Quick Revision Notes: Class 6 Social Science (Geography)

Chapter: The Earth in the Solar System

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### 1. Important Key Terms & Definitions

• Celestial Bodies (खगोलीय पिंड): The sun, the moon, and all those objects shining in the night sky are called celestial bodies.
• Stars (तारे): Celestial bodies that are very big and hot. They are made up of gases and have their own heat and light, which they emit in large amounts. Example: The Sun.
• Constellations (नक्षत्रमंडल): Various patterns formed by different groups of stars in the night sky. Example: Ursa Major or Big Bear, and Saptarishi (a group of seven stars).
• Pole Star (ध्रुव तारा): The North star that indicates the north direction. It always remains in the same position in the sky.
• Planets (ग्रह): Celestial bodies that do not have their own heat and light. They are lit by the light of the stars. The word 'planet' comes from the Greek word "Planetai", which means "wanderers".
• Satellite (उपग्रह): A celestial body that moves around the planets in the same way as the planets move around the sun. Example: The Moon.

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### 2. The Solar System (सौरमंडल)

• Definition: 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 (सूर्य):
  • It is at 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.
  • It is the ultimate source of heat and light for the solar system.

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### 3. The Eight Planets (आठ ग्रह)

In order of their distance from the sun, the planets are:

1. Mercury (बुध) - Nearest to the sun. Takes about 88 days to complete one round along its orbit.
2. Venus (शुक्र) - Considered as "Earth's-twin" because its size and shape are very much similar to that of the earth.
3. Earth (पृथ्वी) - The third nearest planet to the sun and the fifth largest planet. It is a unique planet supporting life.
4. Mars (मंगल) - Appears reddish in colour and is often called the Red Planet.
5. Jupiter (बृहस्पति) - The largest planet in the solar system.
6. Saturn (शनि) - Appears yellowish and has rings around it.
7. Uranus (अरुण) - Discovered in 1781, rolls on its side.
8. Neptune (वरुण) - Farthest planet from the sun in the solar system.

Easy Trick to Remember Planet Order: Mery Visited Every Mother Just Served Us Noodles.
(Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune)

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### 4. The Earth: A Unique Planet (पृथ्वी: एक अनोखा ग्रह)

• Conditions for Life: Earth has conditions favourable to support life. It is neither too hot nor too cold. It has water and air, which are essential for survival.
• Life-supporting Gas: Air has life-supporting gases like oxygen.
• Blue Planet: From outer space, the earth appears blue because two-thirds of its surface is covered by water. Therefore, it is called a blue planet.

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### 5. The Moon (चन्द्रमा)

• Size: Our earth has only one satellite, that is, the moon. Its diameter is only one-quarter that of the earth.
• Distance: It is about 3,84,400 km away from us.
• Movement:
  • 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.
• Environment: It does not have conditions favourable for life. It has neither water nor air. Its surface has mountains, plains, and depressions that cast shadows on the moon's surface.

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### 6. Other Celestial Bodies

• Asteroids (क्षुद्र ग्रह): Numerous tiny bodies that move around the sun, found between the orbits of Mars and Jupiter.
• Meteoroids (उल्कापिंड): Small pieces of rocks which move around the sun. Sometimes these meteoroids come near the earth and tend to drop upon it.
• Galaxy (आकाशगंगा): A huge system of billions of stars, and clouds of dust and gases.
• The Universe (ब्रह्मांड): Millions of galaxies make up the universe.