Hi all,

I'm working on a solar tracker and would like to calculate the
sunrise/sunset position. The aim is to use a PIC controlling a
pair of steppers and gear trains

One algorithm frequently cited is this one

http://williams.best.vwh.net/sunrise_sunset_algorithm.htm

That page is reproduced below

Now, the first problem has arisen. Do I assume correctly that the
C function "floor" is similar to INT ? ie, the decimal part of a result
is discarded ?

http://en.wikipedia.org/wiki/Floor_and_ceiling_functions

15th November 2010, day of year (by addition) 319

31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31  + 15 =3D 319

day =3D 15, month =3D 11, year =3D 10

N1 =3D floor(275 * 11 / 9)
N1 =3D 336

N2 =3D floor((month + 9) / 12)
N2 =3D 1

N3 =3D 5 [ by way of (1 + INT(10 - (4 * (INT(10/4) + 2)/3))) ]

N =3D 336 - (1 * 5) + 15 - 30
N =3D 316

Where did the other 3 days go ? Or what did I miss ?

Working today's date, 28th February 2011, day of year 59

N1 =3D floor(275 * 2 / 9)
N1 =3D 61

N2 =3D floor((2 + 9) /12)
N2 =3D 0

N3 =3D 6 [ by way of (1 + INT(11 - (4 * (INT(11/4) + 2)/3))) ]

N =3D 61 - (0 * 6) + 28 - 30
N =3D 59

Coincidentally correct ?

Joe

For your interest, timeanddate do solar tables (show all columns)

eg, this month in Auckland

http://www.timeanddate.com/worldclock/astronomy.html?n=3D22&month=3D2&year=
=3D2011&obj=3Dsun&afl=3D-1&day=3D1

Compare Quito (equator) and Nome (way north)

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D

Source:

 Almanac for Computers, 1990
 published by Nautical Almanac Office
 United States Naval Observatory
 Washington, DC 20392

Inputs:
 day, month, year:      date of sunrise/sunset
 latitude, longitude:   location for sunrise/sunset
 zenith:                Sun's zenith for sunrise/sunset
   offical      =3D 90 degrees 50'
   civil        =3D 96 degrees
   nautical     =3D 102 degrees
   astronomical =3D 108 degrees

 NOTE: longitude is positive for East and negative for West
        NOTE: the algorithm assumes the use of a calculator with the
        trig functions in "degree" (rather than "radian") mode. Most
        programming languages assume radian arguments, requiring back
        and forth convertions. The factor is 180/pi. So, for instance,
        the equation RA =3D atan(0.91764 * tan(L)) would be coded as RA
        =3D (180/pi)*atan(0.91764 * tan((pi/180)*L)) to give a degree
        answer with a degree input for L.


1. first calculate the day of the year

 N1 =3D floor(275 * month / 9)
 N2 =3D floor((month + 9) / 12)
 N3 =3D (1 + floor((year - 4 * floor(year / 4) + 2) / 3))
 N =3D N1 - (N2 * N3) + day - 30

2. convert the longitude to hour value and calculate an approximate time

 lngHour =3D longitude / 15

 if rising time is desired:
   t =3D N + ((6 - lngHour) / 24)
 if setting time is desired:
   t =3D N + ((18 - lngHour) / 24)

3. calculate the Sun's mean anomaly

 M =3D (0.9856 * t) - 3.289

4. calculate the Sun's true longitude

 L =3D M + (1.916 * sin(M)) + (0.020 * sin(2 * M)) + 282.634
 NOTE: L potentially needs to be adjusted into the range [0,360) by
adding/subtracting 360

5a. calculate the Sun's right ascension

 RA =3D atan(0.91764 * tan(L))
 NOTE: RA potentially needs to be adjusted into the range [0,360) by
adding/subtracting 360

5b. right ascension value needs to be in the same quadrant as L

 Lquadrant  =3D (floor( L/90)) * 90
 RAquadrant =3D (floor(RA/90)) * 90
 RA =3D RA + (Lquadrant - RAquadrant)

5c. right ascension value needs to be converted into hours

 RA =3D RA / 15

6. calculate the Sun's declination

 sinDec =3D 0.39782 * sin(L)
 cosDec =3D cos(asin(sinDec))

7a. calculate the Sun's local hour angle

 cosH =3D (cos(zenith) - (sinDec * sin(latitude))) / (cosDec * cos(latitude=
))

 if (cosH >  1)
   the sun never rises on this location (on the specified date)
 if (cosH < -1)
   the sun never sets on this location (on the specified date)

7b. finish calculating H and convert into hours

 if if rising time is desired:
   H =3D 360 - acos(cosH)
 if setting time is desired:
   H =3D acos(cosH)

 H =3D H / 15

8. calculate local mean time of rising/setting

 T =3D H + RA - (0.06571 * t) - 6.622

9. adjust back to UTC

 UT =3D T - lngHour
 NOTE: UT potentially needs to be adjusted into the range [0,24) by
adding/subtracting 24

10. convert UT value to local time zone of latitude/longitude

 localT =3D UT + localOffset

--=20
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