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July 1, 2014 14:43
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sunPosition <- function(year, month, day, hour=12, min=0, sec=0, | |
lat=46.5, long=6.5) { | |
twopi <- 2 * pi | |
deg2rad <- pi / 180 | |
# Get day of the year, e.g. Feb 1 = 32, Mar 1 = 61 on leap years | |
month.days <- c(0,31,28,31,30,31,30,31,31,30,31,30) | |
day <- day + cumsum(month.days)[month] | |
leapdays <- year %% 4 == 0 & (year %% 400 == 0 | year %% 100 != 0) & | |
day >= 60 & !(month==2 & day==60) | |
day[leapdays] <- day[leapdays] + 1 | |
# Get Julian date - 2400000 | |
hour <- hour + min / 60 + sec / 3600 # hour plus fraction | |
delta <- year - 1949 | |
leap <- trunc(delta / 4) # former leapyears | |
jd <- 32916.5 + delta * 365 + leap + day + hour / 24 | |
# The input to the Atronomer's almanach is the difference between | |
# the Julian date and JD 2451545.0 (noon, 1 January 2000) | |
time <- jd - 51545. | |
# Ecliptic coordinates | |
# Mean longitude | |
mnlong <- 280.460 + .9856474 * time | |
mnlong <- mnlong %% 360 | |
mnlong[mnlong < 0] <- mnlong[mnlong < 0] + 360 | |
# Mean anomaly | |
mnanom <- 357.528 + .9856003 * time | |
mnanom <- mnanom %% 360 | |
mnanom[mnanom < 0] <- mnanom[mnanom < 0] + 360 | |
mnanom <- mnanom * deg2rad | |
# Ecliptic longitude and obliquity of ecliptic | |
eclong <- mnlong + 1.915 * sin(mnanom) + 0.020 * sin(2 * mnanom) | |
eclong <- eclong %% 360 | |
eclong[eclong < 0] <- eclong[eclong < 0] + 360 | |
oblqec <- 23.439 - 0.0000004 * time | |
eclong <- eclong * deg2rad | |
oblqec <- oblqec * deg2rad | |
# Celestial coordinates | |
# Right ascension and declination | |
num <- cos(oblqec) * sin(eclong) | |
den <- cos(eclong) | |
ra <- atan(num / den) | |
ra[den < 0] <- ra[den < 0] + pi | |
ra[den >= 0 & num < 0] <- ra[den >= 0 & num < 0] + twopi | |
dec <- asin(sin(oblqec) * sin(eclong)) | |
# Local coordinates | |
# Greenwich mean sidereal time | |
gmst <- 6.697375 + .0657098242 * time + hour | |
gmst <- gmst %% 24 | |
gmst[gmst < 0] <- gmst[gmst < 0] + 24. | |
# Local mean sidereal time | |
lmst <- gmst + long / 15. | |
lmst <- lmst %% 24. | |
lmst[lmst < 0] <- lmst[lmst < 0] + 24. | |
lmst <- lmst * 15. * deg2rad | |
# Hour angle | |
ha <- lmst - ra | |
ha[ha < -pi] <- ha[ha < -pi] + twopi | |
ha[ha > pi] <- ha[ha > pi] - twopi | |
# Latitude to radians | |
lat <- lat * deg2rad | |
# Azimuth and elevation | |
el <- asin(sin(dec) * sin(lat) + cos(dec) * cos(lat) * cos(ha)) | |
az <- asin(-cos(dec) * sin(ha) / cos(el)) | |
# For logic and names, see Spencer, J.W. 1989. Solar Energy. 42(4):353 | |
cosAzPos <- (0 <= sin(dec) - sin(el) * sin(lat)) | |
sinAzNeg <- (sin(az) < 0) | |
az[cosAzPos & sinAzNeg] <- az[cosAzPos & sinAzNeg] + twopi | |
az[!cosAzPos] <- pi - az[!cosAzPos] | |
# if (0 < sin(dec) - sin(el) * sin(lat)) { | |
# if(sin(az) < 0) az <- az + twopi | |
# } else { | |
# az <- pi - az | |
# } | |
el <- el / deg2rad | |
az <- az / deg2rad | |
lat <- lat / deg2rad | |
return(list(elevation=el, azimuth=az)) | |
} |
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