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 to-report light[day time]
   let psi   51.4408                ;; lat. ehv
   let Gsc   1367                   ;; w/m2 solar constant
   let LC    (15 * 1 - 5.4778) / 15 ;; timezone corrected longitude
   let c     0.21                   ;; fraction of extraterrestrial sunlight that reaches
                                     ; the surface (clear weather
 
   ;; most of the following formulas are from wikipedia, they need to be verified and sourced.
   let Ext   Gsc * (1 + 0.034 * cos(360 / 365 * (day - 2))) ;; day
   let theta 360 * (day - 1) / 365.242
   let d     23.45 * sin(360 * (day + 10) / 365)
   let eqt   0.258 * cos(theta) - 7.416 * sin(theta) - 3.648 * cos(2 * theta) - 9.228 * sin(2 * theta)
   let w     time + eqt / 60 - LC
   let as    asin(cos((w - 12) / 24 * 360) * cos(psi) * cos(d) + sin(psi) * sin(d))
   let Edv   Ext * exp(- c / sin(as)) * sin(90 - as) * (max list 0 as) / as
   report Edv * 93;;lm/w http://physics.ucsd.edu/~tmurphy/papers/lumens-per-watt.pdf
 end