Calculate position of the Sun and Moon from the Earth frame of reference (equatorial coordinates)

sun_moon_position.js

// https://stackoverflow.com/a/11760121/11303
function julian_days() {
  const date = new Date();
  //const jd = Math.floor((date / 86400000) - (date.getTimezoneOffset() / 1440) + 2440587.5);
  const jd = (date / 86400000) - (date.getTimezoneOffset() / 1440) + 2440587.5;
  console.log("jd=" + jd);
  return jd;
}

// https://en.wikipedia.org/wiki/Position_of_the_Sun
function sun_position() {
  const JD = julian_days();

  // Days since J2000
	const n = JD - 2451545.0;

  // Mean longitude of the Sun
  const L = 280.460 + 0.9856474 * n;
  
  // Mean anomaly
  const g = 357.528 + 0.9856003 * n;
  
  const L_mod_360 = L % 360;
  const g_mod_360 = g % 360;

  const ecliptic_longitude = L + 1.915 * Math.sin(g) + 0.020 * Math.sin(2 * g);
  const ecliptic_latitude = 0.0;
  const distance_from_earth = 1.00014 - 0.01671 * Math.cos(g) - 0.00014 * Math.cos(2 * g);

  const obliquity_of_ecliptic = 23.439 - 0.0000004 * n;
  const right_ascension = Math.atan2(Math.cos(obliquity_of_ecliptic) * Math.sin(ecliptic_longitude), Math.cos(ecliptic_longitude));
  const declination = Math.asin(Math.sin(obliquity_of_ecliptic) * Math.sin(ecliptic_longitude));
  
  console.log("ra=" + right_ascension + "  dec=" + declination);
}