WHAT IS ROTATION?

WHAT IS ROTATION?

As you know that the earth has two types of motions, namely rotation, and revolution. Rotation is the movement of the earth on its axis.

The axis of the earth which is an imaginary line makes an angle of 66½° with its orbital plane. The plane formed by the orbit is known as the orbital plane. The earth receives light from the sun.

Due to the spherical shape of the earth, only half of it gets light from the sun at a time (Figure 3.2). The portion facing the sun experiences day while the other half away from the sun experiences night. The circle that divides the day from night on the globe is called the circle of illumination. This circle does not coincide with the axis as you see in Figure 3.2. The earth takes about 24 hours to complete one rotation around its axis. The period of rotation is known as the earthday. This is the daily motion of the earth.

What would happen if the earth did not rotate?

The portion of the earth facing the sun would always experience day, thus bringing continuous warmth to the region. The other half would remain in darkness and be freezing cold all the time. Life would not have been possible in such extreme conditions.

Do you know?

The ancient Indian astronomer Aryabhata had stated that ‘the earth is round and rotates on its own axis’.

Activity:

Let’s Do

Take a ball to represent the earth and a lighted candle to represent the sun. Mark a point on the ball to represent a town X. Place the ball in such a way that the town X is in darkness. Now rotate the ball from left to right. As you move the ball slightly, the town will have its sunrise. As the ball continues to move, the point X gradually gets away from the sun. This is sunset.

WHAT IS REVOLUTION?

WHAT IS REVOLUTION?

The movement of the earth around the sun in a fixed path or orbit is called Revolution.            (or)

The second motion of the earth around the sun in its orbit is called Revolution.

It takes 365¼ days (one year) to revolve around the sun. We consider a year as consisting of 365 days only and ignore six hours for the sake of convenience.

Six hours saved every year are added to make one day (24 hours) over a span of four years. This surplus day is added to the month of February. Thus every fourth year, February is of 29 days instead of 28 days. Such a year with 366 days is called a leap year. Find out when will the next leap year be?

From Figure 3.3, it is clear that the earth is going around the sun in an elliptical orbit.

Notice that throughout its orbit, the earth is inclined in the same direction.

A year is usually divided into summer, winter, spring, and autumn seasons. Seasons change due to the change in the position of the earth around the sun.

Activity:

Let’s Do

Do you know how to draw an ellipse? Take a pencil, two pins, and a loop of thread. Now fix these pins on a paper as shown in the figure. Put the loop on the paper enclosing these two pins inside the loop. Now hold the pencil and draw the line keeping the thread tight and moving the pencil along it. The figure represents an ellipse.

SUMMER SOLSTICE

SUMMER SOLSTICE

On 21^st June, the Northern Hemisphere is tilted towards the sun. The rays of the sun fall directly on the Tropic of Cancer. As a result, these areas receive more heat. The areas near the poles receive less heat as the rays of the sun are slanting. The North Pole is inclined towards the sun and the places beyond the Arctic Circle experience continuous daylight for about six months. Since a large portion of the Northern Hemisphere is getting light from the sun, it is summer in the regions north of the equator. The longest day and the shortest night at these places occur on 21^st June. At this time in the Southern Hemisphere, all these conditions are reversed. It is winter season there. The nights are longer than the days. This position of the earth is called the Summer Solstice.

WINTER SOLSTICE

WINTER SOLSTICE

On 22^nd December, the Tropic of Capricorn receives direct rays of the sun as the South Pole tilts towards it. As the sun’s rays fall vertically at the Tropic of Capricorn (23½° S), a larger portion of the Southern Hemisphere gets light. Therefore, it is summer in the Southern Hemisphere with longer days and shorter nights. The reverse happens in the Northern Hemisphere. This position of the earth is called the Winter Solstice. Do you know that Christmas is celebrated in Australia in the summer season?

EQUINOX

EQUINOX

On 21^st March and September 23^rd, direct rays of the sun fall on the equator. At this position, neither of the poles is tilted towards the sun; so, the whole earth experiences equal days and equal nights. This is called an equinox.

On 23^rd September, it is the Autumn season in the Northern Hemisphere and Spring season in the Southern Hemisphere. The opposite is the case on 21^st March when it is Spring in the Northern Hemisphere and Autumn in the Southern Hemisphere.

Thus, you find that there are days and nights and changes in the seasons because of the rotation and revolution of the earth respectively.

Activity:

Let’s Do

To understand the earth’s inclination in the same direction, draw a big ellipse on the ground and take a flag with a stick. Stand anywhere on the line of the ellipse. Point your flag to a fixed point far away like on a tree-top. Now move along the ellipse keeping your flag always pointing towards that fixed point. In this way, the axis of the earth remains inclined permanently in the same position. The revolution of the earth and the inclination of the earth’s axis in a fixed direction cause season.