Crescent and star lantern on an Arduino Uno

// Crescent and star lantern: a crescent moon of warm LEDs and a twinkling
// star, for a window or a table at Eid, lit by itself at dusk.
//
//   light sensor  module AO -> A0, VCC -> 5 V, GND -> GND
//   crescent      16 WS2812B LEDs in a ring, DIN <- 330 ohms <- pin 6
//   star          pin 5 -> 220 ohms -> white LED -> GND
//
// Eid al-Fitr comes at the end of Ramadan, and Eid al-Adha about ten weeks
// later. Both follow the Islamic lunar calendar, which is about eleven days
// shorter than the solar year, so they move earlier each year and the start
// of each month waits on the new crescent moon. That is why the crescent and
// star are on so many Eid lanterns and cards, and why this sketch makes no
// claim about dates: it is a lantern for every year.
//
// The crescent is part of a ring: ten LEDs lit in a curve, brightest in the
// middle and fading toward the horns, with the other six dark. The star is
// one LED on a PWM pin that twinkles, a quick bright flash now and then over
// a soft glow.
//
// The light module's reading rises as the room gets darker, so the lantern
// lights when it goes above DARK_ABOVE and goes out in daylight.
//
// Things to try. Drag the light sensor's slider down. Make the moon fuller by
// lighting more of the ring with ARC.

#include <Adafruit_NeoPixel.h>

const int STAR_PIN = 5;
const int LEDS = 16;
const int ARC = 10;            // LEDs in the crescent
const int DARK_ABOVE = 700;    // A0 reading: on above this
const int LIGHT_BELOW = 600;   // and off below this

Adafruit_NeoPixel moon(LEDS, 6, NEO_GRB + NEO_KHZ800);
bool lit = false;
int star = 0;

void drawMoon(int glow) {
  moon.clear();
  for (int i = 0; i < ARC; i++) {
    // Brightest in the middle of the arc, dimmer toward the horns.
    int fromMiddle = abs(2 * i - (ARC - 1));
    int v = glow - fromMiddle * glow / (2 * ARC);
    moon.setPixelColor(i, v, v * 3 / 4, v / 4);
  }
  moon.show();
}

void setup() {
  Serial.begin(9600);
  pinMode(STAR_PIN, OUTPUT);
  moon.begin();
  moon.setBrightness(170);
  moon.show();
  Serial.println("Eid Mubarak! The lantern lights at dusk");
}

void loop() {
  int light = analogRead(A0);
  if (!lit && light > DARK_ABOVE) {
    lit = true;
    // Fade the crescent in.
    for (int g = 0; g <= 255; g += 5) {
      drawMoon(g);
      delay(20);
    }
    Serial.println("Dusk: the crescent and star are lit");
  } else if (lit && light < LIGHT_BELOW) {
    lit = false;
    moon.clear();
    moon.show();
    analogWrite(STAR_PIN, 0);
    Serial.println("Daylight: lantern off");
  }
  if (!lit) return;

  // The star: a soft glow with a bright twinkle now and then.
  if (random(0, 60) == 0) star = 255;
  analogWrite(STAR_PIN, star < 60 ? 60 : star);
  if (star > 60) star -= 12;
  delay(20);
}