Notes: Planet: Earth | Social Studies & Pedagogy Paper 2 for CTET & TET Exams - CTET & State TET PDF Download

The Sun, the Moon and all those objects shining in night sky are called celestial  bodies. Some celestial bodies have their own heat and light, which they emit in  large amounts, are called stars. There are countless stars in the sky. Some of the  stars form a group and follow certain pattern which are called constellations. A huge  system of billions of stars, clouds of dust and gases in spiral shape is called a galaxy.  Universe is composed of millions of galaxies and Big Bear is an example of  constellation. Akash Ganga is the galaxy of the Earth. 

• Scientists believe that our solar system is formed with gas, dust and such other particles (solar nebula). Because of the collapsing of nebula due to gravity, it started spinning faster and compressed into a disc. Whatsoever was spread around was pulled inside and Sun was formed. Rest of the particles collided together and formed planets, moons, and so on. The wind of Sun pushed the lighter objects away and the heavier ones remained in innermost level, and those were small and hard.
• It is important to understand that there are no boundaries in space. In our solar system, the entire planets orbit around the Sun. Neptune is the outermost planet that orbits and revolves around 30 astronomical units from the sun. 

• Astronomical unit is calculated as follows:
  One astronomical unit = Distance between Sun and Earth (149 million km)

Planets

The galaxy that we live in is named as ‘Milky Way’, and it has many planets of different nature, shape, and size. Currently, there are eight planets in our solar system including Earth. As of now, Earth is the only planet that supports life from microorganisms to human beings. In this chapter, we will explore about these planets.

Mercury
It is the nearest planet to the Sun. It can be seen clearly in the sky two hours before sunrise. It is the smallest planet in our galaxy and has no satellite. It has the following special characteristics:

• It has a magnetic field. 
• It has lesser density, even less than water. 
• It has extremely hot days and very cool nights. 
• It has temperature difference of approximate 560°C.

Venus

• It is the nearest planet of earth. Brightness is the special characteristic of this planet. It is also known as the evening star and morning star. It revolves clockwise, which is different from other planets, as they revolve anticlockwise. It does not have any satellite.

Mars

• It is also known as red planet, as it is red in colour. The main reason behind its redness is the presence of iron oxide on its surface. It has two poles like earth, that is, North Pole and South Pole. It is bent at an angle of 25°, and hence, it has climate change cycle like Earth. It completes a circuit on its axis in 24 hours, and it takes 687 days to complete one circumambulation of Sun, where for Earth it is 365 days and it is called a year.

Jupiter

• It is the largest planet of the solar system. It takes 10 hours to rotate on its axis and it takes 12 years to complete one circumambulation of Sun. 

Saturn

• It is the second largest planet of the solar system. It seems like a yellow star. It has seven bracelets around it. The biggest satellite of this planet is Titan, which is second largest satellite of the solar system.

Uranus

• It is the third largest planet of the solar system. It has a temperature of –215°C. It was discovered by William Herschel in the year 1781. There are nine bracelets around it, in which five are alfa, Beta, gamma, delta, and epsilon. It rotates from east to west on its axis. Here, Sun rises in the west and sets in the east. All its satellites rotate opposite of the rotation of Earth’s satellite. The biggest satellite of this planet is Titania.

Neptune

• It was discovered in 1846 by German astronomer Johann Galle. In the new solar system, it is the farthest planet from the Sun. It appears green. Cold methane cloud rotates around it. It has more than one satellite, among them satellite Triton is important. It is 17 times larger than the earth.
  
  Solar system

Earth in the Solar System 

• Similar to other planets, Earth is also a member of solar system. Apart from these planets, there are multiple objects in our solar system such as natural satellites, asteroids, comets, and so on. Planets do not have their own light. All planets revolve on their axis. It is important to note that all planets revolve west to east on its axis, except Uranus and Venus.
• The nearest star of Earth is Proxima Centauri, which is the member of Alfa century star family, which is 4.22-light year away from the earth. In its shape and appearance, it is similar to Venus.

Shape and Size of Earth 

• People believe that the shape of Earth is round, but in reality, it is not a complete round. It is oblate spheroid in shape. The diameter of earth on equator is approximately 12,762 km and the polar diameter is of 12,713.6 km.

Moon (Natural Satellite) 

• Moon is the satellite of Earth. It rotates on its axis along with its rotation on the orbit round the Earth. Its axis makes an angle of 58.48° with the axis of earth. Its axis is approximately parallel to the earth. It has a radius of 3480 km, and its mass is 1/8 of the earth. During the rotation, when it is closer to the earth, it is known as perigee, and the distance between the Earth and moon is 356,000 km, and when it is farthest, it is called apogee with a distance of 407,000 km. It completes an orbital rotation in 29.53 days, which is called one synodic month. With respect to the star time, its completion on orbit is called a star month. Mons Huygens is the highest mountain of the moon.
• If in one calendar month, there are two full moons, then the second moon would be called blue moon, which happens in less time than usual, that is, 31 days. It happens usually in two or three years. Last time in year 2012, it was seen, and in the year 2018, it would be seen again, and if in one year, the blue moon happen twice or more, it would be considered as the blue moon year. Therefore, 2018 will be a blue moon year.

Dwarf Planets 

Pluto is one of the examples of dwarf planet. It was discovered in the year 1930 by Clyde Tombaugh. In 2006 International Astronomical Union (IAU) Prague conference, it was categorised as dwarf planet (Bone Graha), because it was not following the criteria of a planet. Its size is smaller than the moon, it is crossing the orbit of Neptune, and its orbit was not circular. Its new name is 134340 Pluto. Another such planet is Ceres. It was discovered by Giuseppe Piazzi, and its new name is 1 Ceres.

Asteroids

Asteroid

• Between the orbits of Mars and Jupiter, there are small celestial bodies that rotate around the Sun, and these are called asteroids. According to astronomers, when a star or planets after completion of their age explodes, asteroids are formed. Sometimes these asteroids come into the atmosphere of earth and collide with its surface forming big digs or pits. One such example is Lonar Lake in Maharashtra.
• Asteroids can be very large and can be 940 km across. The smallest asteroid found was only 20 feet. It is important to know that there is no fixed shape and size of asteroids. They are irregular in shape, and most of them are enclosed in dust.
• Asteroids can be categorised into three types. First is Type-C (carbonaceous). These are greyish in colour. These are made up of clay and stones. Type-S (silicaneous) asteroids are greenish and reddish in colour. These are made up of nickel iron and silicate materials. Type-M is reddish in colour. These are also known as metallic asteroids. These are constructed with nickel iron. All these asteroids dwell in different level of belts, such as type-C dwell in the mail belts of outer region, type-S dwell in the inner belt of asteroid and type-M dwell in the middle groin of the belt of steroid.

Comets

There are millions of heavy bodies on the exteriors of solar system, which are called comets. Basically, these are combinations of gases and dusts. These can be seen in the sky with a bright long tail. These can be seen when they move towards the Sun, which make them observe like shooting stars. The rotational time of the comet is called Halley, which is 76 years. Last time, it was seen in the year 1886 and in the year 1962.

Meteoroids

Meteoroids are seen in the sky as a shining line that shine for a moment and extinct. These are the tiny parts of comets and asteroids.

Constellation

Constellations are units that divide the sky to identify the position of stars. These were named on many mythological characters. There are 88 identified constellations in total. Constellations may be rectangular slices consisting stars within it. In simple words, constellation is defined as organised group of stars that provide a shape or pattern in the sky.
Different types of constellations and their Indian names