Calculate Lunar Phase

moonphase.py

#!/usr/bin/env python
"""
moonphase.py - Calculate Lunar Phase
Author: Sean B. Palmer, inamidst.com
Cf. http://en.wikipedia.org/wiki/Lunar_phase#Lunar_phase_calculation
"""

import math, decimal, datetime
dec = decimal.Decimal

def position(now=None): 
   if now is None: 
      now = datetime.datetime.now()

   diff = now - datetime.datetime(2001, 1, 1)
   days = dec(diff.days) + (dec(diff.seconds) / dec(86400))
   lunations = dec("0.20439731") + (days * dec("0.03386319269"))

   return lunations % dec(1)

def phase(pos): 
   index = (pos * dec(8)) + dec("0.5")
   index = math.floor(index)
   return {
      0: "New Moon", 
      1: "Waxing Crescent", 
      2: "First Quarter", 
      3: "Waxing Gibbous", 
      4: "Full Moon", 
      5: "Waning Gibbous", 
      6: "Last Quarter", 
      7: "Waning Crescent"
   }[int(index) & 7]

def main(): 
   pos = position()
   phasename = phase(pos)

   roundedpos = round(float(pos), 3)
   print "%s (%s)" % (phasename, roundedpos)

if __name__=="__main__": 
   main()

Hi, original author here. The equation that I used was originally posted to Wikipedia by an author named "Monsieur le docteur Ralph" in March 2005. The magic values that I used were from that equation. Later, on the talk page for that article where they discuss removing the equation, they also discussed some potentially more accurate alternatives.

I posted the above code to my own website as moonphase.py, which has a Last-Modified date of 30 November 2005. I've been hosting it for nearly two decades on that site! I'm glad that people are finding the script useful. I and my friends use it in an IRC bot called saxo which I wrote about a decade ago to replace another one which I'd written a decade previously, roughly contemporaneous with moonphase.py.

If anybody comes up with an improved version of this script, with greater accuracy or ported to another language such a Go or Rust, please post the code or link to the code on this thread, and please send me a message. You can contact me on GitHub at @sbp, leave a message on my Wikipedia user talk page, or use whatever email address du jour I've got listed on my homepage.

@sbp howdy! I did leave credit in the script and I honestly don't remember where I came across it or why I originally shared it. I posted this gist 9 years ago! Thanks for pointing folks to the original source and offering support.