This lesson deals with the Universe and the members of the Solar System, the motions of the Earth and their effects, and the four spheres of the Earth. The Earth is the third planet in the Solar System; the Solar System is in the Milky Way Galaxy, and there are millions of such galaxies in the Universe.
1. The Universe
- Numerous stars and celestial bodies came into existence by a massive explosion called the Big Bang. These celestial bodies together are called the Universe, also referred to as the Cosmos.
- The Universe is a vast expanse of space. Most astronomers believe that it came into existence after the Big Bang explosion that took place about 15 billion years ago.
- It consists of billions of galaxies, stars, planets, comets, asteroids, meteoroids and natural satellites, collectively called celestial bodies.
- The study of the Universe is called Cosmology. The term Cosmos is derived from the Greek word 'Kosmos'.
- The stars appear small because they are so far away, but they are really huge in size.