How to calculate the phase of the Moon with only your brain to help you
And maybe some paper and a pen
Or even a calculator

Sometimes, you just want to know what the phase of the Moon is, or will be. Perhaps you need to know in advance for some important reason. And maybe you can't see the Moon because it's cloudy, or you're locked in a cellar. Well, never fear. Just remember these simple instructions and you can be a phase-of-the-moon guru.

  1. Count the number of days since January 1, 2001
  2. Multiply this by 850
  3. Add 5130
  4. Divide by 25101 (This bit may be difficult to do in your head)
  5. That's it!

So, you've got a weird decimal number. "What does it mean?" I hear you cry. Well, it's the number of lunations since a new moon in December 2000, so the fraction after the point tells you how full the moon is. Dicard the number before the point and look at the other bit, if it's:

  • Equal to 0: The moon is new.
  • less than 0.5: The moon is waxing
  • equal to 0.5: The moon is full
  • more than 0.5: The moon is waning

So, just remember this:

---------  (n is the number of days since New Year's Day 2001)

How about an Example?

Really? Oh, OK. Today (at the time of noding) is May 26, 2005...

  1. That's four years from the start of 2001 to the start of this year.
  2. That's 365*4=1460 days +1 for the leap year last year --- 1461
  3. 1461 * 850 is 1241850
  4. 1241850 + 5130 is 1246980.
  5. 1246980 / 25101 is 49.6785ish

So we look at the bit after the point and we see that the moon is waning. Which it is. Well, actually it's too cloudy to see at the moment, but... --- /me rummages for his postcard with the moon phases on --- yep! It was full three days ago.