How Seasons Occur Class 7 Geography Notes Maharashtra State Board
Note the timings of sunrise and sunset in your area, for the following period by direct observation, or using calendars, newspapers, or the Internet. You must have noticed the change in the duration of daytime and the nighttime from 19th to 28th June. It takes 24 hours for the Earth to rotate around itself. It rotates from west to east. The earth’s rotation has enabled us to measure time in terms of days. During a single day, we experience different stages like sunrise, midday, and sunset as well as daytime and nighttime. To understand the changes in the locations of sunrise and sunset on the horizon, let us carry out the activity given on the next.
Paste a large white paper on one side of the table. Place a torch in front of the table in such a way that it wouldn’t move. Stand a rod or a candle between the white paper and the torch. Direct the light beam of the torch in such a way that the shadow of the rod/candle falls on the paper. Mark the position of the shadow on the paper. Move the table along with the rod/candle, from one side to the other slowly. Observe and mark the position of the shadow that falls on the paper. Note the change in the location of the shadow.
From this activity, you will realize that as the position of the table changes, the position of the shadow also changes. If the positions of the sunrise and the sunset on the horizon are observed for the whole year, you will realize that they keep on changing. Let us try to understand the reason behind this with the help of the next activity.
Take a thick stick about 1.5 to 1.7 m long. Keeping some distance, fix the stick near a wall that receives sunlight at the time of sunrise or sunset throughout the year. (Remember that the stick has to be fixed at this spot for about one year.) After the observation, mark the position of the shadow for that date. If the position of the shadow changes, measure and note the distance between the earlier and the changed position. During the period of this activity, observe the place of sunrise and sunset on the horizon as well.
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 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 the sunrise. Let us study the changes in the position of the sunrise and the difference in day and night time.
Apparent Movement of the Sun:
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 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 21 st 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 concerning the northern and the southern hemispheres.
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.
Perihelion and Aphelion Positions of the Earth:
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 figure 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. 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.
As the earth revolves around the sun, the equator receives perpendicular rays on two days in a year. This condition occurs on 21 st March and 23 rd September. On these days, both the poles are at the same distance from the sun. This is called equinox.
The illuminated and dark portions of all the parallels including the equator are shown in figure. 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 21 st March to 21 st June while autumn prevails from 23 rd September to 22nd December. The southern hemisphere has the opposite seasons during these periods. In the northern hemisphere, 21 st 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.
Figure shows the position of the earth with its tilted axis on 21 st June and 22nd December. Its also shows its illuminated and dark portions. 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 21 st June and 22ndDecember the sunrays become perpendicular on the Tropic of Cancer and Tropic of Capricorn respectively. These two days are called Solstice days.
Sunrays are never perpendicular on any of the parallels between Tropic of Cancer and the North Pole or between Tropic of Capricorn and the South Pole. 21 st June is the longest day and it marks the shortest night in the northern hemisphere. Similarly, 22nd December marks the longest day and the shortest night in the southern hemisphere. 22nd December is the shortest day in the northern hemisphere. In the region from the Arctic Circle to the north pole, the sun remains visible for 24 hours or longer. At the north pole, the sun is visible in the sky from 22nd March to 23 rd September, i.e., for six months. Similar situation prevails in the region between the Antarctic Circle and the South Pole in the period from 23 rd September to 21 st March. Note that on the equator, the duration of day and night is same throughout the year. (i.e.,12 hours each).
Seasons have been decided on the basis of the duration of sunlight, equinoxes and solstices. The equatorial region does not experience any change of season. Hence the climate in that region does not change at all in the year. In places beyond the equatorial region in either hemispheres, summers or winters are experienced one after the other, within a year. The occurrence of seasons one after the other in a year leads to the ‘cycle of seasons’. Generally, there are two seasons, summer and winter, on the earth. However, in some places, seasons are taken to be four in all.
The changes in the atmosphere, vapour in the air, the wind and the precipitation also influence the seasons. Continuous occurrence of rain in a specific period gives rise to an additional season besides summer and winter. Due to the local conditions, seasons other than summer and winter are seen to occur in different parts. For example, rains occur in India in a specificperiod. Therefore we consider four seasons : such as summer, the rainy season the period of retreating monsoon and winter. There are four seasons in Europe and North America too. They are summer, autumn, winter and spring.
The Cycle of Season and the Living World:
If the earth’s axis were not tilted, the same climatic conditions would have prevailed throughout year. The seasons would not have occurred. One and the same type of climatic conditions would have prevailed on each of the different parallels. It is the tilt of the axis that leads to occurrence of seasons,change and diversity on the earth. The living world on the earth is affected by the cycle of seasons. For example, in the region between 66°30′ and 90° in both the hemispheres, even the mild sunlight available for a part of the year gives rise to certain flora and fauna. In the Antarctic region, at the southernmost part of the earth, birds like penguins, fish like seal and animals like walruses are found.
In the polar region of the northern hemisphere animals like reindeer, polar bears, Arctic foxes etc. are found. People living in this region too have adapted to the natural conditions prevailing in that region. Our adaptation to climatic conditions is possible only up to a certain limit. That is why organisms prefer a certain habitat. During extreme cold climatic conditions when the food supply becomes scarce, a number of birds and animals migrate temporarily. Trees bear fruit in a particular season. Therefore, agricultural seasons also depend on to the local climatic conditions. In polar areas, snowline shifts north or south according to seasons. This affects migration of birds and animals.
Once the winter at the North Pole becomes severe, this bird travels southwards. When it is summer in the northern hemisphere, it flies back towards the North Pole. It has to travel in search of food. In the course of one year, it travels around 70,000 km. It might be the only species in the world that experiences summer twice in a year.
Due to severe winter and lack of food, cranes from the cold northern regions visit India travelling around 8 to 10 thousand kilometres. Once summer starts in India, they migrate back to the north.
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