The apparent change in the size of the sun at dusk or dawn is an optical illusion caused by atmospheric refraction and contrast with terrestrial objects. The colour of the sun at dawn and dusk appears reddish due to Rayleigh scattering of sunlight, where shorter wavelengths are scattered away, leaving longer wavelengths. The twinkling of stars is due to the atmospheric refraction of starlight as it passes through different layers of varying densities and temperatures. Conversely, the visibility of the moon at dawn and the polestar being visible are real astronomical observations, not optical illusions.