Tue January 31, 2012 By:

Please explain the flattening of sun during sunrise/sunset more descriptively with an illustrated diagram.

Expert Reply
Wed February 01, 2012

Rays from the setting Sun (lower) are refracted by the atmosphere and make it appear higher in the sky.     The lower limb is lifted more than the top, making its image oval.

               Density gradients in the atmosphere cause it to act like a lens.   Rays bend towards the higher density.   Higher density is associated with higher pressure and lower temperature so a useful guideline for atmospheric refraction  is that rays bend towards higher pressure and to cooler air.

Incoming rays from the Sun curve downwards so that it appears higher in the sky than it really is.    The Moon and stars and the whole sky are similarly raised and astronomical observations are routinely corrected for "atmospheric refraction".

The effect is small except near to the horizon when the rays are literally bent a little around the curve of the Earth.    When the lower limb of the Sun just touches the sea horizon the whole Sun has actually already set.

Objects closer to the horizon are raised upwards most and the lower limb of the Sun is raised more than the top making it appear oval.

The flattening is up to about 20% for a normal atmospheric temperature profile and an observer close to sea level.   It varies with atmospheric conditions especially when there are abnormal temperature gradients.  From high aircraft or space the flattening is much greater.
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