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Moon Phase Calculator for the Pebble Watch (C Source)

Written by Michael Earls
 programming  development

A few years ago, I bought a Pebble smartwatch. Mostly I bought it because it was cheap, but I also wanted to write my own apps for it. I wrote a few apps.

Recently, Pebble was bought by FitBit and ceased production of the Pebble smartwatch. However, they are making great progress to ensuring that the watch continues to be viable into the future.

Anyway, as part of the transition, I wanted to release my source code for my moon phase app. What sets this moon phase calculator apart from my first implementation is the ability for it to run when not connected to the phone. I implemented it as a C app.

The moon phase calculation itself was originally written in JavaScript and I had to convert it to C. Sadly, I can't remember where I got the source, so I can't give attribution.

The code is available on my GitHub Page:

pebble-moon-phase on GitHub

Here is the actual calculation for anyone searching for a moon phase calculator in C:

#include "math.h"

#define        PI  3.1415926535897932384626433832795
#define        RAD (PI/180.0)
#define         SMALL_FLOAT    (1e-12)

static double Julian(int year,int month,double day)  
      Returns the number of julian days for the specified day.

    int a,b=0,c,e;
    if (month < 3) {
    month += 12;
    if (year > 1582 || (year  1582 && month>10) ||
    (year  1582 && month==10 && day > 15)) {
    c = 365.25*year;
    e = 30.6001*(month+1);
    return b+c+e+day+1720994.5;

static double sun_position(double j)  
    double n,x,e,l,dl,v;
    //double m2;
    int i;

    if (x<0) x += 360;
    x *= RAD;
    do {
    } while (fabs(dl)>=SMALL_FLOAT);
    return l;

static double moon_position(double j, double ls)  

    double ms,l,mm,n,ev,sms,ae,ec;//,z,x;//,lm,bm,ec;
    //double d;
    //double ds, as, dm;
    int i;

    /* ls = sun_position(j) */
    ms = 0.985647332099*j - 3.762863;
    if (ms < 0) ms += 360.0;
    l = 13.176396*j + 64.975464;
    i = l/360;
    l = l - i*360.0;
    if (l < 0) l += 360.0;
    mm = l-0.1114041*j-349.383063;
    i = mm/360;
    mm -= i*360.0;
    n = 151.950429 - 0.0529539*j;
    i = n/360;
    n -= i*360.0;
    ev = 1.2739*sin((2*(l-ls)-mm)*RAD);
    sms = sin(ms*RAD);
    ae = 0.1858*sms;
    mm += ev-ae- 0.37*sms;
    ec = 6.2886*sin(mm*RAD);
    l += ev+ec-ae+ 0.214*sin(2*mm*RAD);
    l= 0.6583*sin(2*(l-ls)*RAD)+l;
    return l;

static double moon_phase2(int year,int month,int day, double hour, int* ip)  
      Calculates more accurately than Moon_phase , the phase of the moon at
      the given epoch.
      returns the moon phase as a real number (0-1)

    double j= Julian(year,month,(double)day+hour/24.0)-2444238.5;
    double ls = sun_position(j);
    double lm = moon_position(j, ls);

    double t = lm - ls;
    if (t < 0) t += 360;
    *ip = (int)((t + 22.5)/45) & 0x7;
    return (1.0 - cos((lm - ls)*RAD))/2;