index.js

"use strict";
const lunar = require("./lib/lunar");
const solar = require("./lib/solar");

// Convert to/from J2000 dates.
const J1970 = -10957.5;

function ms_to_julian(ms) {
  return J1970 + ms / 86400000;
}

function unix_to_julian(unix) {
  return J1970 + unix / 86400;
}

function date_to_julian(date) {
  return ms_to_julian(date.getTime());
}

function julian_to_ms(t) {
  return (t - J1970) * 86400000;
}

function julian_to_unix(t) {
  return (t - J1970) * 86400;
}

function julian_to_date(t) {
  return new Date(julian_to_ms(t));
}

// Greenwich mean sidereal time, accurate to ~1 second.
// http://aa.usno.navy.mil/faq/docs/GAST.php
const GMST0 = 18.697374558 * Math.PI / 12;
const GMST1 = 24.06570982441908 * Math.PI / 12;

function greenwich_mean_sidereal_time(jt) {
  return GMST0 + GMST1 * jt;
}

function local_sidereal_time(jt, lon_r) {
  return greenwich_mean_sidereal_time(jt) + lon_r;
}

function hour_angle(jt, lon_r) {
  let angle = local_sidereal_time(jt, lon_r) - solar.right_ascension(jt);
  angle -= Math.floor(angle / (2 * Math.PI)) * (2 * Math.PI);
  if(angle > Math.PI) {
    angle -= 2 * Math.PI;
  }
  return angle;
}

exports.ms_to_julian = ms_to_julian;
exports.unix_to_julian = unix_to_julian;
exports.date_to_julian = date_to_julian;
exports.julian_to_ms = julian_to_ms;
exports.julian_to_unix = julian_to_unix;
exports.julian_to_date = julian_to_date;
exports.local_sidereal_time = local_sidereal_time;
exports.hour_angle = hour_angle;
exports.solar = solar;
exports.lunar = lunar;