World Geography: the term Geography has evolved from Greek words ‘geo’ the earth and ‘graphein’ to write. Scholars have defined geography in different times in the history of its evolution. However, it is agreed today by most that geography is the study of the earth as a home for mankind.
The Universe: the universe is commonly defined as the totality of everything that can exist or that we know exists. It includes all matter and energy: Planets, stars and galaxies: and the contents of intergalactic space. The study of Universe is called Cosmology. The origin of the universe is explained by the big bang theory, propounded by the Belgian cosmologist Georges Lemaitre.The solar system: the solar system consists of the sun, the eight planets and their satellites (or moons) and thousands of other smaller heavenly bodies such as asteroids, centre of the solar system and all these bodies revolve around it. Planets revolve around the sun in an elliptical orbit. In the solar system, the planet nearest to the sun is mercury and the planet farthest from the sun is Neptune. The sun is also the source of all the energy in the solar system. Pluto is a dwarf planet. Mercury, Venus, earth and mars are called terrestrial planets while Jupiter, Saturn, urns and Neptune are called gaseous planets.
The sun: The sun is at the centre of the solar system. Its diameter is 13, 92,000 km. it is composed of 71% hydrogen, 26.5% helium and 2.5% other elements. Within the sun, hydrogen is converted to helium due to nuclear fusion releasing a tremendous amount of heat and light. It has a surface temperature of about 6000 degree Celsius. The temperature at the centre is around 15,000,000 degree Celsius. Light at the speed of 300,000 kilometer per second takes about 8.3 minutes to reach the Earth from the sun. The sun is about 150 million kilometer away from the Earth.
The Planets: these are not transmitting bodies which continuously revolve around and are lit by the sun. The sequence of planets (eight planets) according to their distance from the sun is Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Jupiter is the biggest and Mercury is the smallest planets of our solar system. The four planets nearest to the sun-Mercury, Venus, Earth, and Mars-are called terrestrial planets, because their structure is similar to the Earth, other planets Jupiter, Saturn, Uranus, and Neptune are called Jovian planets.
Mercury: it is the closest planet to the sun. It is an extremely hot planet. It does not have water. It has no gases like CO2, N2, H2 and O2 which may act as building blocks of life.
Venus: it is the second distant planet from the sun. It is nearest to the earth and is also the brightest planet. It is known as the “Evening Star” as well as “morning Star”. It is like the earth in size and mass, and hence also known as the “Earth’s twin”.
The Earth: it is the largest of the inner planets. It is 231/2 degree tilted on its axis and thus makes 661/2 degree angle. It takes 23 hours, 56 minutes and 4.091 seconds to rotate on its axis. It takes 365 days, 5 hours and 48 minutes to revolve around the sun. Earth is known as the “watery Planet” or the “blue planet” due to the presence of huge amount of water on it. It is the only known planet which provides sustenance or life on it. It has all the essential elements like carbon (in the form of co2), hydrogen (h2), nitrogen (n2) and oxygen (o2) which act as building blocks of life. It has protective blankets of ozone layer in its atmosphere to save life from the harmful ultraviolet radiations.
The Moon: it is the only satellite of the earth. It has a diameter of 3476 km. and its circumference is 10917 km. while its orbit is elliptical. The maximum distance of the moon form the earth is 4, 06,395 km. and the minimum distance is 3, 64,000 km approximately, only 59% of the total surface of the moon is visible from the earth. The highest mountain on the moon is Liebuity Mountain, which is 10, 660 meters high. The moon has no atmosphere, no twilight and no sound. The light from the Moon takes about 1.3 seconds to reach the earth. Gravitational pull of the moon is one sixth that of the earth. The study of the Moon is called “scientology”. The moon is also known as the fossil Planet.
Mars: Iron-rich red soil and pink sky of Mars give it the name “red planet”. Phobos and Demos are the two satellites of Mars. Jupiter: it is the largest planet (71% of the total mass of all planets) of the solar system. It is also known as winter planet as its average temperature is very low (-148 degree Celsius). It has 67 satellites. Ganymede is the largest satellite of the solar system.
Saturn: it is the second largest planet in the solar system. It has bright concentric rings which are made up of ice and ice covered dust particles which revolve around it. It has 62 satellites. Titan the satellite of Saturn is the second largest satellite in the solar system.
Uranus: It is about four times the size of the earth. It appears greenish in color because methane gas is present in its atmosphere. Uranus was discovered in 1781 by Sir William Herschel. Uranus is the first planet to have been discovered by the use of a telescope
Neptune: Neptune is very similar to Uranus and can be considered as its twin. It is surrounded by methane rings of sub zero temperature. Trojan asteroids are found in two clouds moving ahead of it and the other moving behind it.
Meteors and Meteorites: Meteors and Meteorites are also called shooting stars. They are fragments of rocks coming toward the earth, formed due to the collision of asteroids with one another. They are usually small and burn up before they reach the earth’s surface. When meteors are large and do not burn up completely, they land on the earth’s surface and are known s meteorites. All meteorites are believed to originate in the asteroid belt, where a sudden collision may send them towards the earth whose gravity attracts them towards its surface.
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