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The phrase "Moon phase" is ambiguous, because sometimes it means relative ecliptic longitude, other times it means illuminated fraction. The "moonphase" demos were only calculating the relative ecliptic longitude, which was confusing. Now they calculate both.
105 lines
2.5 KiB
C
105 lines
2.5 KiB
C
/*
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moonphase.c - by Don Cross - 2019-05-25
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Example C program for Astronomy Engine:
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https://github.com/cosinekitty/astronomy
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This program calculates the Moon's phase for a given date and time,
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or the computer's current date and time if none is given.
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It also finds the dates and times of the subsequent 10 quarter phase changes.
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*/
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#include <stdio.h>
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#include "astro_demo_common.h"
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static const char *QuarterName(int quarter)
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{
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switch (quarter)
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{
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case 0: return "New Moon";
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case 1: return "First Quarter";
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case 2: return "Full Moon";
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case 3: return "Third Quarter";
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default: return "INVALID QUARTER";
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}
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}
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int main(int argc, const char *argv[])
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{
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astro_time_t time;
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astro_angle_result_t phase;
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astro_moon_quarter_t mq;
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astro_illum_t illum;
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int i;
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switch (argc)
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{
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case 1:
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time = Astronomy_CurrentTime();
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break;
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case 2:
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if (ParseTime(argv[1], &time))
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return 1;
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break;
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default:
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fprintf(stderr, "USAGE: moonphase [date]\n");
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return 1;
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}
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/*
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Calculate the Moon's ecliptic phase angle,
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which ranges from 0 to 360 degrees.
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0 = new moon,
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90 = first quarter,
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180 = full moon,
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270 = third quarter.
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*/
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phase = Astronomy_MoonPhase(time);
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if (phase.status != ASTRO_SUCCESS)
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{
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printf("Astronomy_MoonPhase error %d\n", phase.status);
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return 1;
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}
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PrintTime(time);
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printf(" : Moon's ecliptic phase angle = %0.3lf degrees.\n", phase.angle);
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/*
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Calculate the percentage of the Moon's disc that is illuminated
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from the Earth's point of view.
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*/
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illum = Astronomy_Illumination(BODY_MOON, time);
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if (illum.status != ASTRO_SUCCESS)
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{
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printf("Astronomy_Illumination error %d\n", illum.status);
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return 1;
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}
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PrintTime(time);
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printf(" : Moon's illuminated fraction = %0.2lf%%.", 100.0 * illum.phase_fraction);
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/* Find the next 10 lunar quarter phases. */
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printf("\nThe next 10 lunar quarters are:\n");
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for (i=0; i < 10; ++i)
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{
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if (i == 0)
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mq = Astronomy_SearchMoonQuarter(time);
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else
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mq = Astronomy_NextMoonQuarter(mq);
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if (mq.status != ASTRO_SUCCESS)
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{
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printf("Error %d trying to find moon quarter.\n", mq.status);
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return 1;
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}
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PrintTime(mq.time);
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printf(" : %s\n", QuarterName(mq.quarter));
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}
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return 0;
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}
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