R/dayLength.R

In tanerumit/hydrosystems: Tools for hydrologic and water systems modeling & analysis

dayLength <- function (lat, doy) 
{
  if (class(doy) == "Date" | class(doy) == "character") {
    doy <- as.character(doy)
    doy <- as.numeric(format(as.Date(doy), "%j"))
  }
  else {
    doy <- (doy - 1)%%365 + 1
  }
  lat[lat > 90 | lat < -90] <- NA
  P <- asin(0.39795 * cos(0.2163108 + 2 * atan(0.9671396 * 
                                                 tan(0.0086 * (doy - 186)))))
  a <- (sin(0.8333 * pi/180) + sin(lat * pi/180) * sin(P))/(cos(lat * 
                                                                  pi/180) * cos(P))
  a <- pmin(pmax(a, -1), 1)
  DL <- 24 - (24/pi) * acos(a)
  return(DL)
}