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In mathematics, a degree is 1/360 part of a circle. Imagine a round pie. Cut it through the middle in two parts. Now cut the halves in two equal parts. Each of the four parts has a right angle where the middle of the pie was. If each of these parts was in 90 equal slices they will all have an angle of one degree. 180 degrees would be half of the pie. 270 degrees would be three-quarters of the pie.
Why ninety and not one hundred?
Ninety is easier to make calculations with mentally. Ninety can be split into equal pieces in many different ways.
- 2×45, 3×30, 5×18, 6×15, 10×9, 9×10, 15×6, 18×5, 30×3, 45×2 - 10 ways
One hundred can only be split in less ways:
- 2×50, 4×25, 5×20, 10×10, 20×5, 25×4, 50×2 - only 7 ways.
In times when people made calculations in their head a lot, they liked the numbers 360, 90, and 12 better than 10 and 100 for this reason. That is also why there are 24 hours in a day (twice twelve), and why a troy pound is 12 ounces.