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October 28, 2013
Rotation & Revolution of Earth - Explained
(Geography for civil service exams, APPSC Material, APPSC Notification, Bankpo & IBPS Exam) AXIS AND ITS INCLINATION: The earth's axis is an imaginary line that passes through its centre. Its two ends are the North Pole and the South Pole. This axis is inclined to the plane of the earth's orbit at an angle of 6672°.
Besides moving round the sun, our earth also moves on its own axis. Thus, our earth has two kinds of motions — one on its axis, which we call 'Rotation' and the other around the sun, which we call 'Revolution'.
ROTATION: The movement of the earth round its axis in 24 hours is called Rotation. Because of rotation the days and nights are caused.
Day and Night and Circle of Illumination: It is because of the earth's rotation that the sun seems to us rising and setting. When the earth moves on its axis, half of its surface faces the sun at a time while the other half remains away from the sun. That part of the earth which receives sunlight is called the circle of illumination.
As a result, in that part of the earth that receives illumination or sunlight, it is the day while in the other part it is the night. This phenomenon of alternating day and night is the result of the earth's movement on its axis, which we call 'Rotation' and which it completes in 24 hours or in one day.
Suppose if the earth stops rotating, then one half of its part would be permanently in daylight, while the other half would have a perpetual night. Thus, it is because of the rotation of the earth that days and nights are caused and they follow each other in all parts of the earth.
The earth moves from west to east. It is also because of this movement or rotation of the earth that the sun, the moon and the stars appear to us as if they move round the earth from east to west.
Both in the morning and the evening, the rays of the sun are slanting. They fall on a large area of the earth. So there is lesser heat during these timings. But at noon, the rays of the sun are more or less vertical. They fall on a smaller area and make it very hot. So, noons are comparatively hotter as compared to both mornings and evenings.
Comparison of Morning and Evening: In the morning, the rays of the sun appear on the eastern horizon while in the evening they appear on the western horizon.
Tropic of Cancer — An imaginary circle on the earth's surface in the Northern Hemisphere at an angular distance of 23 1/2° from the Equator.
Tropic of Capricorn — An imaginary circle on the earth's surface in the Southern Hemisphere at an angular distance of 23 1/2° from the Equator.
Arctic Circle — An imaginary circle on the earth's surface in the Northern Hemisphere at an angular distance of 66 1/2° from the Equator.
The earth revolves round the sun at an enormous speed of 100,000 kilometers per hour. The path of the Earth in which it moves round the sun is called its orbit. The Earth completes one circle round the Sun in approximately 365 days and 6 hours. But for the sake of convenience we calculate a year in complete days, i.e. 365 days. In this way, we add one day (6 x 4 = 24 hours) in the fourth year. This year is called the Leap Year. Thus, every fourth year in the Solar Calendar has 366 days. This extra day is added to the month of February. In an ordinary year, February has 28 days while in a leap year it has 29 days.
Comparison of Rotation and Revolution
Rotation is the spinning of the earth round its own axis while revolution is the motion of the earth round the sun. Secondly, while a rotation is completed in a day, the revolution round the sun is completed in a year. Thus, there is much difference in the duration of these movements. Thirdly, as a result of the revolution, seasons like the summer, the autumn, the winter and the spring are caused. As a result of rotation days and nights are caused.
How are Days and Nights Caused?
Earth's axis is inclined to the plane of its orbit at 66 1/2° and it is always inclined in the same direction. As a result, one hemisphere of the earth leans towards the sun for six months. During this period, greater area of this hemisphere receives sunlight for a longer time. Thus, the days here are longer, while in the other hemisphere the days are shorter and the nights are longer. Hence, when the days are longer in the Northern Hemisphere, nights are longer in the Southern Hemisphere. For the next six months when nights are longer in the Northern Hemisphere, the days are longer in the Southern Hemisphere.
Can you imagine what would happen if the earth's axis was really perpendicular to the plane of the earth's orbit instead of inclining at 66 1/2°?
In such a situation, there would be nights and days of equal duration in both the hemispheres. Moreover, there would be only two seasons, i.e., the summer and winter in the whole year.
Also, the days and nights are always of equal length on the Equator because throughout the year the sun shines vertically on the Equator. The lengths of days and nights increase or decrease as we move away from the Equator either southwards or northwards.
Due to the inclination of the earth's axis in the Northern Hemisphere from March 21 to September 23, the North Pole receives sunlight continuously for six months while on the South Pole, the sun shines continuously for the other six months. In this way, at the Poles, there are days and nights of six months' duration each.
Days and nights are equal throughout the world on March 21 and September 23 because on these two days neither of the two poles is inclined towards the sun. These days are called Equinoxes.