Chapter 1 The Earth In The Solar System (GIST of NCERT)
The sun, the moon and all those objects shining in the night sky are called celestial bodies.
Some celestial bodies are very big and hot. They are made up of gases. They have their own heat and light, which they emit in large amounts. These celestial bodies are called stars. The sun is a star.
While watching the night sky, you may notice various patterns formed by different groups of stars. These are called constellations. Ursa Major or Big Bear is one such constellation. One of the most easily recognisable constellation is the Saptarishi (Sapta-seven, rishi-sages). It is a group of seven stars that forms a part of Ursa Major Constellation. Ask someone elder in your family or neighbourhood to show you more stars, planets and constellations in the sky.
In ancient times, people used to determine directions during the night with the help of stars. The North star indicates the north direction. It is also called the Pole Star. It always remains in the same position in the sky. We can locate the position of the Pole Star with the help of the Saptarishi. You will notice that, if an imaginary line is drawn joining the pointer stars and extended further, it will point to the Pole Star.
Some celestial bodies do not have their own heat and light. They are lit by the light of the stars. Such bodies are called planets. The word ‘planet’ comes from the Greek word “Planetai” which means ‘wanderers’. The earth on which we live is a planet. It gets all its heat and light from the sun, which is our nearest star. If we look at the earth from a great distance, say the moon, it will appear to be shining just as the moon.
Jupiter, Saturn and Uranus have rings around them. These are belts of small debris. These rings may be seen from the earth with the help of powerful telescopes.
The moon that we see in the sky is a satellite. It is a companion of our earth and moves round it. Like our earth, there are eight other planets that get heat and light from the sun. Some of them have their moons too.
The solar system
The sun, eight planets, satellites and some other celestial bodies known as asteroids and meteoroids form the solar system. We often call it a solar family, with the sun as its Head.
Many words used in a language may have been taken from some otherlanguage. Geography, for example, is an English word. It has its origin in Greek, which relates to the description of the earth. It is made of two Greek words, ‘ge’ meaning’ ‘earth’ and ‘graphia’ meaning ‘writing’. Find out more about the earth.
Geology = Geo + logy = Study of the earth
Geometry = Geo + metry = Measurement of earth
Geoid = Geo + oid = resembling the shape or form of theearth
The sun is in the centre of the solar system. It is huge and made up of extremely hot gases. It provides the pulling force that binds the solar system. The sun is the ultimate source of heat and light for the solar system. But that tremendous heat is not felt so much by us because despite being our nearest star, it is far away from us. The sun is about 150 million km away from the earth.
There are eight planets in our solar system. In order of their distance from the sun, they are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. An easy way to memorise the name of the planets in order of their distance from the sun is: My Very Efficient Mother Just Served Us Nuts.
All the eight planets of the solar system move around the sun in fixed paths. These paths are elongated. They are called orbits. Mercury is nearest to the sun. It takes only about 88 days to complete one round along its orbit. Venus is considered as ‘Earth’s-twin’ because its size and shape are very much similar to that of the earth. Till recently (August 2006), Pluto was also considered a planet. However, in a meeting of the International Astronomical Union, a decision was taken that Pluto like other celestial objects (Ceres, 2003 UB313) discovered in recent past may be called ‘dwarf planets.”
The earth is the third nearest planet to the sun. In size, it is the fifth largest planet. It is slightly flattened at the poles. That is why, its shape is described as a Geoid. Geoid means an earth-like shape. Conditions favourable to support life are probably found only on the earth. The earth is neither too hot nor too cold. It has water and air, which are very essential for our survival. The air has life-supporting gases like oxygen. Because of these reasons, the earth is a unique planet in the solar system. From the outer space, the earth appears blue because its two-thirds surface is covered by water. It is, therefore, called a blue planet.
Do you know? Light travels at the speed of about 300, 000 km per second. Yet, even with this speed, the light of the sun takes about eight minutes to reach the earth.
A Satellite is a celestial body that moves around the planets in the same way as the planets move around the sun. A Human-made Satellite is an artificial body. It is designed by scientists to gather information about the universe or for communication. It is carried by a rocket and placed in the orbit around the earth. Some of the Indian satellites in space are INSAT, IRS, EDUSAT, etc.?
Our earth has only one satellite, that is, the moon. Its diametre is only one-quarter that of the earth. It appears so big because it is nearer to our planet than other celestial bodies. It is about 3, 84, 400 km away from us. Now you can compare the distance of the earth from the sun and that from the moon. The moon moves around the earth in about 27 days. It takes exactly the same time to complete one spin. As a result, only one side of the moon is visible to us on the earth. The moon does not have conditions favourable for life. It has neither water nor air. It has mountains, plains and depressions on its surface. These cast shadows on the moon’s surface. Look at the full moon and observe these shadows. You can see the full moon only once in about a month’s time. It is Full moon night or Poornima. A fortnight later, you cannot see it at all. It is a New moon night or Amavasya. On this day, you can watch the night sky best, provided it is a clear night.
Apart from the stars, planets and satellites, there are numerous tiny bodies which also move around the sun. These bodies are called asteroids. They are found between the orbits of Mars and Jupiter. Scientists are of the view that asteroids are parts of a planet which exploded many years back.
The small pieces of rocks which move around the sun are called meteoroids. Sometimes these meteoroids come near the earth and tend to drop upon it. During this process due to friction with the air they get heated up and burn. It causes a flash of light. Sometimes, a meteor without being completely burnt, falls on the earth and creates a hollow.
Do you see a whitish broad band, like a white glowing path across the sky on a clear starry night? It is a cluster of millions of stars. This band is the Milky Way galaxy. Our solar system is a part of this galaxy. In ancient India, it was imagined to be a river of light flowing in the sky. Thus, it was named Akash Ganga. A galaxy is a huge system of billions of stars, and clouds of dust and gases. There are millions of such galaxies that make the Universe. It is difficult to imagine how big the universe is. Scientists are still trying to find out more and more about it. We are not certain about its size but we know that all of us – you and I belong to this universe.
Important Terms Full Moon Night: A night when the moon is visible from the earth as a full sphere is called the Full Moon night, and it occurs once a month. New Moon Night: The fifteenth night after the Full Moon night, when the moon is not visible at all in the sky, is called the New Moon night. Celestial Body: An object in the universe (but not on the earth) is said to be a celestial body. Examples are Sun, Earth, Moon, Stars, etc. Star: A celestial body that is characterised by being very hot and big, and made of gases is a star. A star typically has its own heat and light. Inner Planets: The Inner Planets are the planets that orbit around the sun between the sun and the asteroid belt, that is, are close to the sun. These are: Mercury, Venus, Earth and Mars. Outer Planets: The Outer Planets are the planets that orbit the sun beyond the asteroid belt, that is, are very far away from the sun. These are: Jupiter, Saturn, Uranus and Neptune. • Asteroids They are numerous tiny bodies which move around the sun between the orbits of Mars and Jupiter. The largest asteroid is the Ceres. • Meteoroids The small pieces of rocks which move around the sun are called meteoroids. Our solar system is a part of Milky Way galaxy. Milky Way galaxy was named Akash Ganga. There are millions of galaxies that make the Universe.