(It is because of the earths rotation and revolution, the systematic changes happen)
Inclination of the earth’s axis
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| Tilt of the Axis |
We know that the axis of our earth is imaginary. The earth is tilted by an angle of 23 and half degree from the vertical line or perpendicular line to the orbital plane. Because of this tilt the earth is making an angle of 66 and half degree with its plane. This tilting of the earth’s axis is referred to as the Inclination of the earth’s axis.
Days and Nights
We know that, our earth is always in motion. It has two types of movements I.e. rotates on its own axis and revolves around the sun. The earth takes approximately 23 hours and 56 minutes for one rotation on its axis and this rotation is the cause of day night occur on earth. During the rotation, all parts of the earth do not experience day and night at the same time. It is because of all parts do not get sunlight at the same time. The part of the earth’s surface which faces the sun experience day and the other part which is away from the sun experiences night. So Day and night are the systematic changes that takes place everyday.
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| Days and Nights |
In ancient times people thought that day and nights occurred due to the revolution of the sun around the earth. But it is Aryabhatta, the great ancient Indian astronomer only clarified this misconception that the earth has its own rotation. In ancient times, sunrise is marked as the beginning of the day, but now-a-days we consider 12 midnight as the beginning of the day.
What will happen, if the earth’s axis is flat i.e. not tilted? The answer is, there would be six months of day and six months of night all over the year on the earth. This situation would change only after when earth completes one revolution. Do you know, this kind of situation existed on which planet? This situation prevails in the planet Uranus.
Leap Year
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| Leap Year |
Like rotation, direction of the earth’s revolution is also anti-clockwise i.e. from west to east. The earth revolves round the sun at a speed of about 100,000 km. per hour. The earth takes 365 days and about 6 hours to complete one revolution. For the sake of convenience we consider a year is of 365 days. The additional 6 hours, after 4 years, become 24 hours or one additional day and we add this to the month of February every 4 years. Thus, the month of February has 29 days instead of 28 days once in every 4 years. That particular year is called a leap-year. Leap-year is always divisible by four.
Seasonal Changes
Seasons occurred due to earth’s revolution and it is not uniform everywhere. Like the day and nights, there are seasonal changes every year. Generally there are four seasons. They are Spring, Summer, Autumn and Winter. The path which earth takes to revolve around the sun is elliptic. Hence at one position, earth is close to the Sun and other position it is away from the sun. Generally during July, the earth is very far from the sun and during the January, the earth is close to the sun. If seasons are caused due to this, then there would be summer in January and winter in July. So it is clear that earth’s elliptic orbit is not responsible for the seasons. So how seasonal changes occur? It is because of the inclination of the earth’s axis
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| Seasonal Changes |
Because of the tilt of the earth’s axis, one hemisphere come to face the sun first and then come the other hemisphere. Therefore, variation in the weather phenomena is observed which we call seasons. When the northern hemisphere faces the sun it receives direct and vertical rays from the sun. Sunlight received in those parts during the day is longer. This is summer in northern hemisphere. But the same hemisphere, when remain away from the sun’s rays, direct sunlight does not reach there. Those regions experience lesser hours of sunlight, it is winter condition in those regions. While revolving around the sun, the earth comes in four critical positions. These changing positions are responsible for the cycle of season.
Summer Solstice: On June 21, the northern hemisphere is tilted towards the sun. The sun is directly over the 23 ½0 N latitude or the tropic of cancer. On this date maximum hours of sunlight is available in this hemisphere. This situation is called the summer solstice. During this period, the sun ray is vertical on the Tropic of Cancer and gives more heat. This is the summer condition in the northern hemisphere and winter condition in the southern hemisphere.
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| Changing Positions of the Earth |
Autumnal Equinox: On September 23, as the earth’s revolution continues, equator faces the sun directly. On this date day and nights are equal all over the world. Therefore it is called Equinox. As autumn season prevails during this time, it is called Autumnal Equinox in the northern hemisphere.
Winter Solstice: On December 22, the sun is directly over the 23 ½0 South latitude or the Tropic of Capricorn. This is summer solstice in the southern hemisphere. On this day the northern hemisphere experiences shortest day and it is called winter solstice in the northern hemisphere.
Vernal Equinox: As the earth moves forward, on March 21, the equator again comes to face the sun, which is directly over it. Day and night again become equal on this day. This is called Vernal Equinox in the northern hemisphere.
Position of the rising Sun – Uttarayan and Dakshinayan
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| Uttarayan & Dakshinayan |
East is the direction in which the sun rises. This statement is partially true because the position of the rising sun changes every day. From summer solstice (around 21 June), the point of sunrise gradually shift towards the south. The sun is then said to be in dakshinayan (moving south). It keeps doing so till Winter Solstice (around 22 December). Thereafter, the point of sunrise changes direction and starts moving towards north. The sun is said to be in uttarayan (moving north). From the equator, only on two days, on the days of the equinoxes (around 21 March and 23 September) the sun rises in the east. On all other days, it rises either north of east or south of east.
Day and Night on the Poles
Nights as well as days on the poles extend for six months, because the sun is never high above the horizon in the Polar Regions. It is only on the equator that the day and the night are always of equal length. As we move away from the equator, either northward or southward, the difference between the length of day and that of night generally goes on increasing.
Incredible work..
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