The apparent change in the size of the sun at dusk or dawn is due to atmospheric refraction, creating an illusion of a larger disc. Similarly, the colour of the sun appears reddish at dawn and dusk because atmospheric scattering of sunlight causes shorter blue wavelengths to be dispersed, leaving longer red wavelengths to reach our eyes. The twinkling of stars is also an optical phenomenon caused by the refraction of starlight through atmospheric layers of varying densities and temperatures.