biometeoR: BIOMETEOROLOGICAL INDEX

#' sunposition
#'
#' @description Function to assess sun azimuth and elevation given the time in GMT format.
#' 
#' @param timenow time indicat in time format equivalent to "POSIXct" or "POSIXt" object.
#' @param  lat numeric latitude in decimal degrees
#' @param lon numeric  longitude in decimal degrees
#' @references 
#' Istituto di Biometeorologia Firenze Italy
#' Centro Interdipartimentale di Bioloclimatologia University of Florence
#' \url{http://stackoverflow.com/questions/8708048/position-of-the-sun-given-time-of-day-latitude-and-longitude}
#' @author  Alfonso crisci \email{a.crisci@@ibimet.cnr.it} Marco Morabito \email{m.morabito@@unifi.it} 
#' 
#' 
#' @export



sunposition <- function(timenow,lat, long) {
    
    time=as.POSIXlt(timenow, "GMT")
    year=as.numeric(format(time,"%Y"))
    hour=as.numeric(format(time,"%H"))
    day=as.numeric(format(time,"%d"))
    month=as.numeric(format(time,"%m"))
    min=as.numeric(format(time,"%M"))
    sec=as.numeric(format(time,"%S"))

    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]

  

    el <- el / deg2rad
    az <- az / deg2rad
    lat <- lat / deg2rad

    return(list(elevation=el, azimuth=az))
}