source: buchla-68k/orig/DATE/MOON.C@ d21fc6f

/*
   =============================================================================
        moon.c -- calculate the phase of the moon
        Version 3 -- 1988-12-05 -- D.N. Lynx Crowe

        Modified version of the public domain code "mphase.c" from Usenet.
        Original author unknown.

        Modified to run under GEMDOS.
   =============================================================================
*/

#define MAIN            1

#include "stddefs.h"

/*
   =============================================================================
        mphase() -- return the phase of the moon

        returns 0..7  where:  0 = new, 4 = full
   =============================================================================
*/

int
mphase(year, diy)
register int year, diy;
{
        /* moon period: 29.5306 days */
        /* year: 365.2422 days */

        register int epact, golden;

        golden = (year % 19) + 1;

        epact = (11 * golden + 18) % 30;

        if (((epact EQ 25) AND (golden > 11)) OR (epact EQ 24))
                epact++;

        return( (((((diy + epact) * 6) + 11) % 177) / 22) & 7);
}

/* 

*/

#if     MAIN

#include "stdio.h"

char    *mph[] = {

        "new",
        "waxing, in its first quarter",
        "in its first quarter",
        "waxing full",
        "full",
        "waning full",
        "in its last quarter",
        "waning, in its last quarter" };

main()
{
        register int year;              /* year */
        register int diy;               /* day in year */

        if (argc NE 3) {

                printf("usage:  moon year day-in-year\n");
                exit(1);
        }

        year = atoi(argv[1]);
        diy  = atoi(argv[2]);

        printf("The moon is %s.\n", mph[mphase(year, diy)]);
}

#endif