Paheli and Boojho visited their grandparents’ village during the summer break. After dinner, they went on to the roof of the house. It was a clear cloudless night. They were surprised to see a large number of bright stars in the sky. They had never seen such a beautiful scene in their city (Fig. 17.1).

Paheli wondered, why the village sky was so different from the night sky in big cities? Her grandfather explained that due to bright light, smoke and dust, the sky in big cities is rarely clear.

He also identified some objects seen in the night sky and told stories associated with them. Night sky watching can be a fascinating experience at a place where there are no bright lights and the atmosphere is clear.

Look at the sky on a dark, clear night. You see the entire sky dotted with countless stars, some bright and some not so bright. Observe them carefully. Do all of them appear to twinkle? Do you find any star-like object which does not twinkle? The objects which do not twinkle are planets.

The moon is the brightest object in the night sky. The stars, the planets, the moon and many other objects in the sky are called celestial objects.

The study of celestial objects and associated phenomena is called astronomy. In acient India our ancestors performed methodical observations of sky. Their knowledge of astronomy was highly advanced for their time. Passage of the Sun, stars, moon and planets in the sky helped them to devise calenders and almanacs. These were often used by people in their day to day conduct and a better understanding of climate and rainfall patterns for timely sowing and choice of crops, fixing the dates of seasons and festivals.

Let us also make some observations of celestial objects and learn about them.

17.1 The Moon

Activity 17.1

Observe the moon continuously for several nights, preferably from one full moon to the next. Make a sketch of the moon every night in your note book and note the day from the day of the full moon. Also note everyday the part of the sky (east or west) in which the moon is seen.

Is there a change in the shape of the moon everyday? Are there days when the shape of the moon appears to be perfectly round? Are there days when the moon cannot be seen at all even if the sky is clear?

The day on which the whole disc of the moon is visible is known as the full moon day. Thereafter, every night the size of the bright part of the moon appears to become thinner and thinner. On the fifteenth day the moon is not visible. This day is known as the ‘new moon day’. The next day, only a small portion of the moon appears in the sky. This is known as the crescent moon. Then again the moon grows larger every day. On the fifteenth day once again we get a full view of the moon.

The various shapes of the bright part of the moon as seen during a month are called phases of the moon (Fig. 17.2). Phases of the moon play an important role in our social life. Almost all festivals in India are celebrated according to the phases of the moon. For example, Diwali is celebrated on the new moon day; Budh Poornima and Guru Nanak’s birthday are celebrated on full moon day; Maha Shivratri is celebrated on thirteenth night of waning moon; Eid-ul-Fitr is observed on the day following the sighting of crescent moon.

Fig.17.2 : Phases of the moon

Let us try to understand why phases of the moon occur. You have studied in Chapter 16 that the moon does not produce its own light, whereas the Sun and other stars do. We see the moon because the sunlight falling on it gets reflected towards us (Fig. 17.3). We, therefore, see only that part of the moon, from which the light of the Sun is reflected towards us.

The Moon’s Surface

The moon is a fascinating object for poets and story-tellers. But when astronauts landed on the moon, they found that the moon’s surface is dusty and barren. There are many craters of different sizes. It also has a large number of steep and high mountains (Fig. 17.7). Some of these are as high as the highest mountains on the Earth.

The moon has no atmosphere. It has no water. Can any life exist on the moon?

We learnt in Chapter 13 that sound cannot travel when there is no medium. Then, how can we hear any sound on the moon?

On July 21, 1969 (Indian time) the American astronaut, Neil Armstrong, landed on the moon for the first time. He was followed by Edwin Aldrin.

17.2 The Stars

What other objects do you see in the night sky? There is a large number of stars in the sky. Observe carefully on a dark night and from a place away from a big city. Are all the stars equally bright? Are they of the same colour? In fact, stars emit light of their own. The Sun is also a star. Why does it appear so large compared to the other stars?

Which appears bigger, a football placed near you, or a football placed at a distance of 100 m? The stars are millions of times farther away than the Sun. Therefore, the stars appear to us like points.

The Sun is nearly 150,000,000 kilometres (150 million km) away from the Earth. The next nearest star is Alpha Centauri. It is at a distance of about 40,000,000,000,000 km from the Earth. Can you read this distance in kilometres conveniently? Some stars are even further away. Such large distances are expressed in another unit known as light year. It is the distance travelled by light in one year. Remember that the speed of light is about 300,000 km per second. Thus, the distance of the Sun from the Earth may be said to be about 8 light minutes. The distance of Alpha Centauri is about 4.3 light years.

If light from stars takes years to reach us, I wonder if we are looking into the past when we look at stars.

In fact, the stars are present in the sky during the day-time also. However, they are not visible then because of the bright sunlight.

Observe some prominent star or a group of stars in the sky for about two hours or more. What do you find? Do you find any change in the positions of stars in the sky?

You will find that the stars appear to move from east to west. A star which rises in the east in the evening, sets in the west in the early morning.

Why do stars appear to move from east to west? Let us find out.

Activity 17.4

Stand in the centre of a big room and start rotating. In which direction will the objects in the room appear to move? Do you see them moving in the direction opposite to your motion?

Paheli recalls that when she is in a moving train the nearby trees and buildings appear to move in the backward direction.

If the stars appear to move from east to west, could it mean that the Earth, rotates from west to east?

Activity 17.5

Take an umbrella and open it. Make about 10-15 stars out of white paper. Paste one star at the position of the central rod of the umbrella and others at different places onthe cloth near the end of each spoke (Fig. 17.9).

Now rotate the umbrella by holding its central rod in your hand. Observe the stars on the umbrella. Is there any star which does not appear to move? Where is this star located?

If there were a star located where the axis of rotation of the Earth meets the sky, could this star also be stationary?

There is actually a star, the pole star, which is situated in the direction of the earth’s axis. It does not appear to move (Fig. 17.10).

17.3 Constellations

Look at the sky for some time. Can you see some stars forming groups with shapes like those in Fig. 17.11.

The stars forming a group that has a recognisable shape is called a constellation.

You can easily identify some constellations in the night sky. For this, you should know what a particular constellation looks like and where to look for it in the night sky.

One of the most famous constellations which you can see during summer time in the early part of the night is Ursa Major [Fig.17.11 (a)].

Fig. 17.11 : Some constellations in the night sky

Note that the Pole star is not visible from the southern hemisphere. Some of the northern constellations like Ursa Major may also not be visible from some points in the southern hemisphere.

Orion is another well-known constellation that can be seen during winter in the late evenings. It is one of the most magnificent constellations in the sky. It also has seven or eight bright stars [Fig. 17.11(b)] Orion is also called the Hunter. The three middle stars represent the belt of the hunter. The four bright stars appear to be arranged in the form of a quadrilateral.

The star Sirius, which is the brightest star in the sky, is located close to Orion. To locate Sirius, imagine a straight line passing through the three middle stars of Orion. Look along this line towards the east. This line will lead you to a very bright star. It is Sirius. (Fig. 17.15).

 

Cassiopeia is another prominent constellation in the northern sky. It is visible during winter in the early part of the night. It looks like a distorted letter W or M [Fig. 17.11(c)].

17.4 The Solar System

The Sun and the celestial bodies which revolve around it form the solar system. It consists of large number of bodies such as planets, comets, asteroids and meteors. The gravitational attraction between the Sun and these objects keeps them revolving around it.

The Earth, as you know, also revolves around the Sun. It is a member of the solar system. It is a planet. There are seven other planets that revolve around the Sun. The eight planets in their order of distance from the Sun are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

Figure 17.17 shows a schematic view of the solar system.

Let us know about some members of the solar family.

The Sun

The Sun is the nearest star from us. It is continuously emitting huge amounts of heat and light. The Sun is the source of almost all energy on the Earth. In fact, the Sun is the main source of heat and light for all the planets.

The Planets

The planets look like stars, but they do not have light of their own. They merely reflect the sunlight that falls on them. Can you distinguish between planets and stars?

The simplest method of identifying planets from stars is that stars twinkle, whereas planets do not. Also the planets keep changing their positions with respect to the stars.

A planet has a definite path in which it revolves around the Sun. This path is called an orbit. The time taken by a planet to complete one revolution is called its period of revolution. The period of revolution increases as the distance of the planet increases from the sun.

I wonder why the planets do not collide while revolving around the Sun.

Activity 17.9

Go out into the playground with four or five of your friends. Draw four circles of radii 1m, 1.8m, 2.5m and 3.8m, all having a common centre (Fig. 17.18).

Ask one of your friends to stand in the centre and represent the Sun. Your other four friends may represent Mercury, Venus, Earth and Mars.

Ask your friends to move around the Sun in anti-clockwise direction in their own orbits (Fig. 17.18). Do they collide with one another?

Besides revolving around the Sun, a planet also rotates on its own axis like a top (Fig. 17.19). The time taken by a planet to complete one rotation is called its period of rotation.

Some planets are known to have moons/satellites revolving round them. Any celestial body revolving around another celestial body is called its satellite.

There are many man-made satellites revolving round the Earth. These are called artificial satellites.

Mercury (Budh)

The planet mercury is nearest to the Sun. It is the smallest planet of our solar system. Because Mercury is very close to the Sun, it is very difficult to observe it, as most of the time it is hidden in the glare of the Sun. However, it can be observed just before sunrise or just after sunset, near the horizon. So it is visible only at places where trees or buildings do not obstruct the view of the horizon. Mercury has no satellite of its own.

Venus (Shukra)

Venus is earth’s nearest planetary neighbour. It is the brightest planet in the night sky.

Activity 17.10

Find out from some newspaper or from an almanac the time when Venus is visible in the sky. You can easily recognise Venus by its brightness. Remember that Venus cannot be seen very high in the sky. You must try to observe Venus either 1-3 hours before sunrise or 1-3 hours after sunset.

Venus has no moon or satellite of its own. Rotation of Venus on its axis is somewhat unusual. It rotates from east to west while the Earth rotates from west to east.

The Earth (Prithvi)

The Earth is the only planet in the solar system on which life is known to exist. Some special environmental conditions are responsible for the existence and continuation of life on the Earth. These include just the right distance from the Sun, so that it has the right temperature range, the presence of water and suitable atmosphere and a blanket of ozone.

 

The axis of rotation of the Earth is not perpendicular to the plane of its orbit. The tilt is responsible for the change of seasons on the Earth. The Earth has only one moon.

Mars (Mangal)

The next planet, the first outside the orbit of the Earth is Mars. It appears slightly reddish and, therefore, it is also called the red planet. Mars has two small natural satellites.

 

Jupiter (Brihaspati)

Jupiter is the largest planet of the solar system. It is so large that about 1300 earths can be placed inside this giant planet. However, the mass of Jupiter is about 318 times that of our Earth. It rotates very rapidly on its axis.

Jupiter has a large number of satellites. It also has faint rings around it. You can easily recognise Jupiter as it appears quite bright in the sky. If you observe it with the help of a telescope, you can also see four of its large moons (Fig. 17.22)

Saturn (Shani)

Beyond Jupiter is Saturn which appears yellowish in colour. What makes it unique in the solar system is its beautiful rings. These rings are not visible with the naked eye. You can observe them with a small telescope. Saturn also has a large number of satellites.

One interesting thing about Saturn is that it is the least dense among all the planets. Its density is less than that of water.

Uranus and Neptune

These are the outermost planets of the solar system. They can be seen only with the help of large telescopes. Like Venus, Uranus also rotates from east to west. The most remarkable feature of Uranus is that it has highly tilted rotational axis (Fig. 17.24). As a result, in its orbital motion it appears to roll on its side.

The first four planets, Mercury, Venus, Earth and Mars are much nearer the Sun than the other four planets. They are called the inner planets. The inner planets have very few moons.

The planets outside the orbit of Mars, namely Jupiter, Saturn, Uranus and Neptune are much farther off than the inner planets. They are called the outer planets. They have a ring system around them. The outer planets have large number of moons.

17.5 Some Other Members of the Solar System

There are some other bodies which revolve around the Sun. They are also members of the solar system. Let us learn about some of them.

Asteroids

There is a large gap in between the orbits of Mars and Jupiter (Fig. 17.25). This gap is occupied by a large number of small objects that revolve around the Sun. These are called asteroids. Asteroids can only be seen through  large telescopes.

Comets

Comets are also members of our solar system. They revolve around the Sun in highly elliptical orbits. However, their period of revolution round the Sun is usually very long. A Comet appears generally as a bright head with a long tail. The length of the tail grows in size as it approaches the sun. The tail of a comet is always directed away from the sun (Fig. 17.26).

Fig. 17.26 : Different position of a Comet

 

Superstitions about the Comets

Some people think that comets are messengers of disasters, such as wars, epidemics and floods. But these are all myths and superstitions. Appearance of a comet is a natural phenomenon. We have no reason to be afraid of it.

 

Meteors and Meteorites

At night, when the sky is clear and the moon is not there, you may sometimes see bright streaks of light in the sky (Fig. 17.27). These are commonly known as shooting stars, although they are not stars. They are called meteors. A meteor is usually a small object that occasionally enters the earth’s atmosphere. At that time it has a very high speed. The friction due to the atmosphere heats it up. It glows and evaporates quickly. That is why the bright steak lasts for a very short time.

Fig. 17.27 : Streak of a Meteor

Some meteors are large and so they can reach the Earth before they evaporate completely. The body that reaches the Earth is called a meteorite. Meteorites help scientists in investigating the nature of the material from which the solar system was formed.

Meteor Showers

When the Earth crosses the tail of a comet, swarms of meteors are seen. These are known as meteor showers. Some meteor showers occur at regular intervals each year. You can find the time of their appearance from a scientific magazine or from the internet.

Artificial Satellites

You must have heard that there are a number of artificial satellites which are orbiting the Earth. You might wonder how artificial satellites are different from natural satellites. Artificial satellites are man-made. They are launched from the Earth. They revolve around the Earth much closer than earth’s natural satellite, the moon.

India has built and launched several artificial satellites. Aryabhata was the first Indian satellite. Some other Indian satellites are INSAT, IRS, Kalpana-1, EDUSAT, etc. (Fig. 17.28).

Artificial satellites have many practical applications. They are used for forecasting weather, transmitting television and radio signals. They are also used for telecommunication and remote sensing.