Everyday the sun rises in the east and sets in the west. This process led many people in the past to think that the sun is moving and the earth is fixed. The sun moves around the earth. But with the advancement of science, this has been proved that the sun itself is moving and the earth has also motion. Later it has been revealed that the sun does not move around the earth, rather the earth moves around the sun. The earth not only moves around the sun, it also rotates on its own axis. The motion of the earth is of two types : (1) to rotate on its own axis, and (2) to revolve around the sun in its own orbit. The rotational motion of the earth is known as Diurnal motion and the revolutionary motion is called the Annual motion.

• Diurnal Motion

The earth has motion. Centering the sun, the earth moves from west to east on its axis. This motion of the earth is known as rotation. The earth takes 23 hours 56 minutes and 4 seconds or nearly 24 hours to rotate once on its axis. This period of 24 hours is regarded as one day. This diurnal movement of the earth is the rotation. One such full rotation of the earth is known as solar day. The earth takes a full day to complete rotation once on her axis. The speed of rotation is different in different places. The speed of rotation at the equator is highest. Here the speed of rotation is more than 1,610 kilometres per hour. This speed decreases towards north and south of the equator. The speed of the rotation is nearly zero at the poles. Though the diurnal motion exists, but still we don’t feel due to the following reasons:

1. Compared to the size of the earth, we are very much insignificant, so we do not feel the speed.
2. No fixed or movable object is available in front of the earth in the space which can be taken as the basis to understand the rotational speed of the earth.
3. Keeping pace with the speed of the earth, the atmosphere also moves from west to east. So, we, do not feel the rotational speed of the earth.
4. The rotational speed of each of the places of the earth is fixed.

Evidences of Diurnal Rotation

In the old days Ptolemy and his follower astronomers used to think that the earth was motionless. But subsequently astronomers like Copernicus, Kepler, Galileo and Newton gave their opinions in favour of the rotation of the earth.

1. Everyday the sun rises in the east and sets in the west. From this phenomenon we can infer that either the sun of the earth is moving. This motion is accomplished within one day or 24 hours. The sun is stationed about 15 crore kilometres away from the earth in the space. Being located at such a long distance, it is not possible for the sun to revolve round the earth within 24 hours. Moreover the tremendous speed the sun requires to move round the earth will be even more than the speed of the light, which is simply impossible. Because no object can move faster than light. So, it is not the sun but the earth which rotates on its own axis in 24 hours.
2. The shape of the earth : The earth is inflated in the middle and is compressed towards the poles. If any pliable object or matter rotates around its axis, it acquires such a condition. The present shape of the earth is the result of such rotational speed. Scientist Newton had the opinion that due to the rotational motion, the earth acquired the present shape.
3. Due to rotation of the earth, changes also occur in ocean currents and wind system. According to Ferrel’s Law, it is due to the rotation of the earth that the air and the ocean currents are deflected to the right in the northern hemisphere and to the left in the southern hemisphere.
4. It is observed that if a stone is thrown from a high place, it does not reach the ground vertically, rather it moves slightly towards the east. This proves that the earth rotates from west to east or the earth has rotational motion.
5. Primarily, tide occurs due to the attraction of the moon. The moon takes twenty seven days to complete one revolution around the earth. So, it is expected that the high tide would occur once every 27 days. But practically we observe that the high tide occurs every day which proves the rotational movement of the earth.
6. French scientist Foucault, with the help of a pendulum proved the diurnal motion of the earth. He attached a pin at the bottom of the pendulum and hang it on the top of St. Pantheon church of Paris with a fixed motion, so that it would oscillate from north to south. Due to oscillation, the pin at the bottom of the pendulum started marking on the ground. Mr. Foucault observed that every time the marks have been deflected towards east. So, it can be proved that due to diurnal motion the earth rotates from west to east.

Effects of Earth’s Rotation

1. Occurrence of day and night: One of the effects of the earth’s rotation is the occurrence of day and night. We know that the earth is round and it does not have any light of its own. The earth becomes illuminated by the light of the sun. Due to diurnal motion, that part of the earth which faces the sun becomes illuminated. It is day in the illuminated portion of the earth. Sunlight does not reach the opposite part of the illuminated portion. So, it remains dark and it is night in the dark portion of the earth. 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 are changed. When the dark portion is illuminated, it becomes day there. Similarly the illuminated portion turns dark, it becomes night there. Thus, day and night has been occurring alternately. So, in some places it is 12 hours of day and 12 hours of night. If the shape of the earth would have been flat instead of being round, the rotation of the earth would create only day or only night. In other words, either only day or only night would exist simultaneously over the globe. On a specific date, in some parts of the earth it is day whereas in other parts it is night. That means when a portion of the earth remains dark the other part becomes illuminated. The border of the lighted and the dark portion is called the shadow circle. Due to rotation, when the dark portion, after crossing the shadow circle faces the light, it is known as morning. The feeble light which precedes the morning is known as dawn and similarly the feeble light that precedes the evening is called twilight. In any place, when the sun reaches the highest elevation of the sky it is known as midday or noon and the time recorded is 12 O’ clock. But in the opposite point of that place it is midnight.

2. Temperature differences: It is due to the spherical shape and also for rotation that a comparatively small portion of the earth, located in the tropical areas between 23.5° North latitude to 23.5° South latitude gets direct sunlight only for some time of the year. The region where the sun gives its rays perpendicularly gets more heat. On the other hand, the areas located north and south of the region mentioned above get slanting rays and so these areas get less heat.
3. Ocean currents, Tide and ebb and wind circulation: Diurnal motion of the earth creates rotational force which is responsible to change the direction of wind and ocean currents. So, as an effect of earth rotation, ocean current and the winds are deflected towards right in the northern hemisphere and towards left in the southern hemisphere.
4. Determining the time: Due to rotation of the earth, it has become easier to calculate the time, since the period of one complete rotation of the earth can be divided into 24 parts and each of the parts can be calculated as an hour. Again the hour can be sub-divided to sixty divisions and the period of each of the divisions can be taken as a minute. Similarly, the minute can again be divided into sixty divisions taking each division as one second.
5. Influence on nature and living creatures: If due to rotation of the earth, any change occurs in the field of temperature and light then such change influences the production, growth and character of living organisms. Had there been no diurnal motion some of the regions would remain ever dark and no plant could grow, while some other regions due to continuous sunshine would have been converted into desert.

• Annual Motion

According to Newton’s Law of Gravitation, the planets move around the stars due to their attraction. A law is also applicable to the solar system. In solar system each of the planets moves round the sun. The earth being a planet of the solar system also moves on an elliptical orbit round the sun. The earth having a full rotation on her axis for 24 hours, simultaneously moves around the sun on a set route i.e. orbit once in a year. This movement of the earth is known as the annual motion. The earth takes 365 days 6 hours to revolve around the sun in her orbit. The period, the earth takes to complete the revolution is known as the solar year.

The circumference of the earth’s orbit is slightly more than 93 crore 80 lakhs and 827 kilometres. The average speed of revolution is 1, 06,260 kilometres per hour. In other words, the earth moves around the sun at a speed of 30 kilometres per second. Considering this speed, it requires 365 days 6 hours 48 minutes and 47 seconds to move round the sun for one time. But the solar year is calculated to be of 365 days.

So, there remains a shortfall of about 24 hours or one day. To make up this deficit one extra day is added to the calendar in every fourth year. And that year is calculated as of 366 days. The month of February is calculated to be of 29 days instead of 28 days. This year is regarded as Leap Year and the leap year is taken to be of 366 days.

Revolution of the earth is responsible to create fluctuation in the duration of day and night. It is also responsible to bring changes in the march of seasons.

Effects of Annual Motion

• Fluctuation of day and night: The duration of the days and nights fluctuate in the same time in the different parts of the year due to the revolution of the earth. As a result, it is observed that in the same latitude, the length of day and night fluctuate in different periods of the year.

In the process of revolutionary motion the earth, on 21st June reaches in such a position that the sun’s rays fall vertically on the northern most position of the earth, that means, on 23.5° North latitude or on the Tropic of Cancer. At this time, the northern hemisphere remains inclined more towards the sun and the southern hemisphere remains at the furthest distance from the sun. That is why the length of the day is greater and the temperature is higher in the northern hemisphere. So, 21st June is the longest day and shortest night in the northern hemisphere. The sun reaches its northern most limits on 21st June and the date is known as Summer Solstice.

The sun shines vertically over the equator on 21st March and 23rd September. On these two dates, the sun remains in equal distance from the north and south poles. So, on these two dates, the duration of the day and night is equal (12 hours day and 12 hours night) over the entire world. The day when the day and night are equal over the globe, is known as the Equinoxes. 21st March is spring in northern hemisphere. So, 21st March is called the Vernal Equinox, and 23rd September is Autumn northern hemisphere, so 23rd September is known as Autumnal Equinox.

The southern hemisphere shows maximum inclination towards the sun on 22nd December. So, on that date it is the longest day and the shortest night in the southern hemisphere. The sun reaches its southernmost position on this day and the date is known as the Winter Solstice. The sun gives its rays vertically over 23.5° South latitude or Tropic of Capricorn.

Proofs of Annual Motion

1. The apparent motion of the stars: As we cannot feel the occurrence of earth’s rotation, similarly we cannot perceive the existence of earth’s revolution. But with the changing locations of the stars from east to west at night sky, we can comprehend that the earth has an annual motion or the revolution. By looking at the stars in a clear sky one can understand the gradual movement of the stars from east to west.
2. The changing locations of the sun in the sky: The sun is seen to be located in different positions in different periods of the year. We can observe that on 21st March and 23rd September, the sun rises at a point due east on the horizon and sets at a point due west on the horizon. Again, we can see that on 21st June and 22nd December, the sun rises at north-eastern horizon and south-eastern horizon in the sky respectively. From 21st June to 23rd December, the sun seems to have an apparent motion towards south. This is known as southerly movement of the sun. Again from 22nd December to 21st June, it seems that the sun is apparently moving towards north and this movement of the sun is known as northerly movement.
3. The revolutionary motion of different planets: It has been observed through telescope that all the planets are revolving around the sun. The earth as a planet also has revolutionary speed or annual motion.
4. Direct observation: Recently the spaceman from spaceship has observed the revolution of the earth.
5. The Law of Gravitation: In comparison to the sun, the earth is very small. The sun is 13 lakh times greater than the earth. So, it is naturally thought that due to gravitation the earth moves round the sun.

Change of Seasons

The whole year is being divided into 4 divisions on the basis of the variation of temperature. Each of the divisions is known as a season. These are the Summer, the Autumn, the Winter and the Spring. It is to be noted here that when summer prevails in the northern hemisphere, winter sets in the southern hemisphere. Again, when there is winter in the northern hemisphere, it is summer in the southern hemisphere.

Similarly, when it is spring in the northern hemisphere, it is autumn in the southern hemisphere and it is spring in the southern hemisphere when it is autumn in the northern hemisphere.

Reasons for change of seasons: Due to the difference of temperature, the change of seasons does occur. The reasons for difference of temperature and as well as that of seasons are the following :

1. Difference in solar heat;
2. The earth is round. So, some places get direct sunshine, whereas in other places it falls at a slanting position. As a result, it creates difference in temperature which in turn causes to change the season.
3. The orbit of the earth is elliptical. So the distance of the earth from the sun fluctuates. As a result, it creates difference in temperature which ultimately brings changes in season.
4. The axis of the earth makes a fixed inclination of about 66.5° with the plane of the ecliptic. As a result, once in a year the North Pole or the South Pole shows inclination towards the sun. Temperature increases in the hemisphere when it remains inclined towards the sun and in opposite case the temperature decreases. As a result, the season changes.