Calculating the Moon Phase In Your Head
for the years 2000 through 2009

step 1: this area intentionally blank Total the four digits in the year. 
step 2:   Multiply by 11. 
step 3:   Subtract the largest possible multiple of 30 (e.g. 0, 30, 60, 90, or 120). 
step 4:   Add the number for the month. 
step 5:   If the month is January or February, add 1. 
step 6:   Subtract the largest possible multiple of 30. 
step 7:   Add the number for the day of the month. 
step 8:   Subtract the largest possible multiple of 30.

You now have the age of the moon in days for midday (UT) of your chosen date, thus your result may be interpreted as follows.

= new moon 
~7  = first quarter 
~15  = full moon 
~22  = last quarter 
~30  = new moon

This algorithm is accurate within 2 days for any date 2000-2009.  For super accurate results, use the source formula from the specifications on page 2 of the Moonstick Instructions.

Example 1:  2005 September 4

step 1: this area intentionally blank 2+0+0+5=7 
step 2:   711=77 
step 3:   77–60=17 
step 4:   17+9=26 
step 5:   do nothing 
step 6:   26–0=26 
step 7:   26+4=30 
step 8:   30–30=0 (new moon 2 days)

Example 2:  2008 February 29

step 1: this area intentionally blank 2+0+0+8=10 
step 2:   1011=110 
step 3:   110–90=20 
step 4:   20+2=22 
step 5:   22+1=23 
step 6:   23–0=23 
step 7:   23+29=52 
step 8:   52–30=22 (22-day-old moon 2 days (~ last quarter))

March 20, 2000 –  I have checked this algorithm for accuracy on all of its valid dates.  No accuracy violations were found.  –author

This page is part of the Moonstick Information Site.
copyright 2000 Sean Barton, all rights reserved

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