ephemeris/src/astronomy/moshier/epsilon.js

JavaScript implementation of Moshier's ephemeris calculations for sun, planets, comets, asteroids and stars.

var constant = require('./constant')

var epsilon = {
  /* The results of the program are returned in these
   * global variables:
   */
  /** Date for which obliquity was last computed */
  jdeps: -1.0,
  /** The computed obliquity in radians */
  eps: 0.0,
  /** Cosine of the obliquity */
  coseps: 0.0,
  /** Sine of the obliquity */
  sineps: 0.0
}

epsilon.calc = function (date) {
  if (date.julian == this.jdeps) {
    return
  }
  var T = (date.julian - 2451545) / 36525

  /* DE403 values. */
  T /= 10
  this.eps = (((((((((2.45e-10 * T + 5.79e-9) * T + 2.787e-7) * T
    + 7.12e-7) * T - 3.905e-5) * T - 2.4967e-3) * T
    - 5.138e-3) * T + 1.9989) * T - 0.0175) * T - 468.33960) * T
    + 84381.406173
  this.eps *= constant.STR

  this.coseps = Math.cos(this.eps)
  this.sineps = Math.sin(this.eps)
  this.jdeps = date.julian
}

module.exports = epsilon