Atmospheric refraction – The stars are lower than we think

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Refraction means that an electromagnetic wave changes its direction when passing from a medium of a given optical density to one of a different density. The wave undergoes refractive action towards the perpendicular due to a change in its propagation velocity at this point. This results in a change in the wavelength, but not in the frequency. We can observe a variety of light refraction phenomena in the various high air layers of the atmosphere.

I like to sleep late in the morning, and that’s why my archive has much fewer sunrise images than would be good for it. – Getting up early is just torture for me! In the meantime, however, I have found a justification for my preference for sunset: Due to the refraction of light in the atmosphere, the sun seems to stay above the horizon a little longer in the evening than in the morning, when it seems to rise accelerated by the same effect – about four minutes more time for good pictures, then! But all kidding aside.

Due to the more powerful air layer near the horizon and the longer path of the light rays, atmospheric refraction is most pronounced near the horizon, resulting in the sun geometrically passing the horizon line when it only touches it with its lower edge at sunset.

But curvature is in this case actually a better expression than refraction, because in the atmosphere the density differences are small and continuous, which is why the light rays are continuously refracted in tiny steps and cover a just rather curved path. As a result, we perceive the objects to be slightly elevated above the horizon, which is not the case in reality.

This explains yet another peculiarity of the low-standing sun and the moon. Perhaps you have also observed through your longest focal length that both celestial bodies look strangely out of round, almost flattened, when they reach the horizon with their underside. This is also a result of refraction, which is less pronounced the higher the object is above the horizon. Accordingly, the lower edge of the sun is raised by 35, but its upper edge by 29 minutes of arc, which of course does not leave its beautiful roundness unscathed.

But this is not enough. The color of the sun also reflects the different refractive properties of the atmosphere: the more different its red and orange hues, the greater the temperature differences of the air layers above the horizon, and the more the paths of the light rays and the degree of their scattering and absorption deviate from each other.

Next Rainbows – The whole spectrum of colors

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