In a medium different to vacuum, the speed of light is different than the
speed of light in vacuum. Now, if light from one medium with a certain speed
of light passes in an angle into another medium with a different speed of
light, then it will be deflected according to Snell's law. Hence, if light
from the near vacuum of outer space goes through the atmospheric air of the
Earth, then it will be deflected as well. This process is called refraction.
As the density of the Earth's atmosphere varies, especially with altitude, so
does the degree of deflection, depending on the incoming angle and the
refractive index of the local air (ie its density and chemical composition).
To calculate this process, the refraction, I devised a new model which is described in:
Balluch, M., and Lary, D.J., 1997:"Refraction and Atmospheric
Photochemistry". Journal of Geophysical Research - Atmospheres,
In this article the effects on the chemistry are discussed as well. As an
example of the results, the following figure shows how much refraction
is changing the apparent position of the sun in the sky:
The figure shows how many degrees the sun is apparently displaced in a light
of 175nm for different altitudes and true solar zenith angles. For example, on
a mountain top of 4000 metres with the sun truely having just passed over the
horizon, the sun will look as if it is still above the horizon, shining
straight into one's eye and putting all surrounding peaks into a glowing red.
Indeed, due to refraction the days become quite a bit longer. The following
picture gives the difference in sunlit day due to refraction for equinox
(solid curve), winter solstice (dashed curve) and summer solstice
(dash-dotted curve) in minutes for different latitudes.