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:

{

/*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;

// adjust for leap years

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