how 1 degree is equal to 60 mins?
They defined it that way.
A more detailed explanation on the mysteries of degrees and minutes is related in the wikipedia article:
Where they mention a circle having 360 degrees and the year having 360 days in some ancient calendar systems. Also 360 is a number with a lot of factors making it a useful choice for measurement systems. The 60 minutes to a degree may be the result of the base 60 number system in use by the Babylonians.
An interesting maritime feature of the minute is that it was used to define the nautical mile until they realized the variation of length if measured in the low latitudes vs the high latitudes.
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