[aprssig] Re: metrics
Steve Dimse steve at dimse.comSat Sep 8 12:31:16 UTC 2007
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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 perspective. 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 possible. 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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