# Motions of the Earth

The ancient people had always the sun, the moon and other heavenly bodies in the sky. They believed that the earth was stationary and other heavenly bodies moved around it.

Nicolaus Copernicus, a Polish astronomer, in the sixteenth century, Nicolaus in the west in the evening. This was the first to say that the earth only appeared to be stationary. Actually the Suppose we are travelling in a train. We set earth not only spins on its axis but also moves through the window that the trees and around the sun.

Thus, the earth has two types of motions. past in the opposite direction. We know it The movement of the earth on its axis is called the train that is moving and not the objects rotation and the movement of the earth outside. In the same way, it is the earth that is around the sun is called revolution.

ROTATION OF THE EARTH

The earth rotates on its axis from west to east. The axis of the earth is an imaginary line which passes through its centre. The axis is not vertical. It is inclined at an angle of 66% with the plane of the earth's orbit. This is known as the inclination of the earth's axis (See Fig. 3.3). The inclination is always in the same direction, ie. towards the Pole Star. Every morning, we notice that the sun rises in the east, travel across the sky during the day, and finally sets due to the rotation of the earth.

The earth takes about 24 hours or one day to complete one rotation on its axis. Thus. rotation is also called the daily motion of the earth. The circumference of the earth at the equator is about 40,000 km. The earth covers, this distance in about 24 hours.

EFFECTS OF THE EARTH'S ROTATION

It makes us feel that the sun, the moon and other heavenly bodies move from east to west.

It causes the bulging of the earth at the equator and flattering at the poles, It causes days and nights on earth.

It gives us the concept of direction based on sunrise and sunset.

It causes deflection in the direction of winds and ocean currents

It gives us a day of 24 hours.

It causes tides twice a day.

The direction of the rotation of the earth is anticlockwise at the North Pole and at the equator (See Fig. 3.4).

THE CIRCLE OF ILLUMINATION

The sun is the main source of heat and light for the earth. Due to the spherical shape of the earth, only one half of it gets light from the sun at a time and the other half remains in darkness (See Fig. 3.5).

The circle of illumination is an imaginary line which separates the lighted part of the earth from the dark one. Thus, the portion of the earth that receives sunlight experiences day, while the portion which is in darkness experiences night.

Let us try to understand how days and  nights are caused with the help of an activity.

In a dark room, place a globe in front of an electric bulb.

Switch on the light. You will see that half of the globe is lighted, while the other half is dark.

Mark a point A in the dark portion of the globe.

Now rotate the globe on its axis from west to east through 90.

Point A will turn towards the sun and will get some diffuse light.

The speed of the rotation of the earth at the equator is about 1.675 kr per hour. This is the highest speed of the earth's rotation. This speed decreases towards the poles, where it is zero.

In Figure 3.6 a, the North Pole is inclined towards the sun and the South Pole is turned away from the sun. Thus, a larger part of the Northem Hemisphere receives longer days and shorter reverse in the Southern Hemisphere during

Twilight is the diffused light after sunet sunlight. This results in

We normally notice such light before the nights in the Northern Hemisphere. It is the actual sunrise. This is called dawn.

Move the globe further from west to east .You will notice that now A has sunrise.

On further rotation of the globe, A will face the bulb. At this position, no shadows are noticed. This is noon time.

As we continue to rotate the globe, we will notice darkness approaching A. This is called sunset

At this position, it is not completely dark but there is some diffuse light. This diffused light after sunset is called dusk or twilight.

When A reaches exactly the opposite side of the bulb, it is midnight

LENGTH OF DAY AND NIGHT

We notice that the length of days and nights is not the same throughout the year. This variation is due to the inclination of the earth's axis. Due to this inclination, the Northern Hemisphere remains inclined towards the sun for half of the year and the Southern Hemisphere during the other half.

In Figure 3.6 a, the North Pole is inclined towards the sun and the South Pole is turned away from the sun. Thus, a larger part of the Northem Hemisphere receives longer days and shorter reverse in the Southern Hemisphere during this time.

n Figure 3.6 h, the South Pole is inclined towards the sun and the North Pole is turned away from the sun. Thus, a larger part of the Southern Hemisphere receives sunlight. This results in longer days and shorter nights in Southern Hemisphere. It is the reverse in the Northern Hemisphere during this time.

Throughout the year the length of days and night is equal at the equator. It is f 12 hours each

What would happen if the axis of the earth was vertical? The length of days and nights would be equal (12 hours each) at all places on the earth, throughout the year.

REVOLUTION OF THE EARTH

The earth, while cotating on its axis, also revolves around the sun This motion of the earth is called revolution. The earth revolves around the sun from west to east. The speed of the earth's revolution is about 30 km per The earth takes 365 days, 5 hours 48 minutes and 56 seconds (t.e. about 365 1/4 days) to complete one revolution around the sun. The time taken by the earth to go round the sun once is called a year. Thus, revolution is also called the annual motion of the earth.

For our convenience, we count only 365 days in a year. The balance of about 6 hours is computed to one extra day after every four years. This additional day is added to the month of February in that year. Thus, February has 29 days and that year has 366 days. Such a year is called a leap year and the extra day in the month of February is called a leap day.

The path along which the earth revolves around the sun is called the orbit, which is fixed. The length of the orbit is about 965 million km. The shape of the orbit is oval or elliptical (See Fig. 3.7). Since the orbit is elliptical and the sun occupies one focus of the ellipse, the distance between the earth and the sun varies throughout the year.

When the distance between the earth and the sun is minimum, about 146 million km, the earth is said to be in perihelion (peri' means near and helios' means the sun). The sun is at this position on or around 4 January every year.

When the distance between the earth and the sun is maximum, about 151 million km, the earth is said to be in aphelion l'ap means away from and 'helios means the sun). The sun is at this position on or around 4 July every year.

The distance between the earth and the sun varies only by about three per cent during one complete revoltition. Thus the average distance between the earth and the sun is about 148.5 million km

EFFECTS OF THE EARTH'S REVOLUTION

The revolution of the earth around the sun causes variation in the length of day and night, in the distribution of heat over the surface of  the earth and the phenomenon of seasons. The phenomenon of seasons is the most important effect of the earth's revolution.

THE PHENOMENON OF SEASONS

The earth completes one revolution around the sun in about 365 1/4 days or one year. This period is divided into four seasons-spring summer, autumn and winter. The change of seasons is due to

the rotation of the earth on its axis.

the revolution of the earth around the sun.

the inclination of the earth's axis.

Figure 38 shows the movement of the earth around the sun. The positions 1, 2, 3 and 4 show the position of the earth on four selected days. ie. 21 March, 21 June, 23 September and 22 December. The interval between these dates is about three months

Positions 2 and 4 are called solstices. A solstice is the day, when the midday sun shines vertically overhead at one of the tropics (the Tropic of Cancer or the Tropic of Capricorn) and the duration of the day in longest in that hemisphere.

Positions 1 and 3 are called equinoxes. An equinox is the day, when the midday sun shines vertically overhead at the equator the length of day and night is of equal duration throughout the world. The 'equinox' stands for equal nights.

In the Northern Hemisphere

Position 2 on 21 June is the summer solsticce.

Position 4 on 22 December is the winter solstice.

Position 1 on 21 March is the spring equinox.

Position 3 on 23 September is the autumnal equinox.)

Summer Solstice-21 June

The North Pole is inclined towards the while the South Pole is turned away the sun (See Fig. 3.9).

The rays of the sun fall vertically ove Tropic of Cancer (23 1/2 deg N).

A large portion of the Northern Hemisphere gets heat and light from the sun.

Days are longer than nights in the Northern Hemisphere.

Places in the Northem Hemisphere experience summer season.

The longest day and the shortest night on 21 June in the Northern Hemisphere.

All the above conditions are reversed Southern Hemisphere.

#### Winter Solstice-22 December

The South Pole is inclined towards the Sun, while the North Pole is turned awa the sun (See Fig. 3.10).

The rays of the sun fall vertically over the Tropic of Capricorn (23 1/2deg S).

A large portion of the Southern Hemisphere  gets heat and light from the sun.

Days are longer than nights in the southern Hemisphere.

Places in the Southern Hemisphere experience summer season .

The longest day and the shortest night occur on 22 December in the Southem Hemisphere.

All the above conditions are reversed in the Northern Hemisphere.

#### Spring and Autumnal Equinoxes-21 March and 23 September

On 21 March and 23 September, the rays of the sun fall vertically on the equator.

The North Pole and the South Pole lie at an equal distance from the sun S Fig. 3.11)

Days and nights are of equal duration throughout the world.

It is neither very hot nor very cold in both the hemispheres.

The Northern Hemisphere experiences spring season from 21 March and autumn season from 23 September. It is the reverse in the Southern Hemisphere.

The equinoxes coincide with the time of sunrise at one pole and the time of sunset at the other pole

21 March coincides with the New Year Day of the Indian National Calendar.