Why are key signatures put in a circle?

Question

Why are key signatures arranged in a circle like it is today, why not a triangle... or a square? Who came up with the idea to arrange the key signatures in a circle?

Answer 1

Key signatures are arranged in a circle. This circle is known as the circle of fifths, meaning as you proceed around the circle in a clockwise motion, each consecutive key is five notes apart. If you move around the circle counterclockwise, the interval between keys is a fourth.

Nikolai Diletski created the circle during the 1670s, and it was later revised to the version we use today by Johann David Heinichen in 1728.