Our knowledge about the heavens is dependent on the means of observation. Observations made with the naked eye serve to determine constellations of stars and provide knowledge about the motions of celestial bodies. Looking through a telescope it is apparent that the stars, which are visible to the naked eye, constitute merely a fraction of astronomical objects. Nebulae turn out to be very distant galaxies. Spectral analysis provides information about the composition of stars and leads to the discovery of the redshift of their spectral lines, which increases with distance. Radio telescopes and other detectors of invisible types of radiation discover ever more astronomical objects.