Added demo program galactic.py.

This is the Python version of a demo to convert
galactic coordinates to horizontal coordinates for
a given time and geographic location.

demo/c/README.md

@@ -15,7 +15,7 @@ This could be useful for backyard radio astronomers who know the galactic
 coordinates of a distant radio source and want to aim a radio dish at it.
 Given the galactic coordinates, the geographic coordinates of the observer,
 and the date and time of the observation, this program shows how to
-convert the altitude and azimuth to aim at the radio source.
+obtain the altitude and azimuth to aim the dish at the radio source.
 
 ### [Horizon Intersection](horizon.c)
 This is a more advanced example. It shows how to use coordinate

demo/c/galactic.c

@@ -16,19 +16,19 @@
 
 
 int GalaticToHorizontal(
-    astro_time_t time, 
-    astro_observer_t observer, 
-    double glat, 
-    double glon, 
-    double *altitude, 
+    astro_time_t time,
+    astro_observer_t observer,
+    double glat,
+    double glon,
+    double *altitude,
     double *azimuth)
 {
     astro_rotation_t    rot, adjust_rot;
     astro_spherical_t   gsphere, hsphere;
     astro_vector_t      gvec, hvec;
 
-    /* 
-        Calculate a rotation matrix that converts 
+    /*
+        Calculate a rotation matrix that converts
         galactic coordinates to J2000 equatorial coordinates.
     */
     rot = Astronomy_Rotation_GAL_EQJ();
@@ -36,7 +36,7 @@ int GalaticToHorizontal(
     /*
         Adjust the rotation matrix to convert galatic to horizontal (HOR).
     */
-    adjust_rot = Astronomy_Rotation_EQJ_HOR(time, observer);    
+    adjust_rot = Astronomy_Rotation_EQJ_HOR(time, observer);
     rot = Astronomy_CombineRotation(rot, adjust_rot);
 
     /*
@@ -89,7 +89,7 @@ int main(int argc, const char *argv[])
 
     if (argc < 5 || argc > 6)
     {
-        fprintf(stderr, 
+        fprintf(stderr,
             "\n"
             "USAGE: galactic olat olon glat glon [yyyy-mm-ddThh:mm:ssZ]\n"
             "\n"

demo/python/README.md

@@ -25,6 +25,14 @@ Culmination is also the moment a body crosses the *meridian*, the imaginary semi
 in the sky that passes from due north on the horizon, through the zenith (straight up),
 and then toward due south on the horizon.
 
+### [Galactic to Horizontal Converter](galactic.py)
+A demonstration of how to convert galactic coordinates to horizontal coordinates.
+This could be useful for backyard radio astronomers who know the galactic
+coordinates of a distant radio source and want to aim a radio dish at it.
+Given the galactic coordinates, the geographic coordinates of the observer,
+and the date and time of the observation, this program shows how to
+obtain the altitude and azimuth to aim the dish at the radio source.
+
 ### [Horizon Intersection](horizon.py)
 This is a more advanced example. It shows how to use coordinate
 transforms to find where the ecliptic intersects with an observer's

demo/python/galactic.py

@@ -0,0 +1,76 @@
+#!/usr/bin/env python3
+#
+#   galactic.py  -  by Don Cross - 2021-06-14
+#
+#   Example Python program for Astronomy Engine:
+#   https://github.com/cosinekitty/astronomy
+#
+#   Demo of converting galactic coordinates to horizontal coordinates.
+#
+
+import sys
+import math
+from astronomy import *
+
+UsageText = r'''
+USAGE: galactic olat olon glat glon [yyyy-mm-ddThh:mm:ssZ]
+
+where
+
+    olat = observer's latitude on the Earth
+    olon = observer's longitude on the Earth
+    glat = IAU 1958 galatic latitude of the target
+    glon = IAU 1958 galatic longitude of the target
+    yyyy-mm-ddThh:mm:ssZ = optional UTC date/time
+
+Given the galactic coordinates of a point source in the sky,
+this program calculates horizontal aiming coordinates for an
+observer on or near the Earth's surface.
+
+If the date/time is given on the command line, it is used.
+Otherwise, the computer's current date/time is used.
+'''
+
+
+def GalacticToHorizontal(time, observer, glat, glon):
+    # Calculate a matrix that converts galactic coordinates
+    # to J2000 equatorial coordinates.
+    rot = Rotation_GAL_EQJ()
+
+    # Adjust the rotation matrix to convert galactic to horizontal.
+    adjust_rot = Rotation_EQJ_HOR(time, observer)
+    rot = CombineRotation(rot, adjust_rot)
+
+    # Convert the galactic coordinates from angles to a unit vector.
+    gsphere = Spherical(glat, glon, 1.0)
+    gvec = VectorFromSphere(gsphere, time)
+
+    # Use the rotation matrix to convert the galactic vector to a horizontal vector.
+    hvec = RotateVector(rot, gvec)
+
+    # Convert the horizontal vector back to angular coordinates.
+    # Assume this is a radio source (not optical), do not correct for refraction.
+    hsphere = HorizonFromVector(hvec, Refraction.Airless)
+    return hsphere.lat, hsphere.lon
+
+
+if __name__ == '__main__':
+    if len(sys.argv) not in [5, 6]:
+        print(UsageText)
+        sys.exit(1)
+
+    olat = float(sys.argv[1])
+    olon = float(sys.argv[2])
+    observer = Observer(olat, olon)
+
+    glat = float(sys.argv[3])
+    glon = float(sys.argv[4])
+
+    if len(sys.argv) > 5:
+        time = Time.Parse(sys.argv[5])
+    else:
+        time = Time.Now()
+
+    altitude, azimuth = GalacticToHorizontal(time, observer, glat, glon)
+    print('altitude = {:10.3f}, azimuth = {:10.3f}'.format(altitude, azimuth))
+    sys.exit(0)

demo/python/test/.gitignore

@@ -9,3 +9,4 @@ horizon.txt
 lunar_eclipse.txt
 lunar_angles.txt
 jupiter_moons.txt
+galactic.txt

demo/python/test/galactic_correct.txt

@@ -0,0 +1 @@
+altitude =     74.647, azimuth =    178.955

demo/python/test/test

@@ -51,5 +51,9 @@ echo "Testing example: lunar_angles.py"
 ./lunar_angles.py 2021-05-15 > test/lunar_angles.txt || Fail "Error running lunar_angles.py."
 diff test/lunar_angles.txt test/lunar_angles_correct.txt || Fail "Error comparing lunar_angles.py output."
 
+echo "Testing example: galactic.py"
+./galactic.py 38.92056 -77.0658 22.793498 197.070510 2025-04-06T00:00:00Z > test/galactic.txt || Fail "Error running galactic.py."
+diff test/galactic.txt test/galactic_correct.txt || Fail "Error comparing galactic.py output."
+
 echo "PASS: Python examples"
 exit 0