It is the scattering of sunlight by the atmosphere that is the reason behind redness of sunsets and the blue colour of the sky. Sunlight, which appears as white, is actually a mixture of seven colours.
As sunlight passes through the atmosphere, the dust, clouds, it gets scattered on the way. Only red light, which is of a longer wavelength, survives this and so the setting sun is seen as a lovely red.
Below Sunset info from Wikipedia (Read More):-
Sunset is the daily disappearance of the sun below the horizon as a result of the Earth's rotation. The atmospheric conditions created by the setting of the sun, occurring before and after it disappears below the horizon, are also commonly referred to as "sunset".
In astronomy the time of sunset is defined as the moment the trailing edge of the sun's disk disappears below the horizon in the west. Due to refraction of light in the atmosphere, the ray path of the setting sun is highly distorted near the horizon making the apparent astronomical sunset occur when the sun’s disk is already about one diameter below the horizon. Sunset should not be confused with dusk, which is the moment at which darkness falls, when the sun is about eighteen degrees below the horizon. The period between the astronomical sunset and dusk is called twilight.
http://en.wikipedia.org/wiki/Sunset
