R/jd2date.R

In helixcn/ephem: Creating figures for astronomical almanac

jd2date <-
function(jd) {
  jdate2vect <- function(xxx) {
    year = xxx[[1]]
    month = xxx[[2]]
    day <- xxx[[3]]
    frac <- xxx[[6]] / (60 * 60 * 24) + xxx[[5]] / (60 * 24) + xxx[[4]] / 24
    day.dec <- day + frac
    res <- c(year, month, day.dec)
    names(res) <- c("Year", "Month", "Day")
    return(res)
  }

  D = floor(jd + 0.5)
  F = jd + 0.5 - D
  if (D >= 2299161) {
    c = floor((D - 1867216.25) / 36524.25)
    D = D + 1 + c - floor(c / 4);
  }
  D = D + 1524;
  r.Y = floor((D - 122.1) / 365.25);
  D = D - floor(365.25 * r.Y);
  r.M = floor(D / 30.601);
  D = D - floor(30.601 * r.M);
  r.D = D;
  if (r.M > 13) {
    r.M = r.M - 13
    r.Y = r.Y - 4715;
  }
  else {
    r.M = r.M - 1
    r.Y = r.Y - 4716
  }
  F = F * 24;
  r.h = floor(F);
  F = F - r.h;
  F = F * 60;
  r.m = floor(F);
  F = F - r.m;
  F = F * 60;
  r.s = F;
  res <- list(r.Y, r.M, r.D, r.h, r.m, round(r.s, 2))
  names(res) <- c("Year", "Month", "Day", "Hour", "Minute", "Second")
  return(paste(unlist(jdate2vect(res)), collapse = "-"));
}