Class 8 Science
A group of stars which resembles certain shape is called a constellation. Human beings have an innate ability to recognize shapes and faces even in formless things; like clouds, smoke, etc. This ability of human beings has given rise to various constellations. Different constellations may have different names in various cultures. There are twelve signs of zodiac, viz. Aries, Taurus, Gemini, Cancer, Virgo, Leo, Libra, Scorpio, Capricorn, Sagittarius, Aquarius and Pisces. There are numerous other constellations, e.g. Great Bear, Cassiopeia, Ursa Minor, Orion, etc.
Great Bear: This is also known as Ursa Major and Big Dipper. It resembles the shape of a ladle. The four stars which appear as four vertices of a quadrilateral make the bowl of the ladle and the remaining three stars make the handle of the ladle. Ursa Major appears in the northern sky. The pole star is in line with last two stars of the quadrilateral. Since this constellation appears quite close to the pole star, it looks as if revolving around the pole star.
Orion: Orion is also called the Hunter. The three prominent stars in this constellation mark the belt of the hunter. An elaborate structure shows a man with a club and a shield. This constellation is visible in the southern sky and this can be easily seen during summer months in India.
THE SOLAR SYSTEM
The solar system is made up of the sun, its planets, satellites, asteroids, comets, dwarf planets, etc. All other members of the solar system keep on revolving around the sun. This is possible because of the gravitational attraction between the sun and these bodies. The solar system is 4.6 billion years old. Different members of the solar system are as follows.
The Sun: The sun is the centre of the solar system. The sun is a huge store of heat and light energy. The sun is mainly composed of hydrogen gas. The hydrogen gas in the sun keeps on changing to helium gas and this process releases huge amount of energy. This is like thousands of atom bombs working together.
Planet: A celestial body which revolves around a star is called a planet. There are eight planets in the solar system, viz. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
Satellite: A celestial body which revolves around a planet is called a satellite. Mercury and Venus do not have known satellites. Jupiter has the largest number of known satellites.
Mercury: This is the smallest planet in the solar system and is nearest to the sun. It is at a distance of 0.4 AU from the sun. This is so close to the sun that it cannot be seen without a powerful telescope.
Venus: Venus is at a distance of 0.7 AU from the sun. This planet is almost of the same size as the earth. Venus is the hottest planet on the earth. After moon; this is the brightest object in the night sky. Venus is visible in early evening during the summer months and in early morning during winters. Due to this, Venus is also called the morning and evening star.
Earth: Earth is the only planet where life is known to exist. According to scientists; earth is at a perfect distance from the sun and hence it has the right combination of gases, materials and temperature to support life. A major portion of the earth’s surface is covered with water and hence it appears blue from the space. Due to this; earth is also called the BLUE PLANET.
Mars: Mars is at a distance of 1.5AU from the sun. Mars appears red because of the presence of iron oxide on its surface. Hence, it is also known as the RED PLANET. Deimos and Phobos are the two natural satellites of mars.
The four planets before the asteroid belt are called rocky planets or TERRESTRIAL PLANETS because they are made of rocks. The four remaining planets which are after the asteroid belt are called GASEOUS GIANTS because they are mainly composed of gases.
Jupiter: Jupiter is at a distance of 5.2 AU from the sun. This is the largest planet in the solar system. Jupiter is 1300 times the size of the earth. But because of its lower density; it is just 388 times the mass of the earth. There are 67 known satellites of Jupiter.
Saturn: Saturn is at a distance of 9.5 AU from the sun. Saturn is famous for its rings. The rings around this planet are composed of rocks, gases and vapours. These rings can be seen through a telescope. There are 62 known satellites of Saturn. The density of Saturn is so low that it can float on water.
Uranus: Uranus is at a distance of 19.2 AU from the sun. Uranus rotates from east to west; like Venus. The axis of Uranus is slightly tilted on its orbit. Due to this, Uranus appears to be rolling on its sides. There are 27 known satellites of Uranus.
Neptune: Neptune is at a distance of 30 AU from the sun. There are 14 known satellites of Neptune. This is the farthest planet from the sun.
Asteroid Belt: The asteroid belt is present between the orbits of Mars and Jupiter. This belt extends between 2.3 and 3.3 AU from the sun. Asteroids are made up of rocks, metallic minerals and some ice. They are remnants of formation of the solar system.
Comet: A comet is a celestial body which revolves around the sun in a highly elliptical path. When a comet passes from near the earth, it becomes visible to us. Halley’s Comet and Hale Bopp are some of the famous comets. Halley’s Comet appears after every 76 years. It appeared last time in 1986 and is expected to appear again in 2062.
Meteor: When a small object from the space reaches the atmosphere of the earth; it enters at a very high speed. The high speed creates so much friction that the object burns off before reaching the earth. Such objects are called meteors. They are visible as shooting stars in the night sky. Sometimes, an object can be too big to be completely destroyed by burning. Such objects reach the surface of the earth and are called meteorites. A meteorite can cause huge damage to the earth; like destroying a huge population of living beings.
Artificial Satellites: The man-made satellites are called artificial satellites. These satellites are made for various purposes; like remote sensing, telecommunication, defence, etc. Data from the satellites is utilised by meteorologists to make predictions about weather. Communication satellites help in working of mobile phones, television, etc. Global Positioning System (GPS) works because of these satellites.