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.

Transcript

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

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9. 27.321661 days
   a sidereal month
   

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10. ...two years later...
    

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

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

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

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

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15. ⁉ ⁉ ⁉ ⁉
    

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16. sidereal schmidereal
    

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17. lunar phases are
    synodic months
    

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18. synodic months are
    29.530587981 days
    

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19. averages are lie-candy
    for your brain
    

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20. thanks
    BDAstroCalc!
    

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21. 
    0 0.5 0.9
    

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22.  
          
          
          
       
    

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23.  
          
          
          
       
    

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24.        
           
           
      
    0.5235?
    0.6852?
    0.9855?
    

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25.        
    0.614 0.638 0.652
    Feb 2 Feb 3 Feb 4
    When is the half-moon?
    

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26. midnight
    02/23 midnight
    02/24 midnight
    0.46 0.49 0.52
    
    
    

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27. Victory!!!
    
    

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28. witchy.co
    witchy.co/trig
    andre.arko.net
    twitter.com/sailorhg
    twitter.com/indirect
    

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