We know that the light is formed by the union of some colors. When entering in contact with the atmosphere, it is spread due to existing particles in air. However the waves of each color are spread of different form, depending on its length. The more short, more dispersed they become. The length of the blue wave makes with that it if spreads the sufficient to give to the sky the tonality that we see. No longer final of the afternoon, the sun illuminates obliquely, compelling the rays to make a longer way to arrive at the Land. Such dispersed fact almost total the blue light and becomes visible the red, that possesss a bigger length, giving the spectacle to us of the put-do-sun.