Added nodejs demo for calculating equatorial coordinates from horizontal coordinates.

Author: cosinekitty

demo/nodejs/equatorial.js

@@ -0,0 +1,114 @@
+/*
+    equatorial.js  -  by Don Cross - 2021-03-27
+
+    Example Node.js program for Astronomy Engine:
+    https://github.com/cosinekitty/astronomy
+
+    Given an observer's location on the Earth, a
+    date/time, and horizontal coordinates (azimuth, altitude)
+    for that observer, this program works backwards
+    to figure out the equatorial coordinates for that
+    location in the sky. It provides two solutions:
+    one that includes atmospheric refraction, another
+    that ignores atmospheric refraction.
+
+    To execute, run the command:
+    node equatorial latitude longitude azimuth altitude [date]
+*/
+
+const Astronomy = require('./astronomy.js');
+
+function ParseNumber(text, name) {
+    const x = Number(text);
+    if (!Number.isFinite(x)) {
+        console.error(`ERROR: Not a valid numeric value for ${name}: "${text}"`);
+        process.exit(1);
+    }
+    return x;
+}
+
+function ParseDate(text) {
+    const d = new Date(text);
+    if (!Number.isFinite(d.getTime())) {
+        console.error(`ERROR: Not a valid date: "${text}"`);
+        process.exit(1);
+    }
+    return d;
+}
+
+
+function Format(x, length, digits) {
+    let s = x.toFixed(digits);
+    while (s.length < length)
+        s = ' ' + s;
+    return s;
+}
+
+
+function Solve(refract, observer, time, azimuth, altitude) {
+    // Convert the angular horizontal coordinates (azimuth, altitude)
+    // to a horizontal vector (north, west, zenith).
+    const hor_sphere = new Astronomy.Spherical(altitude, azimuth, 1);
+    const refraction_option = refract ? 'normal' : null;
+    const hor_vec = Astronomy.VectorFromHorizon(hor_sphere, time, refraction_option);
+
+    // Make a rotation matrix for this observer and date/time that converts
+    // horizontal coordinates (HOR) to equatorial coordinates in the J2000 epoch (EQJ).
+    const rot_hor_eqj = Astronomy.Rotation_HOR_EQJ(time, observer);
+
+    // Use the rotation matrix to convert the horizontal vector to an equatorial vector.
+    const eqj_vec = Astronomy.RotateVector(rot_hor_eqj, hor_vec);
+
+    // Convert the equatorial vector to equatorial angular coordinates (RA, DEC).
+    const eqj = Astronomy.EquatorFromVector(eqj_vec);
+
+    // Self-check the answers by converting back to horizontal coordinates,
+    // using a different algorithm that has been tested to work.
+
+    // First we need to convert J2000 equatorial (EQJ) to equator-of-date (EQD),
+    // because the Horizon function expects EQD.
+    const rot_eqj_eqd = Astronomy.Rotation_EQJ_EQD(time);
+    const eqd_vec = Astronomy.RotateVector(rot_eqj_eqd, eqj_vec);
+    const eqd = Astronomy.EquatorFromVector(eqd_vec);
+
+    const check_hor = Astronomy.Horizon(time, observer, eqd.ra, eqd.dec, refraction_option);
+    const alt_error = Math.abs(check_hor.altitude - altitude);
+    const az_error = Math.abs(check_hor.azimuth - azimuth);
+
+    let line = (refract ? '   yes' : '   no ');
+    line += '    ' + Format(eqj.ra, 10, 4);
+    line += ' ' + Format(eqj.dec, 10, 4);
+    line += ' ' + Format(eqd.ra, 10, 4);
+    line += ' ' + Format(eqd.dec, 10, 4);
+    line += ' ' + Format(alt_error, 10, 6);
+    line += ' ' + Format(az_error, 10, 6);
+    console.log(line);
+}
+
+
+function Demo() {
+    if (process.argv.length === 6 || process.argv.length === 7) {
+        const latitude  = ParseNumber(process.argv[2]);
+        const longitude = ParseNumber(process.argv[3]);
+        const observer = new Astronomy.Observer(latitude, longitude, 0);
+        const azimuth = ParseNumber(process.argv[4]);
+        const altitude = ParseNumber(process.argv[5]);
+        const time = Astronomy.MakeTime((process.argv.length === 7) ? ParseDate(process.argv[6]) : new Date());
+
+        // Print a common header for both solutions.
+        console.log('Refract?    J2000_RA  J2000_DEC  OFDATE_RA OFDATE_DEC  ALT_error   AZ_error');
+
+        // Solve once ignoring atmospheric refraction.
+        Solve(false, observer, time, azimuth, altitude);
+
+        // Solve again considering atmospheric refraction.
+        Solve(true, observer, time, azimuth, altitude);
+
+        process.exit(0);
+    } else {
+        console.log('USAGE: node equatorial latitude longitude azimuth altitude [date]');
+        process.exit(1);
+    }
+}
+
+Demo();

demo/nodejs/test/.gitignore

@@ -1,3 +1,4 @@
+equatorial.txt
 camera.txt
 moonphase.txt
 positions.txt

demo/nodejs/test/equatorial_correct.txt

@@ -0,0 +1,3 @@
+Refract?    J2000_RA  J2000_DEC  OFDATE_RA OFDATE_DEC  ALT_error   AZ_error
+   no        22.8101   -19.9671    22.8288   -19.8565   0.000000   0.000000
+   yes       22.8087   -20.0007    22.8275   -19.8901   0.000000   0.000000

demo/nodejs/test/test

@@ -5,7 +5,11 @@ Fail()
     exit 1
 }
 
-rm -f test/{camera,moonphase,positions,riseset,seasons,culminate,horizon,lunar_eclipse}.txt
+rm -f test/{equatorial,camera,moonphase,positions,riseset,seasons,culminate,horizon,lunar_eclipse}.txt
+
+echo "Testing example: equatorial.js"
+node equatorial.js 38.1256 -89.5544 215.7 23.5 2021-03-27T18:45:00Z > test/equatorial.txt || Fail "Error testing equatorial.js."
+diff test/equatorial.txt test/equatorial_correct.txt || Fail "Error comparing equatorial.js output."
 
 echo "Testing example: camera.js"
 node camera.js 29 -81 2021-03-22T02:45:00Z > test/camera.txt || Fail "Error testing camera.js."