MOVEMENT OF THE EARTH

THE MOVEMENT OF THE EARTH

Introduction

Like all other planets in the solar system the Earth also two types of movements. The Earth rotates on an imaginary axis. It takes nearly 24 hours for the earth to complete one rotation. The period taken for one rotation is called a day. Earth also revolves around the sun. The path along which the Earth revolves round the sun is called its orbit. This orbit is not circular. The orbit has an elliptical shape. This means that the distance between the Earth and the sun is not the same during the course of its revolution. One revolution is completed in about 365 days and 6 hours. This period is called a year. The rotations of the Earth have important consequences affecting our environment.

Rotation of the Earth

The rotation of the Earth cause day and night. Only one half of the earth is illuminated by the sun at a given time. The other half is in darkness. Owing to the rotation of the Earth passes through the lighted half of the Earth. This period passes through the darker side of the earth, it experiences night.

As it takes 24 hours for the Earth to complete one rotation, a point on the equator moves a distance of about 40,000 km. During that period. The speed of movement of a point at the equator is about 1670 km per hour. Owing to the spherical shape, the speed of rotation decreases both to the north and south of the equator. As the circumference of 60^o N&S latitudes is only half that of that at the equator. At the two poles, which are the two ends of the imaginary axis, there is no relative movement owing to the rotation.

Effects of rotation of the Earth

The rotation of the Earth is responsible for the following major effects.

1.     Rotation of the Earth causes the alternation of day and night; rotation of the Earth is also responsible for difference in time between different places on the Earth. We know that the difference in time is 4 minutes for each degree of longitude.

2.     Owing to the rotation of the Earth from west to east, the sun, moon, planets and stars appear to move from east to west across the sky. The sun appears to rise in the east and set in the west. This apparent movement of the sun and other heavenly bodies is in a direction opposite to that rotation of the Earth. This is similar to what we observe through the window of a moving bus. The trees, houses and fields on either side of the road appear to move in a direction opposite to that of the speeding bus.

3.     The rotation of the Earth is considered responsible for the slight of the Earth at the two poles and bulging at the equator. The equatorial of the Earth is 43 km longer than the polar diameter.

4.     The rotation of the Earth causes a deflection of winds and ocean currents from their normal path.

5.     The occurrence of tides in the oceans twice daily and their westward progress are the result of rotation of the Earth.

Revolution of the Earth.

The Earth revolves around the sun in an elliptical orbit. An ellipse is an elongated circle. The period taken for one revolution is about 365days and 6 hours. A normal year has only 365 days. The remaining 6 hours is added once in four years to make a leap year. In a leap year, the mouth of February has 29 days.

Owing to its elliptical orbit, the between the Earth and the sun varies from a minimum of about 147 million km to a maximum of 152 million km. The minimum distance id reached in 3^rd January when the Earth is closest to sun. This position is known as the perihelion (peri-near; hellion-sun). The difference in distance between the perihelion and aphelion positions is only about 5 million km. This is quite small considering the average distance of about 150 million km between the Earth and the sun.

Inclination of the Earth's axis

The plane in which the Earth revolves around the sun is called the Plane of the Ecliptic. The Earth's axis is not at right angles to the plane of the ecliptic. The axis is inclined at an angle of 66 1/2^o to the plane of the ecliptic. In other words the Earth's axis is inclined at an angle of 231/2^o from vertical. Earth revolves around the sun on an inclined axis. The inclination of the Earth's axis is an important factor which brings about changes during the occurs of revolution of the Earth around the sun.

Effects of revolution of the Earth

The revolution of the Earth the sun on an inclined axis has two major effects.

The North Pole is inclined towards the sun during a part of the year and the south Pole is inclined towards the sun during another part of the midday sun and changes in the altitude of the midday sun and changes the duration of day and night during the course of a year. Both these changes are the basic causes of different seasons during a year.

Equinoxes

Note the positions of the Earth on March 21^st and September 23^rd. On these two days. The sun's rays are overhead at the equator. The circle of illumination passes through the two poles. Days and nights are of equal duration, normally 12 hours at all places on the Earth. Therefore, these days are known as Equinoxes, meaning equal nights. As March 21^st represents the spring season in the Northern Hemisphere, this equinox is called the vernal Equinox. September 23^rd is referred to as Autumnal Equinox because it represents the autumn season in the Northern Hemisphere.

Solstices

After March 21^st, there is an apparent of the sun to the north of the equator. The apparent northward movement of the sun in the sky continues up to June 21^st when the sun appears overhead at the Tropic of Cancer (231/2^oN). The sun appears to stand still at this position and then moves southwards towards the equator. The position of the sun on June 21s^t is called the summer solstice, as it marks the summer season in the Northern Hemisphere. The word solstice is derived from a Latin word. Which means the sun standing still or reaching the highest point. After June 21^st, the sun appears to move southwards and then crosses the equator on September 23^rd or autumnal equinox. The apparent southward movement of the sun continues beyond the equator till December 22^nd. On this day the sun is overhead at the tropic of Capricorn (23_1/2^o). This position of sun is refereed to as the winter season in the Northern Hemisphere.

Summer solstice

On the summer solstice (June 21^st), the sun's rays are overed at the Tropic of cancer. The Northern Hemisphere is inclined towards the sun. The altitude of the sun is 90^o at 23_1/2^oN. The sun appears to stand still at this position and then moves southwards the equator. The position of the sun on June 21^st is called the summer solstice, as it marks the summer season in the Northern Hemisphere. The word solstice is derived from a Latin word, which means the sun standing still or reaching the highest point. After June 21^st, the sun appears to move southwards and then crosses the equator on September 23^rd or autumnal equinox. The apparent southward movement of the tropic of Capricorn (23_1/2⁰s). This position of sun is refereed to as the winter season in the Northern Hemisphere.

Summer solstice

On the summer solstice (June 21^st), the sun's rays are overed at the Tropic of cancer. The Northern Hemisphere is inclined towards the sun. The altitude of the sun is 90o at 23_1/2o N. at the North Pole on summer solstice is greater than on the equinoxes. The altitude of the sun is (90-23 _1/2)^o. This is less than on the equinoxes.

Winter solstice

The position of the Earth on winter solstice (December 22), is show in figure. On this day the sun is overhead at the Tropic of Capricorn (23 _1/2S). The South Pole is included towards the sun. The altitude of the sun is 90^o at the Tropic of Capricorn. The altitude of the sun at the South Pole is [90-(90-23 _1/2)] or 23. The altitude of the sun also decreases north of the Tropic of Capricorn. At the equator, the altitude of the sun is (90-23 _1/2) or 66 ^o.

In the Northern Hemisphere, the altitude of the sun decrease gradually from 66_1/2^o at the equator to 0^o at the Arctic circles (66_1/2^oS). The sun's rays do not reach the North Polar Region beyond the Arctic Circle. The altitude of the sun is greater in the Southern Hemisphere than in the Northern Hemisphere. This marks the summer seasons in the Southern Hemisphere.

Seasonal changes in the duration of day and night

We have learned earlier that during the equinoxes, all places on the Earth have 12 hours of day and 12 hours of night. Owing to revolution of the Earth around the sun on an inclined axis, the duration of day and night varies according to seasons and the altitude of a place.

On the summer solstice the North Pole is inclined towards the sun. There, the duration of sunlight or daytime increase from 12 hours at the equator to 24 hours at the Arctic Circle and beyond. The region beyond the Artic circle is known as the North Pole, there will be seen always above the horizon at a low angle. At 66_1/2^oN, 24 hours of sunlight can be seen only on June 21. Hamerfest in northern Norway is a place of tourist attraction for observing the phenomenon of the midnight sun. This place has continuous daylight from May 13^th to July 29^th. This place is easily accessible to tourists and has hotels and other facilities.

In the southern, the duration of daylight decreases from 12 hours at the equator to 0 hours beyond the Antarctic Circle. In the South Polar Region, there is 24 hours of darkness. The sun is always below the horizon in the Southern Hemisphere of daylight.

During winter solstice (December 22), the Southern Hemisphere experiences the summer and the Northern Hemisphere has winter. Therefore, the duration of daytime or sunlight is greater in the Southern Hemisphere than in the Northern Hemisphere. The duration of daytime or sunlight is greater in the Southern Hemisphere than in the Northern Hemisphere. The duration of daytime increases from 12 hours at the equator to South Polar Region has 24 hours of sunlight for many days continuously. At the South pole, there will he six months of sunlight. The sun will always be seen at a low angle above the horizon.

In the Northern Hemisphere, the duration of the daylight will decrease from 12 hours at the equator to 0 hours at the Arctic Circle. There are 24 hours of darkness in the North Polar Region. The duration of night is greater than the duration of daylight as one moves northwards from the equator.

The duration of the day at different latitudes in different seasons is given below:

It is evident from the above table that the duration of daylight is 12 hours throughout the year only at the equator. As one moves away from the equator, the seasonal variations in the duration of daylight increase. The seasonal variation in the duration of daylight is maximum at the Polar Regions.

Latitude

The earth rotates on an imaginary axis, the two ends, which appear as points on the Earth's surface. These points are called the poles of the Earth and the pole that points towards the pole star is called the North Pole while the opposite pole star is called the North Pole while the opposite pole is the South Pole.

The equator is the imaginary line that lies midway between the two poles. The plane of the equator intersects the earth's axis at right angles.

The equator divides the Earth into two hemispheres. The Northern Hemisphere lies to the north and the southern Hemisphere lies to the south.

Determination of latitude

You have learnt in the previous chapter how the longitude of a place can be calculated based on the difference in time between two places. Latitude of a place can be determined based on the angle of inclination of the sun in the sky.

A simple instrument called a sextant can determine the angle of inclination of the sun in the sky during midday. The angle of inclination may also be obtained approximately by studying the length of the shadow of a pole fixed on the ground. In the Northern Hemisphere, the latitude can also be determined by observing the Pole star.

By observing the sun

You know that sun is overhead only in the tropical zone between the Tropic of cancer and the Tropic of Capricorn. Even in the tropical zone, the sun is overhead only on one or two days in a year. On other days this appears in the northern part of the sky. When the sun is overhead at the equator on the equinoxes, the sun will appear in the southern sky at places in the northern Hemisphere will see the sun always in the southern sky and places in the Southern Hemisphere will see the sun in the southern sky. Hence, while observing the angle on inclination of the sun. It is necessary to find out whether the sun was seen in the northern sky or the latitude of a place by observing the sun is illustrated with a few worked examples.

Problem 1.

On March 21, the angle of inclination of the midday sun was 48^o at a point P. the sun was seen in the southern sky. Calculate the latitude of the station P.

Study the given diagram. In this diagram, OE represents the equator. As the sun was seen in the southern sky at P, the station must be to the north of the equator. The station P is marked north of the equator. Z marks the zenith at the station P, AB marks the horizontal plane at P, SP and FE represents the sun's rays which are parallel.

Ø The angle SPB represents the angle of inclination of sun.

Ø   The angle SPZ is knows as the Zenith Distance.

Ø   Where is the angle of inclination of the sun.

Ø   Latitude of P is angle POE.

Ø   Angle POE = Angle POZ, SP and FO parallel lines.

Ø   Therefore Latitude of P = Zenith distance at P = (90-48)or 42^oN.

Latitude of 42^o N.

Problem 2.

On June 21, the angle of inclination of the midday sun was observed as 720 at station R. the sun was seen in the southern sky. Find out its latitude.

Ø On June 21, the sun is overed at 231/2^oN.

Ø    Latitude of station = 23^1/2o  Zenith distance.

Ø    Zenith distance at R = (90-72) or 18^o.

Ø    Latitude of R = (23_1/218)^o =or 5^o.

Ø    As the sun was seen to the south or R, the sun must be north of the Tropic of cancer.,

Ø    Therefore, the altitude of R is 42_1/2^oN

Problem 3.

 On June 21. The angle of elevation of the sun was observed as 20^o at station S. The sun was seen in the northern sky. Find out the latitude of S.

          Latitude of S        = 23_1/2^o  Zenith distance.

                             = 23_1/2^o  (90-20).

                             = 93_1/2^o  -46_1/2^o.

As the value of latitude cannot exceed 90^o, the value of 90_1/2^o is absurd and can be deleted.

The correct answer is 461/2o S as the

 Negative sign indicates that the station, the latitude of S should be to the south of the tropic of Cancer.

The latitude of S 451/2oS.

Prpblem 4.

On December 22, the latitude of the midday sun was observed as 48o at a station T. the sun was seen in the northern sky. Find out its latitude.

On December 22, the sun is overhead at 231/2oS

Latitude of T = 231/2o  Zenith distance.

                   = 231/2^o (90-a)

                   = 231/2o  42.

                   65_1/2^o of 19_1/2^o

The latitude of the station can be 65_1/2^oS of 18_1/2⁰

As the sun was seen to the north of the station T, the latitude of T should be south of the Tropic of Capricorn.

The latitude of T is 651/2oS

By measuring the length of shadow of a pole

A pole dixed on the school playground such as a aflagstaff or a goal post casts a shadow on the ground. In the morning the length of the shadow decreases gradually and reaches a minimum during noon when the sun is crossing the meridian at the staion. In the afternoon, the length of the shadow starts increasing once again the pole casts a shadow to the south when the sun is to the south, the shadow is cast to the north. The ground around the pole should be free from building of other obstructions so as to enable the shadow to be observed and measured correctly.

The length of the shadow cast be a pole during midday gives an idea of the angle of inclination of the sin at hot place. When the sun is at a low angle at midday, the length of the shadow at midday would be longer than when the sun is at a high angle in the sky.

PROBLEM 1.

On March 21, a flagstaff 3 meters high casts a shortest shadow of 3 meters length at noon. The shadow is cast on the southern side of the flagstaff. Find out the altitude of the place. Draw a diagram to show the flagstaff and the shadow cast by it. In the diagram, IO represents the pole and Os indicates the shadow in the ground. Measure the angle OSP. This gives the angle if inclination of the sun at the station.

The angle OSP = 45^o

March 21^st is the day of equinox.

On equinox, the latitude of a [;ace = Zenith distance. –(90-a) = (90-=45) or 45^o

As the shadow is cast on the southern side, the sun must be to the south of the stations the sun is overhead at the equator, the station must be in the northern Hemisphere.

The latitude of the station is 45^o N.

By observing the Pole star.

In the Northern Hemisphere, the altitude of a place may be obtained by observing angle of inclination of the Pole star the star is not seen in the Southern Hemisphere. The star is seen overhead at the North Pole star is seen at the horizon.

At any intermediate point between the equator and the North Pole, the angle of inclination of the Pole star is equal to the latitude of the place. Sailors to locate the position of ships during the nighttime use this method. A simple instrument called the sextant can easily measure the angle of inclination of the pole star.