# moon phase

here is my easy way to calculate moon phase

```int moon_phase(int y, int m, int d)
{
/*
calculates the moon phase (0-7), accurate to 1 segment.
0 = > new moon.
4 => full moon.
*/

int c,e;
double jd;
int b;

if (m < 3) {
y--;
m += 12;
}
++m;
c = 365.25*y;
e = 30.6*m;
jd = c+e+d-694039.09;  /* jd is total days elapsed */
jd /= 29.53;           /* divide by the moon cycle (29.53 days) */
b = jd;		   /* int(jd) -> b, take integer part of jd */
jd -= b;		   /* subtract integer part to leave fractional part of original jd */
b = jd*8 + 0.5;	   /* scale fraction from 0-8 and round by adding 0.5 */
b = b & 7;		   /* 0 and 8 are the same so turn 8 into 0 */
return b;
}```
i developed this algorithm using floating point rather than integer for use in pocket calculators and only just managed to fit inside the fx-201p. an older, but integer formula is the following:

int Moon_phase(int year,int month,int day)
{
/*k
Calculates the moon phase (0-7), accurate to 1 segment.
0 = > new moon.
4 => Full moon.
*/

int g, e;

if (month == 1) --day;
else if (month == 2) day += 30;
else // m >= 3
{
day += 28 + (month-2)*3059/100;

if (!(year & 3)) ++day;
if ((year%100) == 0) --day;
}

g = (year-1900)%19 + 1;
e = (11*g + 18) % 30;
if ((e == 25 && g > 11) || e == 24) e++;
return ((((e + day)*6+11)%177)/22 & 7);
}

both formulae are simplified to work from 1900 to 2199 inclusive. however, ive discovered that they disagree. consider september 23, 2002. the second formula claims this a full moon, the first does not. the moon is not full on this night so the first seems more accurate.

here is a command line pc program that is more accurate than the above for reference.

and here's another program to calculate the instant of full moon

fullmoon.exe