If you look at the sky, you can see the Sun during daytime; moon and numerous stars during night time. In the earlier days, before the invention of astronomical instruments, people were able to see the Sun, moon and stars only. Based on their observation, they thought that Earth is the centre of all the objects in the space. This was known as the geocentric model, held by Greek astronomer Ptolemy (2nd Century), Indian astronomer Aryabhatta (5th Century) and many astronomers around the world. Later Polish astronomer Nicolaus Copernicus observed the space more keenly and proposed the heliocentric model (helios = Sun), with Sun at the centre of the solar system. The invention of the telescope in the Netherlands, in 1608, created a revolution in astronomy. The improvement of telescopes led astronomers to realize that our Sun is one of hundreds of billions of stars in a galaxy, what we call the Milky Way. We have millions of galaxies in space. The collection of all the things that exist in space is known as the universe. In this lesson we will study how the universe came into existence and all the things in it, how satellites are put into orbit and also about international space station.