wd9get@pur-ee.UUCP (Brandt) (01/17/85)
I tried to mail this, but I got the infamous "Mail Returned" message.
This is a program to calculate the phase of the moon. It currently
polls the system to get the current time of day, but could easily
be modified to accept any date/time as input. The section numbers in
the comments refer to the sections of "Practical Astronomy with Your
Calculator" by Duffet-Smith. I plan to add further comments and perhaps
give the option of specifying any date, but since there was a request
on the net for this kind of information, I thought I'd pass it on.
The code is not as efficient as it could be since I used the formulas
'cookbook'-style, keeping all intermediate values the same as in PAWYC.
This will make it easier for people who look the formulae up in the
book to follow the code. I might also add that I'm a physiologist, not
a programmer by trade, so please forgive the code.
--Keith E. Brandt
pur-ee!wd9get
/****************************************************************************
pom.c
Phase of the Moon. Calculates the current phase of the moon.
Based on routines from `Practical Astronomy with Your Calculator',
by Duffett-Smith.
Comments give the section from the book that particular piece
of code was adapted from.
-- Keith E. Brandt VIII 1984
****************************************************************************/
#include <stdio.h>
#include <sys/time.h>
#include <math.h>
#define PI 3.141592654
#define EPOCH 1983
#define EPSILONg 279.103035 /* solar ecliptic long at EPOCH */
#define RHOg 282.648015 /* solar ecliptic long of perigee at EPOCH */
#define e 0.01671626 /* solar orbit eccentricity */
#define lzero 106.306091 /* lunar mean long at EPOCH */
#define Pzero 111.481526 /* lunar mean long of perigee at EPOCH */
#define Nzero 93.913033 /* lunar mean long of node at EPOCH */
main() {
double dtor();
double adj360();
double potm();
long *lo = (long *) calloc (1, sizeof(long)); /* used by time calls */
struct tm *pt; /* ptr to time structure */
double days; /* days since EPOCH */
double phase; /* percent of lunar surface illuminated */
double phase2; /* percent of lunar surface illuminated one day later */
int i = EPOCH;
time (lo); /* get system time */
pt = gmtime(lo); /* get ptr to gmt time struct */
cfree(lo);
/* calculate days since EPOCH */
days = (pt->tm_yday +1) + ((pt->tm_hour + (pt->tm_min / 60.0)
+ (pt->tm_sec / 3600.0)) / 24.0);
while (i < pt->tm_year + 1900)
days = days + 365 + ly(i++);
phase = potm(days);
printf("The Moon is ");
if ((int)(phase + .5) == 100) {
printf("Full\n");
}
else if ((int)(phase + 0.5) == 0)
printf("New\n");
else if ((int)(phase + 0.5) == 50) {
phase2 = potm(++days);
if (phase2 > phase)
printf("at the First Quarter\n");
else
printf("at the Last Quarter\n");
}
else if ((int)(phase + 0.5) > 50) {
phase2 = potm(++days);
if (phase2 > phase)
printf("Waxing ");
else
printf("Waning ");
printf("Gibbous (%1.0f%% of Full)\n", phase);
}
else if ((int)(phase + 0.5) < 50) {
phase2 = potm(++days);
if (phase2 > phase)
printf("Waxing ");
else
printf("Waning ");
printf("Crescent (%1.0f%% of Full)\n", phase);
}
}
double potm(days)
double days;
{
double N;
double Msol;
double Ec;
double LambdaSol;
double l;
double Mm;
double Ev;
double Ac;
double A3;
double Mmprime;
double A4;
double lprime;
double V;
double ldprime;
double D;
double Nm;
N = 360 * days / 365.2422; /* sec 42 #3 */
adj360(&N);
Msol = N + EPSILONg - RHOg; /* sec 42 #4 */
adj360(&Msol);
Ec = 360 / PI * e * sin(dtor(Msol)); /* sec 42 #5 */
LambdaSol = N + Ec + EPSILONg; /* sec 42 #6 */
adj360(&LambdaSol);
l = 13.1763966 * days + lzero; /* sec 61 #4 */
adj360(&l);
Mm = l - (0.1114041 * days) - Pzero; /* sec 61 #5 */
adj360(&Mm);
Nm = Nzero - (0.0529539 * days); /* sec 61 #6 */
adj360(&Nm);
Ev = 1.2739 * sin(dtor(2*(l - LambdaSol) - Mm)); /* sec 61 #7 */
Ac = 0.1858 * sin(dtor(Msol)); /* sec 61 #8 */
A3 = 0.37 * sin(dtor(Msol));
Mmprime = Mm + Ev - Ac - A3; /* sec 61 #9 */
Ec = 6.2886 * sin(dtor(Mmprime)); /* sec 61 #10 */
A4 = 0.214 * sin(dtor(2 * Mmprime)); /* sec 61 #11 */
lprime = l + Ev + Ec - Ac + A4; /* sec 61 #12 */
V = 0.6583 * sin(dtor(2 * (lprime - LambdaSol))); /* sec 61 #13 */
ldprime = lprime + V; /* sec 61 #14 */
D = ldprime - LambdaSol; /* sec 63 #2 */
return (50 * (1 - cos(dtor(D)))); /* sec 63 #3 */
}
ly(yr)
int yr;
{
/* returns 1 if leapyear, 0 otherwise */
return (yr % 4 == 0 && yr % 100 != 0 || yr % 400 == 0);
}
double dtor(deg)
double deg;
{
/* convert degrees to radians */
return (deg * PI / 180);
}
double adj360(deg)
double *deg;
{
/* adjust value so 0 <= deg <= 360 */
do if (*deg < 0)
*deg += 360;
else if (*deg > 360)
*deg -= 360;
while (*deg < 0 || *deg > 360);
}