Motion of earth

MOTION OF EARTH

The earth has two types of motion:

• The earth rotates about its own axis
• The earth revolves around the sun

THE EARTH ROTATES ON ITS AXIS:

The earth rotates (or spins) on an imagniary axis-which passes through its north and south poles. The earth completes one rotation on its axis in 24 hours which we call one day. The earth rotates (or spins) on its axis from west to east. The axis of rotation of earth is not perpendiucualr to the plane of the earth’s orbit.

Earth’s rotation around its axis

The axis of rotation of earth is slightly tilted with respect to the plane of its orbit (or path) around the sun. In fact, the axis of earth is tilted at an angle of 23.5 degrees to the perpendicular plane. The earth rotates on its axis in the tilted position and it  also revolves around the sun in the same tilted position throughout. The earth is spherical in shape. We can divide the earth into two half spheres called two hemispheres. The two hemispheres of earth meet at the equator (which is an imaginary line running around the middle part of the earth between the north pole and south pole). The upper half part of the earth below the north pole and above the equator is called Northern Hemisphere. The lower half part of the earth below the equator and above the south pole is called Southern Hemisphere. We (in India) live in the Northern Hemisphere of the earth. On the other hand, Australia is in the Southern Hemisphere of the earth.

The rotation of earth on its axis causes day and night.

An important consequence of the rotation of earth on its axis is that it causes day and night on the earth. The formation of day and night can be explained as follows:

The earth recieves its light from the sun. The light of sun always falls on that half part of earth which is facing the sun.

As the earth continues to rotate (or turn) on its axis, almost every 12 hours, a day changes into a night and a night changes into a day. Since the earth's axis is tilted at an angle, therefore, the length of day and night actually changes throughout the year. From the above discussion we conclude that the day and night are caused by the daily rotation of the earth on its axis. The change in the length of day and night is, however, caused by the tilted of earth’s axis. 

There are four seasons in a year: summer, winter spring and autumn. The hot season of the year is called summer. The coldest season of the year is called winter. The season after winter and before summer is called spring. And the season after summer and before winter is called autumn. The spring and autumn seasons are neither very hot nor very cold, they have a moderate weather. We will now describe how the different seasons on the earth are formed.

THE MOTION OF TILTED EARTH AROUND THE SUN CAUSES SEASONS:

An important consequence of the revolution of the earth around the sun is that it causes seasons on the earth such as summer, winter, spring and autumn. Two factors are responsible for causing different seasons on the earth are:

•  Motion of the earth around the sun once every year.
•  Tilting of the earth's axis to the plane of its orbit around the sun.

The earth and the start of four seasons

The earth rotates (or spins) on its axis and also revolves (or moves) around the sun. The earth takes 1 year (or 365 1/4 days) to complete one revolution around the sun. The north-south axis on which the earth spins is not at right angles to the earth's orbit around the sun. The earth's axis is tilted at an angle of 23.5° to its orbit around the sun. When the earth moves around the sun in its orbit, it remains tilted to its orbit throughout. As the tilted earth orbits the sun every year, it causes different seasons during the different times of the year. Actually, when the tilted earth moves around the sun in its annual orbit, then first its one pole (or other hemisphere) tilts towards the sun. This one by one tilting of the two poles (or two hemispheres) of the earth towards the sun causes the change in the strength of sun's rays reaching on an area of earth's surface and produces different seasons. It also causes the change in the length of day and night. The formation of seasons will become more clear from the following discussion

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Fig. 1

(A) When the earth is at position A(see fig.1) in its orbit around the sun, then the Northern Hemisphere of the earth tilts towards the sun. When the Northern Hemisphere of the earth tilts towards the sun, it gets more heat and light from the sun and has summer season. During this period, at noon, the sun's rays fall almost perpendicularly in the Northern Hemisphere, they have more heating effect and hence the atmosphere becomes very hot. So, it is summer season in the Northern Hemisphere of earth (as in India). The peak summer season occurs around the month of June in the Northern Hemisphere.

When the Northern Hemisphere of earth tilts towards the sun, then at the same time, the Southern Hemisphere tilts away from the sun. When the Southern Hemisphere of the earth tilts away from the sun, it gets less heat and light from the sun and has winter season. During this period, even at noon, the sun's rays fall obliquely in the Southern Hemisphere, they have less heating effect and hence the atmosphere gets very cold. So, it is winter season in the Southern Hemisphere of earth (as in Australia). The peak summer season occurs around the month of June in the Southern Hemisphere. From the discussion we conclude that when it is summer in the Northern Hemisphere of earth, it will be winter in the Southern Hemisphere of earth. Now, India is in the Northern Hemisphere whereas Australia is in the Southern Hemisphere. This means that when it is summer in India, it will be winter in Australia.

(B) The earth is moving continuously in its orbit around the sun. So, after 6 months, the earth travels from the position A to position B(see fig.1) on the other side of the sun and the Northern Hemisphere now tilts away from the sun. When the Northern Hemisphere of the earth tilts away from the sun, it gets less heat and light from the sun and has winter season. The peak winter season occurs around the month of December in the Northern Hemisphere of earth.

When the Northern Hemisphere of earth tilts away from the sun, then at the same time, the Southern Hemisphere tilts towards the sun. When the Southern Hemisphere tilts towards the sun, it gets more heat and light from the sun and has summer season. The peak summer season in the Southern Hemisphere occurs around the month of December. From the above discussion we conclude that when it is winter in the Northern Hemisphere of earth, it will be summer in the Southern Hemisphere of earth. This means that when it is winter in India, it will be summer in Australia.

There are two times in a year when the Northern Hemisphere and the Southern Hemisphere of the earth are neither tilted towards the sun nor tilted away from the sun (see positions C and D). This gives us two other seasons: spring and autumn. When the earth is at position C in its orbit around the sun, then it is spring season in the Northern Hemisphere but autumn season in the Southern Hemisphere. This happens around the month of March. On the other hand, when the earth is at position D in its orbit around the sun, then it is autumn in the Northern Hemisphere but spring in the Southern hemisphere. This happens around the month of September. During spring and autumn seasons, both the hemispheres of the earth get almost equal amounts of heat and light from the sun. At positions C and D, day and night are equal in both the hemispheres.

Please note that the seasons on earth are determined by the angle at which the sun rays fall on its surface and not bythe distance of the earth from the sun. During the summer, at noon time, the sun rays fall almost perpendicularly on the earth's surface due to which the sun appears very hot. On the other hand, during winter even at noon time, the sun rays are inclined at a considerable angle to the earth's surface due to which the sun appears less hot. It is a surprising fact that in India the earth is nearer to the sun during our winter and farther from the sun during our summer. In general, we can say that:

IN THE NORTHERN HEMISPHERE:

• It is winter when the earth is closer to the sun.
• It is summer when the earth is farther from the sun.

IN THE SOUTHERN HEMISPHERE:

• It is summer when the earth is closer to the sun.
• It is winter when the earth is farther from the sun.

Another point to be noted is that due to the tilted axis to the earth, the sun rays fall on the earth for a longer time during summer and for shortertime during winter. Due to this, the days are longer in summer but shorter in winter. Please note that when the day is long then the night is short and when the day is short then the night is long.