Right, I think I really am living a dream. I have the following piece of code which I compile and run on an AIX machine: AIX 3 5 PowerPC_POWER5 processor type IBM XL C/C++ for AIX, V10.1 Version: 10.01.0000.0003 #include <stdio.h> #include <math.h> #define RADIAN(x) ((x) * acos(0.0) / 90.0) double nearest_distance(double radius,double lon1, double lat1, double lon2, double lat2){ double rlat1=RADIAN(lat1); double rlat2=RADIAN(lat2); double rlon1=lon1; double rlon2=lon2; double a=0,b=0,c=0; a = sin(rlat1)*sin(rlat2)+ cos(rlat1)*cos(rlat2)*cos(rlon2-rlon1); printf("%lf\n",a); if (a > 1) { printf("aaaaaaaaaaaaaaaa\n"); } b = acos(a); c = radius * b; return radius*(acos(sin(rlat1)*sin(rlat2)+ cos(rlat1)*cos(rlat2)*cos(rlon2-rlon1))); } int main(int argc, char** argv) { nearest_distance(6367.47,10,64,10,64); return 0; } Now, the value of 'a' after the calculation is reported as being '1'. And, on this AIX machine, it looks like 1 1 is true as my 'if' is entered !!! And my acos of what I think is '1' returns NanQ since 1 is bigger than 1. May I ask how that is even possible ? I do not know what to think anymore ! The code works just fine on other architectures where 'a' really takes the value of what I think is 1 and acos(a) is 0.