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How complicated is it to calculate a surface normal on the spherical approximation of the earths surface pointing towards the sun at a given point in time?

What I try do is to highlight a small area on a world map where the radiation from the sun tangential hits a imaginary sphere around the earth.

This is just to get an idea where this area was at the given time, so this information has not to be very accurate. As a mater of fact I will just incorporated this information in the map if it is easy enough to calculate.

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This small area will be a ring around the Earth, like the Equator (but not the Equator). – mtrencseni Sep 27 '11 at 13:26
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For longitude, just use local noon. For latitude, just treat it as a sine function between +/- 23 degrees, with zero-crossings at the equinoxes. That's a first approximation. A slightly better approximation has the sun making a figure-8 motion. – Mike Dunlavey Sep 27 '11 at 13:30
I think longitude == local noon is a good direction. It will be enough to highlight the meridian where the sunset/sunrise happen at the given time. This should be easy to calculate from the "noon meridan". Thank you. – Fabian Sep 27 '11 at 14:20

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