Green Rim imaged by Aymen Ibrahem (Night Sky over Egypt) from the Plaza of the Bibliotheca Alexandrina, about 20 minutes before sunset on March 12, 2008. Thick haze dimmed the sun. ©Aymen Ibrahem, shown with permission.
Our atmosphere refracts the sun’s rays causing its disk to appear slightly flattened and higher in the sky. Close to the horizon the effect increases dramatically causing sunset to be delayed and sunrise hastened.

Green light refracts more strongly than red and thus the ‘green sun’ appears slightly higher producing an upper green rim. We rarely see a blue rim because when the sun is low so much blue light is scattered away by the atmosphere.

Many websites and textbooks ascribe the green flash to this effect. No so, the green rim is far too narrow to see with the unaided eye. Green flashes require a mirage to vertically magnify the narrow rim.

Flattened sun     Flattened moon
Sunset mirages
Green rim
Green flashes