it is because of the refraction of light as the refractive index increases as it comes towards the earth.the light travels from vaccum to air 
1 5 1
but vaccum and air same i think so
the difference is very less  0.1 or 0.2 there will be a bit of refraction
kk thnku so much
Because of refraction in the earth's atmosphere: The air density in the earth's atmosphere and therefore also the index of refraction of the earth's atmosphere decrease with increasing altitude. Snellius' law of refraction tells us that light is refracted towards the medium with higher index of refraction. Therefore, the light coming from any star that is not situated directly above the observer takes a curved path bent down towards the observer, arriving at the observer at a steeper angle than had it travelled on a straight line.