The Earth in the Solar System

"Our universe is a large and unimaginable expanse of dust, gas, stars, clouds, sun, planets, galaxies and life."

1.0Introduction

• How wonderful it is to watch the sky after sunset! One would first notice one or two bright dots shining in the sky.
• Soon you would see the number increasing. You cannot count them any more.
• The whole sky is filled with tiny shining objects - some are bright, others dim.
• It seems as if the sky is studded with diamonds.
• They all appear to be twinkling.
• But if you look at them carefully you will notice that some of them do not twinkle as others do.
• They simply glow without any flicker just as the moon shines.
  

2.0Full Moon and New Moon

• Along with these bright objects, you may also see the moon on most of the days.
• It may, however, appear at different times, in different shapes and at different positions.
• 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.
  

3.0Celestial Bodies

• 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.
• Countless twinkling stars in the night sky are similar to the sun.
• But we do not feel their heat or light, and they look so tiny because they are very far from us.

4.0Constellations

• 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 (Saptaseven, rishisages).
• It is a group of seven stars that forms a part of Ursa Major Constellation.

5.0Pole Star

• 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.
• Look at the given Figure. You will notice that, if an imaginary line is drawn joining the pointer stars and extended further, it will point to the Pole Star.
  

Saptarishi and the North Star

6.0Planets

• Some celestial bodies do no thave 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.

7.0The Satellite

• A satellite is an object that moves around planets.
• 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 seven other planets that get heat and light from the sun.
• Some of them have their moons too.

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.

8.0The 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.

The Sun

• 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.

9.0Word Origin

Many words used in a language may have been taken from some other language. 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'.

Word Origin

10.0Planets

• 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."

11.0The Earth

• 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.
  

Neil Armstrong was the first man to step on the surface of the moon on 20 July 1969. The Moon

The moon as seen from the space

• 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 $3,84,400\mathrm{km}$ 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 mountains, plains and depressions on its surface.
• These cast shadows on the moon's surface.

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.

12.0Asteroids

• 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.
  

13.0Meteoroids

• 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.
  
  Milky Way
• 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.
  

14.0Glossary

• Studded - Containing a lot of something.
• Flicker - To keep going on and off as it burns or shines.
• Debris - The remains of something broke down or destroyed.
• Dwarf Planet - A round object in space that goes around the sun but is not as large as a planet and does not clear other objects from its path.
• Friction - The rubbing of one surface or thing against another.

15.0MIND MAP