Maharashtra Board Class 7 Geography Chapter 8 How Seasons Occur Part 2 PDF Download

How Seasons Occur - Part 2

Can You Tell?

Discuss the activity you have been carrying out since June. (See Chapter 1.) Make use of the observation tables for the months of June, September and December.

In which month the duration of the day was around 12 hours?

What could be the reason behind it?

Bring out the differences between the duration of daytime in the months of June, September and December.

What could be the reason for the change in the shadow of the stick?

What did you observe about the position of the sun at the horizon at the time of sunrise and sunset?

Which of the following factors could be related with the change in the position of the shadow of the stick and the difference in the duration of daytime?

Rotation of the earth

Distance between the earth and the sun

Revolution of the earth

The axis of the earth.

You must have noticed the longest day, the shortest day and the days with the same duration of day and night time with the help of the observations in the months of June, September and December. Generally, these dates are the same every year. With the help of the shadow experiment, you must have noted the change in the position of sunrise. Let us study the changes in the position of sunrise and the difference in duration of day and the night time.

This year you will study the 'apparent motion of the sun' in your science textbook as well. In that book, the sun's east-west motion from sunrise to sunset, that is, its diurnal apparent motion is also considered. In geography, we consider the apparent (north-south) movement of the sun. In both the motions, the sun only appears to move and does not actually move. The diurnal apparent movement of the sun is related to the rotation of the earth whereas its annual apparent movement is related to the revolution of the earth and the tilt of the earth's axis.

Apparent Movement Of The Sun

Geographical Explanation

You must have realized through your observations that the position of the sun, at sunrise appears to change on the horizon every day. Its position appears to move towards the north or south in the course of a year. However, in reality, the sun does not move anywhere. That is why, this movement of the sun towards the north or south in a year is called the apparent movement of the sun. The position of the rising sun keeps on moving towards the south in the period from 21st June to 22nd December. This period is called Dakshinayan. From 22nd December to 21st June, the sun keeps on moving towards the north. This period is called Uttarayan. The revolution of the earth around the sun and the tilted axis of the earth are the two factors responsible for the apparent movement of the sun.

Seasons occur only with reference to the northern and the southern hemispheres.

Teacher's Note

The sun appears to move north and south during the year. In India, we see this change - in June the sun is higher in the sky at noon, but in December it is lower.

Exam Trick

Remember: Dakshinayan = sun moving south (from June to December). Uttarayan = sun moving north (from December to June). The sun does NOT really move - it only looks like it moves!

Points to Remember

Apparent movement means the sun looks like it is moving north or south, but it is actually the earth's position changing.
Dakshinayan is the period from 21st June to 22nd December when the sun appears to move towards the south.
Uttarayan is the period from 22nd December to 21st June when the sun appears to move towards the north.
The tilt of the earth's axis and its revolution cause this apparent movement of the sun.
Seasons are different in the northern and southern hemispheres at the same time.

Think About It

In which direction will the location of sunrise and sunset appear to move after 22nd of December?

Always Remember

This year you will study the 'apparent motion of the sun' in your science textbook as well. In that book, the sun's east-west motion from sunrise to sunset, that is, its diurnal apparent motion is also considered. In geography, we consider the apparent (north-south) movement of the sun. In both the motions, the sun only appears to move and does not actually move. The diurnal apparent movement of the sun is related to the rotation of the earth whereas its annual apparent movement is related to the revolution of the earth and the tilt of the earth's axis.

Observe Fig. 8.1 Carefully And Answer

How is the northern hemisphere in which you live positioned relative to the sun on the dates given in the figure?

Which season will you experience in the northern hemisphere around 22nd December?

What would be the season in the northern hemisphere around 21st June?

If it is winter in the northern hemisphere, which season will it be in the opposite hemisphere?

At any given point of time, why are the seasons in the northern and southern hemispheres different?

Perihelion And Aphelion Positions Of The Earth

Geographical Explanation

The path of the revolution of the earth around the sun is elliptical. The sun is at one of the two centres of the ellipse. The sun does not change its position. As the earth moves in an ellipse, its distance from the sun does not remain the same. It is at its minimum distance in the first week of January. This is called the perihelion position of the earth. In this position, the southern end of the axis is towards the sun. As against this in the first week of July, the earth is at the farthest point from the sun. This is called the aphelion position. In this position, the northern end of the axis is towards the sun. With the help of fig. 8.1 which shows the position of the earth in relation to the sun, you can guess which season prevails in which hemisphere. Seasons occur due to the revolution of the earth, as well as due to the tilt of its axis of rotation.

Do You Know?

Due to the gravitational forces of the sun and the earth, the speed of the earth gets reduced, during the aphelion position and increases during the perihelion position. As the difference in the distance of the earth in both these positions is not very great, it does not have any effect on the seasons.

Teacher's Note

Perihelion means the earth is closest to the sun in January. Aphelion means the earth is farthest from the sun in July. This distance difference does not cause seasons - the tilt does!

Exam Trick

Remember: Perihelion = closest to sun (January, 147 million km). Aphelion = farthest from sun (July, 152 million km). But seasons do NOT depend on this distance!

Points to Remember

The earth's orbit around the sun is elliptical, not circular.
Perihelion is when the earth is closest to the sun in early January.
Aphelion is when the earth is farthest from the sun in early July.
The distance difference between perihelion and aphelion does not affect seasons much.
The tilt of the earth's axis is the main reason for seasons, not the distance.

Equinox

Geographical Explanation

As the earth revolves around the sun, the equator receives perpendicular rays on two days in a year. This condition occurs on 21st March and 23rd September. On these days, both the poles are at the same distance from the sun. This is called equinox. (See fig 8.3)

The illuminated and dark portions of all the parallels including the equator are shown in figure 8.3. In the figure, the circle of illumination divides all the parallels from the north pole to the south pole equally. Everywhere on earth nighttime and daytime are of equal duration. This condition is called equinox. On equinox, the sun's rays are perpendicular on the equator. In this condition, the circle of illumination coincides with the great circle defined by two opposite meridians. In the northern hemisphere spring prevails from 21st March to 21st June while autumn prevails from 23rd September to 22nd December. The southern hemisphere has the opposite seasons during these periods.

In the northern hemisphere, 21st March is called spring or vernal equinox whereas 23rd September is called autumnal equinox.

The equinox or solstice dates can vary by a day or so.

Teacher's Note

On equinox days, day and night are equal everywhere on Earth - 12 hours each. In India, this happens on 21st March and 23rd September.

Exam Trick

Remember: Equinox = equal day and night. On these two days (21st March and 23rd September), everywhere on earth has 12 hours of day and 12 hours of night.

Points to Remember

Equinox occurs on 21st March and 23rd September every year.
On equinox days, day and night are equal everywhere (12 hours each).
21st March is called Spring Equinox or Vernal Equinox in the northern hemisphere.
23rd September is called Autumnal Equinox in the northern hemisphere.
The sun's rays are perpendicular to the equator on equinox days.

Use Your Brain Power!

On equinox days, the two Poles experience either sunrise or sunset. On which pole will it be sunrise on 21st March?

Can You Tell?

When any one of the poles inclines the most towards the sun, 23°30' parallel of that hemisphere receives perpendicular sunrays. See figure 8.4. The sunrays are perpendicular at the equator on 22nd March and 23rd September which are the days of equinox. After 22nd March, other parallels between the equator and the Tropic of Cancer in the northern hemisphere start receiving perpendicular rays sequentially. Only on 21st June and 22nd December the sunrays become perpendicular on the Tropic of Cancer and Tropic of Capricorn respectively. These two days are called Solstice days.

Figure 8.4 shows the position of the earth with its tilted axis on 21st June and 22nd December. Its also shows its illuminated and dark portions. Observe the figure and answer the following.

In fig 'A' which pole is receiving sunlight?

In fig 'B' which pole is not receiving sunlight?

Which hemisphere has the longest daytime on 21st June?

Geographical Explanation

When any one of the poles inclines the most towards the sun, 23°30' parallel of that hemisphere receives perpendicular sunrays. See figure 8.4. The sunrays are perpendicular at the equator on 22nd March and 23rd September which are the days of equinox. After 22nd March, other parallels between the equator and the Tropic of Cancer in the northern hemisphere start receiving perpendicular rays sequentially. Only on 21st June and 22nd December the sunrays become perpendicular on the Tropic of Cancer and Tropic of Capricorn respectively. These two days are called Solstice days.

Teacher's Note

Solstice days are when the sun is directly over the Tropic of Cancer (21st June) or Tropic of Capricorn (22nd December). On these days, one hemisphere has the longest day and the other has the shortest day.

Exam Trick

Remember: Solstice = sun is directly over the tropics. 21st June = longest day in north, shortest in south. 22nd December = longest day in south, shortest in north.

Points to Remember

Solstice occurs on 21st June and 22nd December every year.
On 21st June, the sun is directly over the Tropic of Cancer.
On 22nd December, the sun is directly over the Tropic of Capricorn.
On solstice days, one hemisphere has the longest day and the other has the shortest day.
The duration of day and night is not equal on solstice days.

Use Your Brain Power!

Which hemisphere will have the longest night on 22nd December?

On which date will the Tropic of Cancer receive perpendicular sunrays?

Considering the position of the north pole, which season will prevail in the northern hemisphere from 22nd March to 23rd September?

The cricket matches in Australia are generally in summer. When does Australia have summers?

In which period is the 'midnight sun' observable in Norway? Which season will there be at that time?

At what time will the midnight sun be visible at the 'Bharati' research station of India at Antarctica? What will be the season there at that time?

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