André Arko
February 24, 2019
410

# How to calculate the phase of the moon very, very badly!!

This talk is about how, despite “common knowledge”, lunar cycles are not 28 days long, and in fact lunar cycles don’t have a single numerical length! You’ll hear about several very bad ways to calculate the phases of the moon, why they seemed reasonable at the time, and exactly how it took three years from the time one app shipped to the App Store before the calendar was even arguably correct.

## André Arko

February 24, 2019

## Transcript

1. Everyone Knows a
Lunar Cycle is 28 Days
or, falsehoods werewolves believe about time

2. 27.321661 days
a sidereal month

3. ...two years later...

4. M T W Th F S Su
    
      
      
      

5. M T W Th F S Su
   
      
      
      
 

6. M T W Th F S Su
  
      
      
      
  

7. M T W Th F S Su
 
      
      
      
   

8. ⁉ ⁉ ⁉ ⁉

9. sidereal schmidereal

10. lunar phases are
synodic months

11. synodic months are
29.530587981 days

12. averages are lie-candy

13. thanks
BDAstroCalc!

14. 0 0.5 0.9

15.  
      
      
      
   

16.  
      
      
      
   

17.        
       
       
  
0.5235?
0.6852?
0.9855?

18.        
0.614 0.638 0.652
Feb 2 Feb 3 Feb 4
When is the half-moon?

19. midnight
02/23 midnight
02/24 midnight
0.46 0.49 0.52

20. Victory!!!

21. witchy.co
witchy.co/trig
andre.arko.net