Abstract: Since its invention 400 years ago, as a scientific instrument the telescope has played a very important role in enlarging our human perspective of the cosmos, and in carving out new territories of knowledge. We first recall the series of entirely new astronomical discoveries made by Galileo with a telescope improved by himself; his discoveries provided crucial evidence to support Copernicus's heliocentric theory. Secondly, it is shown that more and more extraordinary knowledge of the heavens has been acquired along with the telescope's technical developments. This knowledge has helped people grasp the most fundamental laws of nature. Finally, it is discussed how the telescope has played a major role in the process of human exploration of the scale of outer space, helping us to find our true place in the universe.