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<h1 class="page-title">Source: deltat.js</h1>
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<pre class="prettyprint source linenums"><code>/** Computation of an estimate of Delta_T, for astronomical computations with calendars.
Delta_T is defined in this way: UTC = TT - Delta_T, where UTC (formerly GMT) is the master time of all our clocks.
TT is Terrestrial Time, a uniform time scale defined independently from any Earth movements.
Delta_T is erratic and difficult to compute, however, the general trend of Delta_T is due to the braking of the Earth's daily revolution.
This estimate of Deltta_T in seconds from the year expressed in Common Era is a quadratic function of the year.
The computation of the long-term Delta_T is after Morrison and Stephenson 2021.
In this version, the estimated Delta_T is smoothed over time. The result is rounded to the nearest second.
* @module
* @version M2021-03-11
* @author Louis A. de Fouquières https://github.com/Louis-Aime
* @license MIT 2016-2022
*/
// Character set is UTF-8
/* Version M2022-01-17 Fix JSdoc
M2022-01-16 fix comments.
M2021-08-02 Recover comments describing Delta T that had been lost when splitting.
M2021-07-29 extracted and isolated from Lunar.js
M2021-03-11 Update formula for average Delta D after Morrison and Stephenson 2021
*/
/* Copyright Louis A. de Fouquières https://github.com/Louis-Aime 2016-2022
Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:
1. The above copyright notice and this permission notice shall be included
in all copies or substantial portions of the Software.
2. Changes with respect to any former version shall be documented.
The software is provided "as is", without warranty of any kind,
express of implied, including but not limited to the warranties of
merchantability, fitness for a particular purpose and noninfringement.
In no event shall the authors of copyright holders be liable for any
claim, damages or other liability, whether in an action of contract,
tort or otherwise, arising from, out of or in connection with the software
or the use or other dealings in the software.
*/
"use strict";
/** Compute an estimate of Delta T, defined as: UTC = TT - Delta T. The estimate is only the quadratic part of Delta T.
* @static
* @function getDeltaT
* @param {Date} theDate - date where Delta T is estimated (estimation is per exact date, not per year).
* @return {number} Delta T, unit is milliseconds, result reflects an integer signed number of seconds.
*/
export default function getDeltaT (theDate) {
const JULIAN_CENTURY_UNIT = 36525 * 86400000,
SECOND_UNIT = 1000, // used with legacy Date object
ORIGIN = new Date (1825, 0, 1); // 1 January 1825
let century = (theDate.getTime() - ORIGIN.getTime()) / JULIAN_CENTURY_UNIT;
return Math.round(century*century*31.4 - 10) * SECOND_UNIT; // Precision is not less than one second
}
</code></pre>
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