From 86fde0d5cdec8570f991180edff5f0ee00c747e2 Mon Sep 17 00:00:00 2001
From: Don Cross <cosinekitty@gmail.com>
Date: Sun, 12 Feb 2023 12:03:43 -0500
Subject: [PATCH] C demo: Calculate true solar time from the Sun's hour angle.

---
 demo/c/README.md              |  3 +++
 demo/c/build                  |  2 +-
 demo/c/correct/solar_time.txt |  1 +
 demo/c/demotest               |  1 +
 demo/c/solar_time.c           | 49 +++++++++++++++++++++++++++++++++++
 demo/c/test/.gitignore        |  1 +
 6 files changed, 56 insertions(+), 1 deletion(-)
 create mode 100644 demo/c/correct/solar_time.txt
 create mode 100644 demo/c/solar_time.c

diff --git a/demo/c/README.md b/demo/c/README.md
index fa9747d5..e258b81c 100644
--- a/demo/c/README.md
+++ b/demo/c/README.md
@@ -39,6 +39,9 @@ Shows how to calculate sunrise, sunset, moonrise, and moonset times.
 ### [Seasons](seasons.c)
 Calculates the equinoxes and solstices for a given calendar year.
 
+### [Solar Time](solar_time.c)
+An example of how to use the Sun's hour angle to calculate true solar time.
+
 ### [Triangulate](triangulate.c)
 Given the geographic coordinates of two observers, and angular
 directions they are looking in, determines geographic coordinates
diff --git a/demo/c/build b/demo/c/build
index c44f6358..03405589 100755
--- a/demo/c/build
+++ b/demo/c/build
@@ -14,7 +14,7 @@ else
 fi
 
 mkdir -p bin
-for name in gravity galactic camera moonphase positions linux_riseset riseset seasons culminate horizon lunar_eclipse triangulate ecliptic_vector; do
+for name in solar_time gravity galactic camera moonphase positions linux_riseset riseset seasons culminate horizon lunar_eclipse triangulate ecliptic_vector; do
     rm -f bin/${name}
     echo "Compiling ${name}.c"
     gcc ${BUILDOPT} -Wall -Werror -o bin/${name} -I../../source/c ../../source/c/astronomy.c astro_demo_common.c ${name}.c -lm ||
diff --git a/demo/c/correct/solar_time.txt b/demo/c/correct/solar_time.txt
new file mode 100644
index 00000000..f0e2d576
--- /dev/null
+++ b/demo/c/correct/solar_time.txt
@@ -0,0 +1 @@
+True solar time = 11.6286 hours (11:37:42.979)
diff --git a/demo/c/demotest b/demo/c/demotest
index 68395427..9cf6a8c1 100755
--- a/demo/c/demotest
+++ b/demo/c/demotest
@@ -16,6 +16,7 @@ rm -f test/*.txt
 mkdir -p test
 [[ "$1" == "nobuild" ]] || ./build || Fail "Error building example programs."
 
+TestDemo solar_time +38.88 -77.03 2023-02-12T17:00:00Z
 TestDemo camera 29 -81 2021-03-22T02:45:00Z
 TestDemo moonphase 2019-06-15T09:15:32.987Z
 TestDemo positions +45.6 -90.7 2018-11-30T17:55:07.234Z
diff --git a/demo/c/solar_time.c b/demo/c/solar_time.c
new file mode 100644
index 00000000..d178d11e
--- /dev/null
+++ b/demo/c/solar_time.c
@@ -0,0 +1,49 @@
+/*
+    solar_time.c  -  by Don Cross - 2019-06-11
+
+    Example C program for Astronomy Engine:
+    https://github.com/cosinekitty/astronomy
+
+    Given an observer's geographic latitude and longitude,
+    and an optional date and time, this program displays
+    true solar time for that observer and time.
+*/
+
+#include <stdio.h>
+#include <math.h>
+#include "astro_demo_common.h"
+
+int main(int argc, const char *argv[])
+{
+    int error;
+    astro_observer_t observer;
+    astro_time_t time;
+    astro_func_result_t ha;
+    double solarTimeHours;
+    int hour, minute, second, milli;
+
+    error = ParseArgs(argc, argv, &observer, &time);
+    if (error)
+        return error;
+
+    ha = Astronomy_HourAngle(BODY_SUN, &time, observer);
+    if (ha.status != ASTRO_SUCCESS)
+    {
+        printf("ERROR %d in Astronomy_HourAngle().\n", ha.status);
+        return 1;
+    }
+
+    solarTimeHours = fmod(ha.value + 12.0, 24.0);
+
+    milli = (int)round(solarTimeHours * 3600000.0);
+    second = milli / 1000;
+    milli %= 1000;
+    minute = second / 60;
+    second %= 60;
+    hour = minute / 60;
+    minute %= 60;
+    hour %= 24;
+
+    printf("True solar time = %0.4lf hours (%02d:%02d:%02d.%03d)\n", solarTimeHours, hour, minute, second, milli);
+    return 0;
+}
diff --git a/demo/c/test/.gitignore b/demo/c/test/.gitignore
index c231da87..e6e1e6ad 100644
--- a/demo/c/test/.gitignore
+++ b/demo/c/test/.gitignore
@@ -12,3 +12,4 @@ galactic.txt
 triangulate.txt
 gravity.txt
 worldmap.txt
+solar_time.txt