R/SolarDec.R

In MeTo: Meteorological Tools

#' @title Solar declination
#' @description Calculate solar declination for daily and shorther periods.
#' @param x date or day of the year
#' @details
#' \describe{\item{x:}{
#' must be provided as number (1-366) or as a common date-time object (e.g, POSIXct, POSIXlt or Date objects).
#' All formats for which is.timepoint from the lubridate package returns TRUE can be used}}
#' @note eq. 24 of reference
#' @references Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. FAO, Rome, 300(9).
SolarDec <- function(x) {
  doy <- prep.date(x)
0.409*sin((((2*pi)/365)*doy)-1.39)
}