source: buchla-68k/orig/DATE/MPHASE.C

/*
   =============================================================================
        mphase.c -- calculate the phase of the moon
        Version 1 -- 1988-01-14 -- D.N. Lynx Crowe

        Modified version of the public domain code from Usenet.
        Original author unknown.
   =============================================================================
*/

#define MAIN            1

#include "stddefs.h"
#include "time.h"

struct  tm      *lcltime();

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

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

int
mphase(lt)
register struct tm *lt;
{
        /* moon period: 29.5306 days */
        /* year: 365.2422 days */

        register int epact, diy, golden;

        diy = lt->tm_yday;

        golden = (lt->tm_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" };

long    lcldate;
struct  tm      lcltm;

struct tm *
getlt()
{
        time(&lcldate);
        return(lcltime(&lcldate, &lcltm));
}

main()
{
        printf("The moon is %s.\n", mph[mphase(getlt())]);
}

#endif