# The Movement of the Earth

The Earth has two motions or movements, rotation and revolution. It was thought earlier that the earth was stationery and the sun moved round the earth. But it was proved later that the sun is stationery and the earth moves round the sun. The earth moves round the sun on its own axis once every 24 hours a day. The axis of the earth is inclined at an angle of 23 degrees 30 minutes with the perpendicular to its plane of orbit. The earth moves from west to east on its own axis a day. Movement on its own axis a day is called Rotation and movement round the sun a year on orbit is called Revolution.

Rotation

The movement of the earth around the Sun is called its Rotation. The path along which the Earth travels around the Sun is called the Orbit.

Different places on the Earth's surface move at different speeds. Places near the Poles hardly show any movement, while those at the Equator spin around at more than 1,600 kilometres per hour.

We are not thrown away from the earth surface and we don't feel it in spite of the motion of the earth. The reasons are :

1. Humans, animals, air etc. move in the same speed along with the surface of the earth so we don't feel the motion of the earth.

2. The earth attracts all the things towards its centre by its gravitation power, so we are not thrown away from it.

3. We don't feel earth's motion or we are not thrown away as we are too small compared to its huge size.

4. We don't feel the motion as the motion of moving of the earth is certain at every place.

5. If one thing out of two moves and another one keeps standing on earth, then it is understood that it has motion.Thus, there is no matter before the earth which is static or equal though we can understand the motion of the earth.

Proofs of Rotation

The proof that the earth moves from the west towards the east on its own axis is :

1. Pictures sent by spacecraft : Pictures sent from the satellite and spacecraft show that the earth moves from west to east. These pictures are the most modem and correct proof of earth's movement or rotation.

2. The size of the earth :If any delicate round shaped , thing spins on its own axis like a top, then a  centripetal and centrifugal power develops at a time which makes the poles a little bit compressed and middle part a little bit enlarged. By virtue of movement at the time of birth the delicate earth got squeezed in the north and south poles a little bit and centre part swelled up. Scientist Newton said, this size of the earth has become like this due to this movement (fig. 2.14).

3. Cause of day-night : In most parts of the world there is day and night. It means 12 hours day and 12 hours night which are the principal reasons of rotation of the earth. If there was no rotation then one part of the earth would have been dark for ever and the other part would have been sunny.

4. Ferrers law : We know that sea waves and air turns right hand side in the north pole and left hand side in the south pole. This turning away is known as Ferrel's law (fig. 2.15).

The results of Rotation : The changes we see because of the rotation are:

1. Occurrence of day and night in the world : Day and night is the result of rotation. The earth is round and it does not have any light of its own. The earth gets light from the sun. The Earth during movement around its axis, becomes lighted with the rays of the sun. When it faces the Sun, that part is day. And the opposite side remains dark as the sunlight does not reach there. This dark part is night (fig. 2.16).

Due to rotation of the earth, alternately the illuminated portion becomes dark and the dark portion becomes illuminated and that is why the day and night occur. When the dark portion is illuminated, it becomes day and similarly the illuminated portion turns dark, it becomes night there. In this process, day and night occur. So, in some places it is 12 hours of day and 12 hours of night.

2. The cause of the high and low tide : High tide and low tide happen due to rotation of the earth. Tide does not occur at the same time everyday. For sample, time of high tide in two consecutive days may vary by 52 minutes. This time difference occurs because of rotation.

3. The cause of air and sea wave : From the equator to both the poles the diameter of latitude and the motion of earth's movement become slower gradually near the two poles. The air wave or sea wave of the earth turn away towards right side in the north pole and left side in the south without going down directly from high pressure to low pressure zone.

4. The cause of temperature difference : At day time, temperature rises because of sunshine. At night it cools down after radiating heat. If there were no rotation, night would not have come after day and the difference of temperature would not occur. This difference of temperature is one of the results of rotation.

5. Measuring time : It becomes easier to measure time because of rotation. One twenty fourth time which is full round of one movement is taken as one hour and its one sixtieth part is called one minute. And one sixtieth part is counted as one second.

6. Animal and plant world : Because of the rotation of the earth all parts of earth experience sunlight at day time and no light at night time. Sunlight is necessary for plants and animals. They collect energy from the sun at day time and utilize it for their metabolism at night. Some animals collect food at day time while some do it at night. Day and night occur because of earth's rotation and the rules and regulations of plants and animals depend mostly on it.

Revolution

The earth moves continuously on its axis towards a particular direction because of the gravitation of the sun ( opposite of the hands of watch) and moves round the sun at a certain period. This motion of the earth is called Revolution. It takes 365 days 5 hours 48 minutes 47 seconds to move round the sun once. It is called Solar Year. But we take 365 days as one year. According to this account, extra 6 hours remain every year. To bring a balance for this extra time in every 4 years in the month of February, 24 hours or 1 day is added. Thus, in the year in which February is of 29 days there are 366 days and this is called a leap Year.

Task: Find out the Leap Year from 2013 to 2030.

The results of Revolution : The results of Revolution are : (1) the increase and decrease of day and night, (2) season change.

The increase and decrease of day and night : The real causes of the increase and decrease of day and night are :

(a) Earth's round shape with curves in two poles, (b) Earth's parabolic orbit, (c) Earth's continuous rotation and revolution, (d) Location of poles in the one direction, (e) Angular location of the earth on its axis.

At the time of revolution earth leans itself on 66.5° angle towards the Sun. On 21st March the sunshines vertically on the equator. Then, the sunshine goes towards north pole gradually.

On 21st June the earth comes to a certain place where the sun rays fall on the earth's surface at 23.5° north latitude (fig. 2.17). During this time the north pole leans toward the sun and south pole is far away from the sun. For this reason the length of the day is longer and the temperature is higher in the north pole. In the north pole the longest day and shortest night is June 21. On this day, the sun reaches the Tropic of cancer.

On the 23rd September the sun again shines vertically on the equator like 21st March. So, the day and night are equal. From 23rd September the sun starts moving again towards south pole. On December 22 the sun rays fall on the Tropic of capricom causing the longest day in South Pole, and shortest day in the North Pole.

On 21st March and 23rd September the sun shines vertically on equator. These two days and nights are equal all over the world. That day is called Equinox. On March 21 there is Spring in the north pole, so it is called Vernal Equinox. On 23rd September there is Autumn in the north pole, and the day is called Autumnal Equinox.

Proofs of Revolution

1. Displacement, disappearance and reppearing of stars in the night sky : In a cloudless night sky it is seen that the stars change their place from east to west. Then one day they disappear. After one year they come back to their original place. This change proves that the earth has motion.

2. The change of location of the sun in the sky : The sun is seen in different places in different times. It does not rise in the east from the same place everyday and does not set in the west in the same place. Six months of the year the sun moves a little towards south and rise from the east. The rest six months, it moves towards the north and then rises in the east. If the world had not moved and stayed in a fixed place, the sun would have risen from the same place. It happens due to the revolution of the earth.

3. The revolution of different planets : It is seen through the telescope that all the planets are moving round the sun. The earth is a planet, so it also has motion or revolution.

4. Direct experience : The astronauts saw the motion of the earth from the space.

5. Gravitational theory: The earth is very small compared to the size of the Sun. The Sun is 13 lakh times bigger than the earth. So, usually the earth moves round the sun because of its gravitational force.

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