> There are other highly divisible numbers. 60 was probably chosen because of its approximate relationship to the length of the solar year.
I had heard & read the reason was more one of counting convenience, more directly related to divisibility. What sources do you have suggesting that the length of the year is the "probable" reason?
Poking around, I see several suggestions that it might be related to the number of months, which has nothing to do with the number of days. I also found this opinion, which seems to contradict the idea that the number system is related to days in the year:
"Several theories have been based on astronomical events. The suggestion that 60 is the product of the number of months in the year (moons per year) with the number of planets (Mercury, Venus, Mars, Jupiter, Saturn) again seems far fetched as a reason for base 60. That the year was thought to have 360 days was suggested as a reason for the number base of 60 by the historian of mathematics Moritz Cantor. Again the idea is not that convincing since the Sumerians certainly knew that the year was longer than 360 days. Another hypothesis concerns the fact that the sun moves through its diameter 720 times during a day and, with 12 Sumerian hours in a day, one can come up with 60. [...] I [EFR] feel that all of these reasons are really not worth considering seriously."
That completely ignores the fact that ancient communities used the lunar calendar with periodic corrections every few years. 360 is equal to 12 lunar months, to the precision required of the ancients who practiced the insertion of entire leap months to compensate for drift (because the phase of the moon served as a more reliable clock than anything else that was available).
