Numerous stars and celestial bodies came into existence by a massive Explosion called the Big Bang. These Celestial bodies together are called the Universe. It is also referred to as the Cosmos. The stars that you see are so Far away that they appear to be small, but they are really huge in size.

Universe:

The Universe is a vast Expanse of space. Most Astronomers believe that The Universe came into Existence after the Big Bang explosion that took Place about 15 billion years ago. The Universe consists of billions of galaxies, Stars, planets, comets, asteroids, Meteoroids and natural satellites. These Are collectively called as celestial bodies, Which are located far away from each Other. A Light year is the unit used To measure the distance between the Celestial bodies

Galaxy:

A galaxy is a huge cluster of stars which Are held together by gravitational force. Most of the galaxies are scattered in space,But some remain in groups. The Milky Way Galaxy was formed about 5 billion years After the Big Bang explosion. Our solar System is a part of the Milky Way galaxy.

Andromeda galaxy is the nearest to the Earth apart from the ‘Magellanic Clouds’ Galaxy.

The Solar System:

The word ‘solar’ is derived from the Roman word ‘sol’, which means ‘Sun God’. The solar system is believed to have Formed about 4.5 billion years ago. The Solar system is a gravitationally bound System which comprises of the Sun, the Eight planets, dwarf planets, satellites, Comets, asteroids and meteoroids.

The Sun:

The Sun is at the Centre of the solar System. Each member Of the solar system Revolves around the Sun. The Sun is so Huge, that it accounts for 99.8 percent of The entire mass of the solar system. The sun is made up of extremely hot gases like Hydrogen and Helium. The Sun is a star. It is self-luminous; It gives light on its own. The surface Temperature of the Sun is about 6,000° C. It is the source of light and heat energy to the entire solar system. Sunlight takes about 8.3 minutes to Reach the Earth.

Planets:

The word ‘planet’ Means wanderer. There are eight planets In the solar system. They are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. All The planets rotate anti-clockwise (from West to east) on their own axes except Venus and Uranus. The elliptical path in Which the planets move around the Sun is Known as orbit. The eight planets revolve In their respective orbits because of the Gravitational pull of the Sun. They do not Move out of their paths or away from the Solar system.The four planets nearer to the Sun Are called Inner or Terrestrial Planets(Mercury, Venus, Earth and Mars). The inner Planets are comparatively smaller in size And are composed of rocks. The surface of Inner planets has mountains, volcanoes and Craters. The last four planets are called as Outer Planets or Jovian Planets (Jupiter, Saturn, Uranus, and Neptune). They are also Called Gaseous Giants. An asteroid belt is Found between Mars and Jupiter.

Mercury (The Nearest Planet)

Mercury is the Smallest and closest Planet to the Sun. It is named after The Roman deity ‘Mercury’, the Messenger to the Gods. It is an airless And waterless planet. It does not have an Atmosphere and so experiences extremes Of temperature. It has no natural satellites. Mercury can be viewed in the morning and Evening with the naked eye.

Venus (The Hottest Planet)

Venus is the Second planet from The Sun. It is called The Earth’s twin, As it is almost the Same size as the Earth. It has the Longest rotation period (243 days) among the planets in the Solar System. It rotates in the opposite direction To all other planets, except Uranus. It Has no natural satellites, like Mercury. It is named after the Roman goddess of Love and beauty. It is often visible in the Mornings and the evenings and so it is Frequently called as the Morning Star and The Evening Star. After the Moon, it is the Brightest natural object in the night sky.

Earth (The Living Planet):

The Earth is The third planet From the Sun and The fifth largest Planet in the solar System. It is called The ‘blue planet’ Or ‘watery planet’ Because three-fourth of the Earth is Covered by water. The Earth is the only Planet in the solar system which is not Named after any Greek or Roman deity. It Is the only planet known to support life. Life is possible on Earth because of the presence of land, air and water. The polar Diameter of the Earth is 12,714 km and The equatorial diameter is 12,756 km. The Earth revolves around the Sun at a Speed of about 30 km per second. The Only natural satellite of the Earth is the Moon.

Mars (The Red Planet):

Mars is the fourth Planet from the Sun and the second Smallest planet in The solar system, After Mercury. It Is named after the Roman God of war. It appears red in colour Due to the presence of iron oxide on its Surface. So, it is often described as theRed Planet. It has a thin atmosphere. It also has polar ice caps like the Earth.Mars has two natural satellites namely Phobos and Deimos. Many orbiters and Rovers have been launched to explore this Planet.

Jupiter (the Largest Planet):

Jupiter is the Fifth planet from the Sun and the largest Planet in the solar System. It is named After the king of the Roman gods. It is the Third brightest object In the night sky, after moon and Venus. It Is the fastest spinning planet in the solar System. It is called a gas giant planet. Its Atmosphere is mostly made up of Hydrogen And Helium like the Sun. It has the largest Number of natural satellites. Io,Europa, Ganymede and Callisto are a few large Satellites of Jupiter.

Saturn (The Ringed Planet):

Saturn is the sixth planet from the Sun And the second largest planet in the solar System, after Jupiter. It is named after the Roman god of agriculture. Saturn has many Rings around it. These rings are huge and Are mostly made up of ice, rocks and dust Particles. Saturn has 62 natural satellites around It. Titan, Saturn’s largest moon, is the only satellite in the solar system that has clouds And a dense atmosphere composed of Nitrogen and methane. The specific gravity Of Saturn is less than that of water.

Uranus (The Somersaulting Planet):

Uranus is the Seventh planet from The Sun. It was the First to be discovered With a telescope By the astronomer William Herschel in 1781. It appears Green due to the presence of methane Gas. It is named after the Greek god of The sky. It rotates on its axis from east to West like Venus. Its axis is tilted so much That, it appears to orbit the Sun on its sides Like a rolling ball. Uranus has 27 natural Satellites, of which Titania is the largest.

Neptune (The Coldest Planet):

Neptune is the Eighth and the Farthest planet From the Sun. There are strong Winds in this Planet. It is named After the Roman god Of sea. Neptune has 14 natural satellites, The largest being Triton. Because of its Distance from the Sun, Neptune is one of The coldest planets in the solar system.The striking blue and white features Of Neptune help to distinguish it from Uranus.

The Dwarf Planets:

Dwarf planets are small celestial Bodies found beyond the planet Neptune. They are extremely cold and dark. They Are almost spherical in shape, but unlike Planets they can share their orbit with Other dwarf planets. The five dwarf Planets of the solar system are Pluto, Ceres, Eris, Makemake and Haumea.

The Moon:

Earth’s SatelliteSatellites are Celestial objects, Which revolve Around the Planets. The moon Is the Earth’s Only satellite. It Revolves around The Earth once in every 27 days and 8 hours. It takes about the same time for It to complete one rotation around its axis. It has no atmosphere. The surface of the Moon is characterized by craters created By the impact of meteors. The distance Between the moon and the Earth is About 3, 84,400 km. The size of the moon Is one-quarter of the Earth. The Moon Is the only celestial body where humans Have landed.

Asteroids:

Asteroids are small solid objects that Move around the Sun. They are found as A belt between Mars and Jupiter. They are Too small to be called as planets. They Are also known as Planetoids or Minor Planets.

Comets:

A comet is a celestial object made up Of a head and a tail. The head of a comet Consists of solid particles held together By ice and the tail is made up of gases. Halley’s Comet is the most famous comet Which comes close to the Earth every 76 Years. It appeared in 1986 and will appear In 2061.

Meteors and Meteorites:

A meteor is a stone like or metallic Body. When entering into the Earth’s Atmosphere, most of them burn. As they Often appear as streaks of light in the sky, They are also known as Shooting Stars. Meteors which strike the Earth’s surface are called meteorites.

Motions of the Earth:

Have you noticed the Sun in the morning, Afternoon or evening? Is it in the same Place throughout the day? No. It is seen In the east in the morning, overhead in the Afternoon and in the west in the evening. Have you ever thought of the reason behind It? This is because of the constant moving Of the Earth around the Sun. It seems that The Sun is moving, but it is not so. This is Similar to what you experience when you Are travelling in a bus or train. When you Look out of the window, the trees, lamp Posts and other objects seem to be moving, But actually it is you who are moving. To Understand the motions of the Earth better, You need to be familiar with the shape and Inclination of the Earth.

Shape and Inclination of the Earth:

The Earth is spherical in shape. It Rotates on its axis, which is an imaginary Line that runs from the North Pole to the South Pole passing through the centre of The Earth. The Earth’s axis is always tilted Or inclined from the vertical by an angle Of 23½°. It makes an angle of 66½° with The plane of the Earth’s orbit.

Rotation:

It is the spinning Movement of the Earth on its axis. The Earth rotates From west to east (anti-clockwise) And takes 23 hours 56 minutes and 4.09 seconds to Complete one rotation. The time taken by The Earth to complete one rotation is called A day. The rotation of the Earth causes Day and night. As the Earth is spherical in Shape, only one half of it is illuminated by The Sun at a time. The other half remains Dark. The illuminated portion of the Earth Experiences day, whereas the darkened Part of the Earth experiences night. The Line which divides the surface of the Earth Into a lighted half and a dark half is called The Terminator Line. 

Revolution:

It is the movement of the Earth Around the Sun on its elliptical path. The Earth takes 365 ¼ days to complete one Revolution. It revolves around the Sun at a Speed of 30 km per second. For the sake Of convenience, we take it as 365 days and call it a year. The remaining quarter day  Is added once in every four years in the Month of February. That is why February Has 29 days once in four years. It is called A Leap Year. The inclination of the Earth On its axis and its revolution around the Sun cause different seasons.

The Northern Hemisphere is inclined Towards the Sun for six months from 21^st March to 23^rd September while the Southern Hemisphere is tilted away from the Sun.From Sep 23^rd to March 21^st the Southern hemisphere is inclined towards The Sun and the northern hemisphere Faces away from the Sun. The changing Position of the Earth in its orbit during revolution gives the impression that the Sun is continuously moving north and South of the equator. The equator faces The Sun directly on 21 March and 23 September. These two days are called Equinoxes, during which the day and Night are equal throughout the Earth.

On 21^st June, the Tropic of Cancer Faces the Sun. This is known as Summer Solstice. It is the longest day in the Northern Hemisphere and longest Night (shortest day) in the Southern Hemisphere. On 22^nd December, the Tropic Of Capricorn faces the Sun. It is called as Winter Solstice. It is the longest day In the Southern Hemisphere and longest Night (shortest day) in the Northern Hemisphere.