Write three differences between summer, monsoon and winter.
Are seasons (particularly summer and winter) experienced everywhere on the Earth?
Do you think if it is summer in India it is summer everywhere in the world? If no, how do seasons differ in different parts of the world?
Why do we have seasons on the Earth?
(Click on icon to write)
A season is a division of the year marked by changes in weather, ecology, and amount of daylight. In India we usually divide the year into three seasons: summer, monsoon and winter. On an average, the temperature remains high in summers and low in winter. Monsoon is a season of rains. The monsoon is a speciality of tropical regions, particularly South Asia (India, Pakistan, Bangladesh etc.), and its English name ‘monsoon’ is derived from Hindi / Arabic word ‘mausam’ (मौसम). Summer and winter, however, are experienced all over the world. You might have learned about the water cycle and how we get rains. Now let us learn why we have summer and winter.
Before we go ahead, think about how summer and winter happen. Why do you think it is hotter in some part of the year and colder in another?
Here are the two reasons:
I. Variation in duration of day and night: Have you noticed that the Sun rises early and sets late in the summer, but it rises late and sets early in the winter? Effectively, we get sunlight for a longer time in the summer than in winter. The sun follows a shorter arc in winter and a longer arc in summer (Figure 2).
Figure 2: Changes in the path of the Sun over a year on the Tropic of Cancer
Activity 2: Change in Path of the Sun over the Year (Gesture)
Procedure:
Locate north, south, east and west in your classroom.
Mimic the Sun’s apparent path in the sky in different seasons by moving your hand from east to west. Since we are in the northern hemisphere, the Sun will always be to the south of the meridian. The Sun comes overhead at noon only twice below the Tropic of Cancer. So at any latitude above 23.50 the Sun will always be on the south of zenith.
II. Variation in intensity (strength) of sunlight: You might have noticed that, in summer, the Sun appears quite high in the sky at midday. In winter, however, the Sun remains relatively low even at noon (See Figure 2). The intensity of the Sun’s rays is maximum when the rays are perpendicular to the surface. The following example will help you understand this point. Suppose it is raining and you want to collect rainwater in a container. How will you hold the container? Perpendicular to the direction of rainfall (Figure 3a) or at an angle (Figure 3b)? You will collect maximum amount of water if you hold the container perpendicular to the rainfall (as shown in Figure 3a). As the angle decreases, the amount of water collected will be reduced. If you hold the container parallel to the direction of rainfall (as shown in Figure 3c), no water will be collected.
Figure 3: Intensity depends upon the angle of incidence
The same is true for the sunrays. In summer, when the angle between sunrays and surface of the Earth (land) is higher, more sunlight will fall on that part of the Earth. In winter, when the angle between sunrays and surface of the Earth is lower, less sunlight will fall on that part of the Earth.
That’s why temperatures are higher in the summer and lower in the winter. Now why does the path of the Sun change over the year?
The axis about which the Earth rotates is tilted by an angle of 23.5 degrees. Thus, the Earth’s axis makes an angle of 66.5 degrees with the plane of the Earth’s orbit. As you know, it is directed towards the Pole Star. The Pole Star is very far away (more than 433 light years); so the direction of the axis remains same everywhere in the orbit (See Figure 4).
Figure 4: The Earth’s axis makes an angle of 66.5° to the plane of its orbit.
When the North Pole is inclined towards the Sun (and the South Pole away from the Sun), that is, at position B in Figure 4, the sunrays fall perpendicular on the Tropic of Cancer (See Figure 5). The angle of incidence of sunrays is smaller (0° at the Tropic of Cancer) in the northern hemisphere than they are in the southern hemisphere. Also, the northern hemisphere will experience longer days than the southern hemisphere. This extreme position, when the day is longest in the northern hemisphere occurs around June 21 and is called the ‘June Solstice’.
At this position (and three months before this position i.e. from 20 March to three months after this position i.e. till 23 September) the days will be longer than 12 hours in the northern hemisphere. Hence, during this period, it will summer in northern hemisphere and winter in southern hemisphere.
Figure 5: June Solstice
Draw a similar diagram when the South Pole is inclined towards the Sun and the North Pole is away from the Sun (position A in Figure 4). Where would the sunrays be perpendicular? This extreme position, when the day is longest in the southern hemisphere, occurs around December 22 and called the ‘December Solstice’. During the entire period from 23 September to 20 March, the days are shorter than 12 hours in the northern hemisphere; so it will be winter in the northern hemisphere and summer in southern hemisphere.
In the Earth’s orbit, there will be two positions when neither the North Pole nor the South Pole will be inclined towards the Sun. At these positions, the plane of Earth’s equator passes through the centre of the Sun (in Figure 5 imagine that the Sun is in front of the screen, where your head is, instead of on the left2). The day and night are of equal duration (12 hours) at these two positions. As you can guess, these two days are 20 March and 23 September; these two days are known as the ‘equinoxes’.
Draw the Earth in its orbit. Mark the positions of the two solstices and two equinoxes.
Use your notebook for drawing. Please write following on page where you are drawing- The Basic Astronomy Module: Unit 1: Lesson 3: Activity 2 & your Login ID
2If the sunrays are coming from the left as shown in Figure 5, the axis would be coming out in the plane perpendicular to the plane of the screen.
Glossary
Seasons
Can you answer the following questions?
(Click on icon to write)
A season is a division of the year marked by changes in weather, ecology, and amount of daylight. In India we usually divide the year into three seasons: summer, monsoon and winter. On an average, the temperature remains high in summers and low in winter. Monsoon is a season of rains. The monsoon is a speciality of tropical regions, particularly South Asia (India, Pakistan, Bangladesh etc.), and its English name ‘monsoon’ is derived from Hindi / Arabic word ‘mausam’ (मौसम). Summer and winter, however, are experienced all over the world. You might have learned about the water cycle and how we get rains. Now let us learn why we have summer and winter.
Before we go ahead, think about how summer and winter happen. Why do you think it is hotter in some part of the year and colder in another?
Here are the two reasons:
I. Variation in duration of day and night: Have you noticed that the Sun rises early and sets late in the summer, but it rises late and sets early in the winter? Effectively, we get sunlight for a longer time in the summer than in winter. The sun follows a shorter arc in winter and a longer arc in summer (Figure 2).
Figure 2: Changes in the path of the Sun over a year on the Tropic of Cancer
Activity 2: Change in Path of the Sun over the Year (Gesture)
Procedure:
Locate north, south, east and west in your classroom.
Mimic the Sun’s apparent path in the sky in different seasons by moving your hand from east to west. Since we are in the northern hemisphere, the Sun will always be to the south of the meridian. The Sun comes overhead at noon only twice below the Tropic of Cancer. So at any latitude above 23.50 the Sun will always be on the south of zenith.
II. Variation in intensity (strength) of sunlight: You might have noticed that, in summer, the Sun appears quite high in the sky at midday. In winter, however, the Sun remains relatively low even at noon (See Figure 2). The intensity of the Sun’s rays is maximum when the rays are perpendicular to the surface. The following example will help you understand this point. Suppose it is raining and you want to collect rainwater in a container. How will you hold the container? Perpendicular to the direction of rainfall (Figure 3a) or at an angle (Figure 3b)? You will collect maximum amount of water if you hold the container perpendicular to the rainfall (as shown in Figure 3a). As the angle decreases, the amount of water collected will be reduced. If you hold the container parallel to the direction of rainfall (as shown in Figure 3c), no water will be collected.
Figure 3: Intensity depends upon the angle of incidence
The same is true for the sunrays. In summer, when the angle between sunrays and surface of the Earth (land) is higher, more sunlight will fall on that part of the Earth. In winter, when the angle between sunrays and surface of the Earth is lower, less sunlight will fall on that part of the Earth.
That’s why temperatures are higher in the summer and lower in the winter. Now why does the path of the Sun change over the year?
The axis about which the Earth rotates is tilted by an angle of 23.5 degrees. Thus, the Earth’s axis makes an angle of 66.5 degrees with the plane of the Earth’s orbit. As you know, it is directed towards the Pole Star. The Pole Star is very far away (more than 433 light years); so the direction of the axis remains same everywhere in the orbit (See Figure 4).
Figure 4: The Earth’s axis makes an angle of 66.5° to the plane of its orbit.
When the North Pole is inclined towards the Sun (and the South Pole away from the Sun), that is, at position B in Figure 4, the sunrays fall perpendicular on the Tropic of Cancer (See Figure 5). The angle of incidence of sunrays is smaller (0° at the Tropic of Cancer) in the northern hemisphere than they are in the southern hemisphere. Also, the northern hemisphere will experience longer days than the southern hemisphere. This extreme position, when the day is longest in the northern hemisphere occurs around June 21 and is called the ‘June Solstice’.
At this position (and three months before this position i.e. from 20 March to three months after this position i.e. till 23 September) the days will be longer than 12 hours in the northern hemisphere. Hence, during this period, it will summer in northern hemisphere and winter in southern hemisphere.
Figure 5: June Solstice
In the Earth’s orbit, there will be two positions when neither the North Pole nor the South Pole will be inclined towards the Sun. At these positions, the plane of Earth’s equator passes through the centre of the Sun (in Figure 5 imagine that the Sun is in front of the screen, where your head is, instead of on the left2). The day and night are of equal duration (12 hours) at these two positions. As you can guess, these two days are 20 March and 23 September; these two days are known as the ‘equinoxes’.
Use your notebook for drawing. Please write following on page where you are drawing- The Basic Astronomy Module: Unit 1: Lesson 3: Activity 2 & your Login ID
2If the sunrays are coming from the left as shown in Figure 5, the axis would be coming out in the plane perpendicular to the plane of the screen.