[GRASS-dev] [GRASS GIS] #2876: r.sun (mode 1): data are not timestamped correctly

[GRASS GIS]

#2876: r.sun (mode 1): data are not timestamped correctly
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  Reporter:  fabriziosossan  |      Owner:  grass-dev@…
      Type:  defect          |     Status:  new
  Priority:  normal          |  Milestone:  7.0.4
 Component:  Raster          |    Version:  6.4.5
Resolution:                  |   Keywords:  r.sun
       CPU:  x86-64          |   Platform:  Linux
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Comment (by fabriziosossan):

 Hi,

 This is our logic explanation for changing the time offset sign from
 positive to negative.

 It should be noted first that there is no agreed convention for the sign
 of the Equation of Time (EoT in the following) and can appear in both the
 forms shown in [1] and [2]. The EoT implemented in the code is as former,
 where positive y values denotes that a wall clock is slower than a
 sundial, and vice-versa.
 For example, assuming to be i) at greenwich latitude ii) on the 1st
 January (when EoT is approx. -3 minutes) and iii) with the sun exactly at
 south (solar noon), a wall clock would appear faster and would mark 12:03
 of civil time. Therefore the additive offset to pass from the solar time
 to civil time should be positive, namely the EoT expression should be with
 inverted sign.

 Inverting the EoT sign in order to obtain, for example, a positive offset
 for the 1st of January, is coherent with the correction that is applied
 for the longitude displacement later in code:

 longitTime = -longitude / 15.

 In this case, when moving eastwise from the greenwich meridian (this means
 progressively increasing positive values of the longitude), the sun
 position will move in the sky to west: in this case  the clock would
 appear slower than a sundial and the offset, conversely to the previous
 case, should be indeed strictly negative.

 Finally, one might wonder if later in the code the signs of the discussion
 expressions are changed: this is not the case as these two contributions
 are summed up together.

 [1]
 https://en.wikipedia.org/wiki/Equation_of_time#/media/File:Equation_of_time.svg
 [2] https://en.wikipedia.org/wiki/Equation_of_time#/media/File
 :Tijdvereffening-equation_of_time-en.jpg

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Ticket URL: <http://trac.osgeo.org/grass/ticket/2876#comment:4>
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