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[aprssig] Re: metrics

Steve Dimse steve at dimse.com
Sat Sep 8 12:31:16 UTC 2007

On Sep 8, 2007, at 3:58 AM, Mark Fellhauer wrote:

> A circle has 360 degrees because a year is 13 Lunar periods of 28  
> days.

  The moonth (not a misspelling, that is the origin of month) is  
29.53 days when viewed from the earth, and 27.322 days when viewed  
from a fixed point in space. The difference is the earth moves along  
its orbit, so the moon has to rotate further for us to see the same  
phase. Note that NEITHER is 28 days. Using the actual lunar rotation  
time of 27.322, 13 periods is just over 355 days. Twelve earth- 
apparent moon rotations gets you to a nearby value a bit over 354  
days. The moon has rotated thirteen times, the earth going once  
around its orbit makes the moon do the extra turn not visible from  
our vantage point. Humans, in their usual self-centered manner,  
choose to define a moonth on their perception, rather than the moon's  

The truth behind 360 is, again, more fascinating to me than the myth.  
The Babylonians began a number of long-lived non-decimal systems,  
which included the number of minutes in an hour, hours in a day, and  
degrees in a circle. The reason was they never developed fractional  
arithmetic. Instead, to make their life easier they used "magic  
numbers", numbers which were evenly divisible by as many factors as  

24 is evenly divisible by 1, 2, 3, 4, 6, 8, 12, and 24.

60 is evenly divisible by 1, 2, 3, 4, 5, 6, 10, 12, 20, 30, and 60.

360 is left as an exercise for the reader, suffice it to say it is 60  
* 6, so it has all the factors of 60 and then some.

Some scholars believe the Babylonians chose 360 as the magic number  
to use because it was close to the number of days in a year, which is  
independent of the lunar cycle. Others believe it was chosen because  
it was closest to their ability to sub-divide a circle. I've never  
read a source that equated it to the lunar cycle, but we do not know  
for sure, and if you want to say they rounded up from TWELVE lunar  
cycles I have no basis to argue. But it is indisputably the  
Babylonians and not the Greeks that broke the circle into 360  
degrees, and the reason for that number was their inability to do any  
fractional math. And if you want to equate it to the lunar cycle, it  
has to be twelve, because they did not know the earth orbited the sun.

Sorry for prolonging the off-topic thread, but I really hate it when  
people perpetuate erroneous information. Repeat after me, bad data is  
worse than no data at all!

Steve K4HG

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