astronomia/es/eqtime.js

/**
 * @copyright 2013 Sonia Keys
 * @copyright 2016 commenthol
 * @license MIT
 * @module eqtime
 */
/**
 * Eqtime: Chapter 28, Equation of time.
 */

import base from './base';
import coord from './coord';
import nutation from './nutation';
import solar from './solar';
var cos = Math.cos,
    sin = Math.sin,
    tan = Math.tan;

/**
 * e computes the "equation of time" for the given JDE.
 *
 * Parameter planet must be a planetposition.Planet object for Earth obtained
 * with `new planetposition.Planet('earth')`.
 *
 * @param {Number} jde - Julian ephemeris day
 * @param {planetposition.Planet} earth - VSOP87 planet
 * @returns {Number} equation of time as an hour angle in radians.
 */

export function e(jde, earth) {
  var τ = base.J2000Century(jde) * 0.1;
  var L0 = l0(τ);
  // code duplicated from solar.ApparentEquatorialVSOP87 so that
  // we can keep Δψ and cε

  var _solar$trueVSOP = solar.trueVSOP87(earth, jde),
      lon = _solar$trueVSOP.lon,
      lat = _solar$trueVSOP.lat,
      range = _solar$trueVSOP.range;

  var _nutation$nutation = nutation.nutation(jde),
      Δψ = _nutation$nutation[0],
      Δε = _nutation$nutation[1];

  var a = -20.4898 / 3600 * Math.PI / 180 / range;
  var λ = lon + Δψ + a;
  var ε = nutation.meanObliquity(jde) + Δε;
  var eq = new coord.Ecliptic(λ, lat).toEquatorial(ε);
  // (28.1) p. 183
  var E = L0 - 0.0057183 * Math.PI / 180 - eq.ra + Δψ * cos(ε);
  return base.pmod(E + Math.PI, 2 * Math.PI) - Math.PI;
}

/**
 * (28.2) p. 183
 */
var l0 = function l0(τ) {
  return base.horner(τ, 280.4664567, 360007.6982779, 0.03032028, 1.0 / 49931, -1.0 / 15300, -1.0 / 2000000) * Math.PI / 180;
};

/**
 * eSmart computes the "equation of time" for the given JDE.
 *
 * Result is less accurate that e() but the function has the advantage
 * of not requiring the V87Planet object.
 *
 * @param {Number} jde - Julian ephemeris day
 * @returns {Number} equation of time as an hour angle in radians.
 */
export function eSmart(jde) {
  var ε = nutation.meanObliquity(jde);
  var t = tan(ε * 0.5);
  var y = t * t;
  var T = base.J2000Century(jde);
  var L0 = l0(T * 0.1);
  var e = solar.eccentricity(T);
  var M = solar.meanAnomaly(T);

  var _base$sincos = base.sincos(2 * L0),
      sin2L0 = _base$sincos[0],
      cos2L0 = _base$sincos[1];

  var sinM = sin(M);
  // (28.3) p. 185
  return y * sin2L0 - 2 * e * sinM + 4 * e * y * sinM * cos2L0 - y * y * sin2L0 * cos2L0 - 1.25 * e * e * sin(2 * M);
}

export default {
  e: e,
  eSmart: eSmart
};