A small C program to add the current moon phase to your prompt

moon.c

#include <stdio.h>
#include <string.h>
#include <time.h>

/**
 * Output the current moon phase as an emoji for your prompt.
 *
 * Build:
 *
 *   gcc -o moon moon.c
 *
 * Place in your path, call from your shell. 
 * Requires UTF-8 terminal and UTF-8 locale.
 *
 * Examples:
 *
 *   bash:
 *     export PS1='$(moon)\!\$\[\e[0;37m\]'
 *
 *   zsh:
 *     setopt PROMPT_SUBST
 *     export PROMPT='$(moon)%#'
 *
 *   fish:
 *     function fish_prompt
 *       echo -n (moon) '> '
 *     end
 *     
 * Disclaimer:
 *
 * Moon phase calculation function found online and may be 
 * inaccurate. Not intended for astrogation, or for treatment 
 * or management of lycanthropy.
 *
 * See http://www.voidware.com/moon_phase.htm
 *
 * Additional code by [email protected]
 */

int moon_phase(int y, int m, int d)
{
  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;
}

char *moon_emoji(int p) {
  switch (p) {
    case 0:
      return "\xf0\x9f\x8c\x91";
    case 1:
      return "\xf0\x9f\x8c\x92";
    case 2:
      return "\xf0\x9f\x8c\x93";
    case 3:
      return "\xf0\x9f\x8c\x95";
    case 4:
      return "\xf0\x9f\x8c\x96";
    case 5:
      return "\xf0\x9f\x8c\x94";
    case 6:
      return "\xf0\x9f\x8c\x97";
    case 7:
      return "\xf0\x9f\x8c\x98";
  }
}

int main() {
  time_t t = time(NULL);
  struct tm tm = *localtime(&t);
  int p = moon_phase(tm.tm_year + 1900, tm.tm_mon + 1, tm.tm_mday);
  char *moon = moon_emoji(p);
  printf(moon);
}

moon.png