Atmospheric Refraction

Atmospheric refraction is the shift in apparent direction of a celestial object caused by the refraction of light rays as they pass through Earth’s atmosphere.

The twinkling of  stars and variation in size of the Sun are due to atmospheric refraction of starlight.

Twinkling of Stars

Do stars really twinkle? No, stars do not twinkle. Let us now learn about why they appear to do so.
The rays of light coming from the stars travel through the layers of air of varying densities.
These rays get refracted continuously and they bend towards the normal as the refraction is from a rarer to a denser medium. The movements of air and convection currents cause a change in the density of the layers of air. As a result, the position of the image of the star goes on changing after every short interval.
These different positions of the images formed at short intervals of time give the impression that the star is twinkling.

Variation in the Size of the Sun

At dusk or dawn the Sun appears to be larger than at noon. This is because when the sun is near the horizon the rays of light coming from the sun have to pass through layers of air of increasing density.
Due to continuous bending of light the sun appears to be larger. At noon, the sun appears to be smaller than at dusk or dawn. This is because the rays of light that fall normally on the surface of the earth do not get refracted.