> ... I'm not sure why a sine would be the right shape for this, but 
> does it account for the fact that the atmoshphere is curved?

It wouldn't :-).
But light fall off is vaguely sine shaped and it was an available and 
easy function which gave some sort of half realistic feel for things. 
It may even be closer to reality than one has a right to hope. And may 
not.

Atmospheric path length is arguably h/cos(X) where X is angle from 
vertical and h is atmospheric scale height or some other number that 
works :-). So ratio of path length to noon length = 1/cos(X). This is 
only truish for a flat earth but adds are it is flat enough at the 
angles concerned for this expression to be useful. AFAIR above 
atmosphere insolation is about 1.3 kW/m^2 whereas a figure of more 
like 1 kW/m^2 is usually used at full sun terrestially. You can get 
solar equivalent kWh/day tagbles for all latitudes.

Attenuation can be expected to rise faster than this as large angles 
include morning  and evening mists etc. Also at low angles hills and 
buildings tend to be impressively opaque :-).



        Russell


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