astronomia/es/refraction.js

/**
 * @copyright 2013 Sonia Keys
 * @copyright 2016 commenthol
 * @license MIT
 * @module refraction
 */
/**
 * Refraction: Chapter 16: Atmospheric Refraction.
 *
 * Functions here assume atmospheric pressure of 1010 mb, temperature of
 * 10°C, and yellow light.
 */
import sexa from './sexagesimal';
var sin = Math.sin,
    tan = Math.tan;

var D2R = Math.PI / 180;

var gt15true1 = new sexa.Angle(false, 0, 0, 58.294).rad();
var gt15true2 = new sexa.Angle(false, 0, 0, 0.0668).rad();
var gt15app1 = new sexa.Angle(false, 0, 0, 58.276).rad();
var gt15app2 = new sexa.Angle(false, 0, 0, 0.0824).rad();

/**
 * gt15True returns refraction for obtaining true altitude when altitude
 * is greater than 15 degrees (about 0.26 radians.)
 *
 * h0 must be a measured apparent altitude of a celestial body in radians.
 *
 * Result is refraction to be subtracted from h0 to obtain the true altitude
 * of the body.  Unit is radians.
 */
export function gt15True(h0) {
  // (h0 float64)  float64
  // (16.1) p. 105
  var t = tan(Math.PI / 2 - h0);
  return gt15true1 * t - gt15true2 * t * t * t;
}

/**
 * gt15Apparent returns refraction for obtaining apparent altitude when
 * altitude is greater than 15 degrees (about 0.26 radians.)
 *
 * h must be a computed true "airless" altitude of a celestial body in radians.
 *
 * Result is refraction to be added to h to obtain the apparent altitude
 * of the body.  Unit is radians.
 */
export function gt15Apparent(h) {
  // (h float64)  float64
  // (16.2) p. 105
  var t = tan(Math.PI / 2 - h);
  return gt15app1 * t - gt15app2 * t * t * t;
}

/**
 * Bennett returns refraction for obtaining true altitude.
 *
 * h0 must be a measured apparent altitude of a celestial body in radians.
 *
 * Results are accurate to 0.07 arc min from horizon to zenith.
 *
 * Result is refraction to be subtracted from h0 to obtain the true altitude
 * of the body.  Unit is radians.
 */
export function bennett(h0) {
  // (h0 float64)  float64
  // (16.3) p. 106
  var c1 = D2R / 60;
  var c731 = 7.31 * D2R * D2R;
  var c44 = 4.4 * D2R;
  return c1 / tan(h0 + c731 / (h0 + c44));
}

/**
 * Bennett2 returns refraction for obtaining true altitude.
 *
 * Similar to Bennett, but a correction is applied to give a more accurate
 * result.
 *
 * Results are accurate to 0.015 arc min.  Result unit is radians.
 */
export function bennett2(h0) {
  // (h0 float64)  float64
  var cMin = 60 / D2R;
  var c06 = 0.06 / cMin;
  var c147 = 14.7 * cMin * D2R;
  var c13 = 13 * D2R;
  var R = bennett(h0);
  return R - c06 * sin(c147 * R + c13);
}

/**
 * Saemundsson returns refraction for obtaining apparent altitude.
 *
 * h must be a computed true "airless" altitude of a celestial body in radians.
 *
 * Result is refraction to be added to h to obtain the apparent altitude
 * of the body.
 *
 * Results are consistent with Bennett to within 4 arc sec.
 * Result unit is radians.
 */
export function saemundsson(h) {
  // (h float64)  float64
  // (16.4) p. 106
  var c102 = 1.02 * D2R / 60;
  var c103 = 10.3 * D2R * D2R;
  var c511 = 5.11 * D2R;
  return c102 / tan(h + c103 / (h + c511));
}

export default {
  gt15True: gt15True,
  gt15Apparent: gt15Apparent,
  bennett: bennett,
  bennett2: bennett2,
  saemundsson: saemundsson
};