At night, the sky is full of stars, the moon and sometimes bright planets. These heavenly bodies that we see in the sky are part of the universe. The universe is a vast space that contains everything – galaxies, stars, planets, moons, asteroids, comets and all forms of matter and energy. The earth, our home, is a tiny part of this huge universe.
We live on the earth, which is a planet. It goes around the sun along with seven other planets. The sun, the eight planets and their moons, along with asteroids and other bodies, together form the solar system. The sun is at the centre of the solar system, and all the planets revolve around it.
In this chapter, we will learn about the celestial bodies, the solar system, the sun and the planets, the earth and the moon, and the difference between stars, planets and satellites.
The objects that we see in the sky are called celestial bodies (from the Latin word "caelum", meaning the sky). The sun, the moon, the stars, the planets and the asteroids are all celestial bodies.
The sun is a star. Stars are huge balls of hot, glowing gases that give out their own light and heat. The sun looks bigger than the other stars only because it is much nearer to us. The other stars are very far away, so they look like tiny points of light, twinkling in the night sky. The moon and the planets do not have their own light; they shine because the light of the sun falls on them.
There are three important kinds of celestial bodies that we must understand clearly:
The sun is a star at the centre of the solar system. It is a huge ball of hot, glowing gases and is the nearest star to the earth.
The sun is the source of almost all energy for the earth. It provides us with light and heat, which are essential for life. Plants need sunlight to make their food, and without the sun, life on the earth would be impossible. The sun is about 150 million kilometres away from the earth, and its light takes about 8 minutes to reach us. The sun is about 100 times bigger than the earth in size.
There are eight planets in our solar system. In order of their distance from the sun, they are:
The first four planets (Mercury, Venus, Earth and Mars) are called the inner planets or terrestrial planets. The last four (Jupiter, Saturn, Uranus and Neptune) are called the outer planets or giant planets.
The earth is the third planet from the sun and the only planet in the solar system where life exists. This is because the earth has the right conditions for life: a suitable temperature, water and air.
The earth is neither too hot nor too cold, and it receives just the right amount of sunlight and heat. It has water in liquid form, and an atmosphere that protects it. The earth revolves around the sun and rotates on its own axis. The shape of the earth is not a perfect sphere; it is slightly flattened at the poles and bulging at the equator. This shape is called an oblate spheroid or a geoid.
The moon is the natural satellite of the earth. It revolves around the earth and shines because of the light of the sun falling on it. The moon is about 3,84,400 kilometres away from the earth.
The moon is the only natural satellite of the earth, and it takes about 27 days to revolve around the earth. The moon has no light and heat of its own. Its surface has many craters, mountains and plains. Neil Armstrong was the first person to step on the moon, on 21 July 1969, in the American spacecraft Apollo 11. The moon does not have any atmosphere or water, so no life can exist on it.
Besides the sun, the planets and the moon, the solar system also contains other members:
Our solar system is a tiny part of a huge galaxy called the Milky Way or Akash Ganga. A galaxy is a huge system of billions of stars, along with dust, gas and other matter, held together by gravity.
The universe contains countless galaxies, each with billions of stars. The Milky Way galaxy, in which our solar system lies, is a spiral galaxy. The earth and the other planets revolve around the sun, the sun revolves around the centre of the galaxy, and the whole universe is in constant motion. The universe is so vast that the distances in it are measured in light years – the distance that light travels in one year (about 9.46 x 10^12 km).
| Position | Planet | Special Feature |
|---|---|---|
| 1 | Mercury | Smallest, nearest to the sun |
| 2 | Venus | Brightest; called morning/evening star |
| 3 | Earth | Only planet with life |
| 4 | Mars | Called the Red Planet |
| 5 | Jupiter | Largest planet |
| 6 | Saturn | Has beautiful rings |
| 7 | Uranus | Spins on its side |
| 8 | Neptune | Farthest from the sun |
| Body | Definition | Example |
|---|---|---|
| Star | Huge ball of hot gases with its own light | Sun |
| Planet | Body that revolves around a star | Earth |
| Satellite | Body that revolves around a planet | Moon |
| Asteroid | Small rocky body revolving around the sun | Ceres |
| Comet | Small icy body with a glowing tail | Halley's comet |
| Galaxy | Huge system of billions of stars | Milky Way |
The earth is a tiny part of the vast universe. It belongs to the solar system, where the sun, eight planets, their moons, asteroids and comets all revolve in their own paths. The sun is a star and the source of light and heat for the earth. Among all the planets, the earth is unique because it alone has the conditions for life.
The moon is the earth's faithful companion and natural satellite, and its exploration by Neil Armstrong and Apollo 11 was a historic achievement. Understanding the earth in the solar system helps us appreciate how special our planet is and why we must protect its air, water and other resources.