Why Does Refraction Cause an Elliptical Sun?

[Pranav Jha]

Why does sun appear elliptical during sunset and sunrise?

I have read that when the Sun (or Moon) is near the horizon, the rays from the lower edge are bent more than the rays from the upper edge. This produces a shortening of the vertical diameter, causing the Sun to appear elliptical
BUT i don't understand why the rays from lower edge are more bent due to refraction.

 

[K^2]

There is a lensing effect of atmosphere that makes this effect really noticeable, but even if you forget that, just think of atmosphere as a layer of glass. When you watch Sun near horizon, the light from top edge hits Earth near the place where you are standing more straight on than light from bottom edge. And by laws of refraction, the light that meets surface at greater angle from perpendicular is refracted more. Look up Snell's Law.

 

[D H]

The effect falls off sharply with increased elevation. The rays from the top of the Sun are going through less atmosphere than are the rays from the bottom of the Sun.