Explanation: Atmospheric refraction is the reason behind the twinkling of stars. The starlight on entering earth surface undergoes refraction continuously before it reaches to earth. The starlight bend towards the normal as it enters from rarer to denser medium. The air near the earth surface is denser than upper layers. Therefore the apparent position of the stars are slightly different from its actual position, when it is viewed from horizon. The apparent position of stars are not stationary and keeps on changing slightly, because the physical condition of the atmosphere are not stationary. Because of this the stars sometimes appears brighter and sometimes appear fainter.