an idea of calculating time.

Deflection of planetary wind : In 1835, Gustave-Gaspard de Coriolis explained that air flows do not move in a straight path because the spinning of the Earth on its axis causes the air masses to get deflected. The tendency of deflection of wind due to the Earth’s rotation is known as the Coriolis effects or the Coriolis force. After investigating and analysing the Coriolis force, this concept was developed by the American climatologist William Ferrel in 1855.

According to Ferrel, due to the effect of Coriolis force, wind and ocean currents are deflected towards the right in the northern hemisphere and towards the left in the southern hemisphere. This is known as Ferrel’s Law.

Alternation or Formation of Day and Night –

As the Earth is a sphere, only half of it, facing the Sun, is lit-up by sunrays. The other half remains dark. As a result, day occurs in the lit-half of the earth and night in the dark-half of it. The imaginary line that separates the lit-up half from the darkened half is called circle of Illumination or shadow circle. Each place of the surface of the Earth is swept over twice by the shadow circle once at dawn and again at twilight everyday.

The Earth’s rotation does not allow either of the two halves to remain always lit or unlit. They keep moving from sunlight to darkness and again from darkness to sunlight after almost regular intervals. The process of this change is not sudden. Transformation from sunlit day to night darkness and again from night darkness to sun-lit day at each place is uniform and ceaselessly gradual at almost regular intervals. During each 24 hour day, each place or meridian appears to shift its position from west to east. Hence from any place or meridian the Sun appears to move from east to west. This phenomenon may be cited as a proof that the earth rotates from west to east in respect of the Sun.

A Causes of dawn, sunrise, noon, evening, twilight (or dusk) and mid-night: All these phenomena occur in a complete 24 hour day. No place on the Earth has an abrupt change from day to night or vice versa. There is a transition period between day and night as well as night and day. During this transition period, there is diffused light produced by scattering and reflection by clouds, dust and other particles present in the atmosphere. These occur both before sunrise and after sunset.

Dawn : The period of receiving refracted and reflected sunlight before sunrise is called dawn. However the period of diffused light before sunrise is called dawn. Dawn lasts until the sun rises and begins when the centre of the sun arrives below the apparent horizon 6° (for civil dawn) / 12° (for nautical dawn) / 18° (for astronomical dawn). Dawn at the equator is the shortest as the sun rises there in vertical path, at the temperate latitudes longer and at the poles it much longer.

Sunrise: The time at which the upper edge of the sun appears above the apparent horizon of a place on the Earth’s surface on a clear day is called sunrise. The Earth rotates around its axis from west to east makes the Sun appear to rise in the east and it is known as sunrise.

Noon : The time at which the Sun stands at its highest position or at in vertical position above the horizon of a place on the Earth surface is called noon.

Evening : Similarly the time at which the upper edge of the Sun appears to sink below the apparent horizon of a place on the Earth’s surface is called evening. Thus when the Sun sets at a particular place it is called evening. D

Twilight or Dusk : The period on diffused light before sunset is called twilight. Twiligh’t or dusk at a place or the Earth’s surface begins after sunset and continues till the centre of the Sun sinks below the horizon 6° (for civil twilight) / 12° (for nautical twilight) / 18° (for astronomical twilight).

Account for the unequal length of day and night : The tilting of the Earth axis is called the inclination of the Earth’s axis. The length of day and night varies throughout the year. It is due to the inclination of the Earth’s axis. Due to inclined axis, one hemisphere leans towards the Sun for the six months, the other hemisphere leans towards the Sun for the next six months.

Effect of the Inclined Axis –

Assume the Earth’s axis to be vertical –

1. Place A on the Equator would be on position A’ after 12 hours
2. Place B on the Tropic
of Cancer would be at position B’ after 12 hours.
Distance BX = Distance XB’
∴ Length of day = Length of night.
3. Similarly at the poles, or at any other place on the Earth, lengths of day and night would be equal if the axis of the Earth was not inclined.

Now consider the effect of the inclined axis –

1. Place A at the Equator would be at position
A’ after 12 hours
Distance AO = Distance OA’
∴ Length of day = Length of night

2. Place B on the Tropic of Cancer will be at position B’ after 12 hours.
But distance BX is not equal to distance XB’. BX is greater than BX’. This means that the length of day is greater than the length of night.
∴ Days are longer than nights

3. Place C on the Arctic Circle will be at position C’ after 12 hours. Even after 12 hours, place C remains in light (C’). In summer, the Sun does not set for six months in the region beyond the Arctic Circle.
∴ Day of 24 hours near the poles

4. Place P on the Tropic of Capricorn in the southern hemisphere will be at position P’ after 12 hours. Distance PQ (in light) is less than the distance QR (in darkness)
∴ Days are shorter than nights
Place S remains in continuous darkness.

Shadow Circle or Circle of Illumination : The Earth is spherical in shape. Its circumference is about 40,000 km while its diameter is about 12756 km in the west-east direction, the direction in which the Earth rotates. The Earth completes one round of rotation in about twenty-four hours. As the Earth rotates, the sun-lit side gradually moves away from the Sun into darkness (at sunset) and the side that was away from the Sun and had darkness gradually moves into daylight (at sunrise).

The circular boundary between the illuminated or sun-lit part of the sphere and the part in darkness is known as shadow circle or the circle of illumination.

Earth’s Rotation and Concept of Local Time : Owing to the spherical shape of the Earth only one half of the Earth is illuminated by the Sun at any given time. The other half is dark and experiences night. As the Earth rotates from west to east, the zone of illumination also moves slowly towards the east. Local time is one which is fixed in relation to the position on the Sun at that place or that meridian. When the Sun is overhead in the sky, it is mid-day or 12 noon.

We keep time in synch with the apparent daily movement of the Sun. The Earth completes one rotation approximately in 24 hours. 1 minute is obtained as we split each hour into 60 equal parts and 1 second is obtained by splitting a minute into 60 equal parts.

Change of Time and Date :

Ferdinand Magellan and his team who started their circumnavigation expedition in 1519 and when they returned at the end of the voyage, they found out something weird. The calendars back home were not at all matching with their calculations of time and date. They showed Tuesday when the mariners thought it was Monday.

Owing to the spherical shape of the Earth, only one half of the Earth is illuminated by the Sun at any given time. The other half is dark and experiences night. As the Earth rotates from west to east, the zone of illumination also moves slowly towards the east. This accounts for the variation of time and also the date. The Sun rises in the east as the earth rotates from west to east once in every 24 hours. So, places on the Earth located more towards the east are ahead in time than those to the west.

Sense of Date: By a common consensus the longitude of Greenwich, a place near London in England (U.K.), has been accepted as 0°. The Earth rotates on its axis once in 24 hours. It means that it takes 24 hours to pass through 360 degrees or 360° of longitude. In other words, it takes 24 hours to pass through 1° longitude. Thus, if we know the meridians of longitude of two places, we can find out the difference in their local times.

The local time at a place is fixed with reference to the apparent movement of the Sun. When the Sun is overhead in the sky, it is mid-day or 12 noon. In the morning, the length of the shadow is long. As the Sun moves up in the sky, the length of the shadow goes on decreasing. The length of the shadow is shortest when the position of the Sun is overhead at noon. In the afternoon, as the Sun moves down, the length of the shadow increases gradually. All places along a line of longitude have the same local time. The local time at places on different longitudes.

International Date Line : 180° is the same longitude which is numbered as 180° E and 180° W. This longitude is just opposite to that of the Greenwich Meridian [0°]. A peculiar situation arises at 180° long.

This may be illustrated with an example. When the time is 6 p.m. on Monday, 25th December at Greenwich, the time at 180° E long will be 12 hours ahead of G.M.T. The time at 180° E will be 6 a.m. on Tuesday, 26th December. The time at 180° W longitude will be 12 hours. behind that of the G.M.T. or 6 a.m. on Monday, 25th December. Thus when the time is computed eastwards, the time is 6 a.m. on Tuesday, 26th December. When the time is computed westwards, it is 6 a.m. on Monday, 25th December. Thus on either side of the 180° longitude there is a time difference of 24 hours or one day. In which longitude should the correction of date be made ? By international agreement, the correction of date is made along the 180° longitude line. This called the International Date Line.

While travelling eastwards, a date is repeated. For example, the time is 6.am. on Tuesday, 26th December, at 180° E, but on crossing the longitude, the time is 6 a.m. on Monday, 25th December. As a result of crossing at the international Date Line eastwards, the Christmas day gets repeated. The person celebrates two Christmas days on successive days. Again if a person crosses the International Date Line westwards, one day is lost. For example, it is 6 a.m. on 25 th December at 180° W. After crossing International Date Line, the time is 6 a.m. on 26th December. The Christmas day is lost owing to the crossing on the Date Line.

Why is the International Date Line not straight ?

The International Date Line passes through the Pacific Ocean. It deviates from 180° longitude in some places in order to keep all the islands under one administration on one side of the Date Line. The Date Line goes zigzag in some places to avoid land and to leave island groups wholly on the same side of the line. The Dare Line deviates eastwards in the Bering Straits between Alaska and Siberia. The line deviates westwards (7°) of 180° longitude to include the entire Aleutian islands to the east of the line. Further south, the Date line deviates eastwards (11°) of 180° around Fiji and Tonga islands. These islands keep the same date as New Zealand.

Duration of twilight :

The duration of twilight at a place depends on the path of the Sun in the sky.
(i) In the low latitudes, where the sun’s path is almost vertical, the duration of twilight is short. The sun sinks below the horizon rapidly.
(ii) In the middle and high latitudes, where the sun has an inclined path at a low angle, the duration of twilight is

Interna-tional Date Line :

The International Date Line may be defined as an imaginary line drawn almost along 180° meridian extending from the north pole to the south pole from which each new date starts and each date ends at last.