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[Alexandr Korol]

This is a constant value for any circle in plane geometry. Whatever circle you take —
small or huge — the ratio of the length of its boundary to the diameter will always be the
same number, pi. The circle is the most perfect figure. In Euclidean geometry, all points
are at an equal distance from the centre. Of the interesting facts: ancient peoples knew
pi approximately — the Egyptians, 3.16; Archimedes calculated it with high precision —
3.1408–3.1429. The Chinese and the Indians also came close. In the Middle Ages it was
used for astronomical calculations of the positions of the planets, of omens. It’s infinite
and non-periodic in its decimal notation, like an infinite circle. Also, in antiquity and
medieval thought, the circle and the sphere symbolized perfection, the divine order,
and eternity. The Primum Mobile is the most perfect outer sphere. The number pi is the
mathematical key to this perfection. Plato and the Pythagoreans saw a divine harmony
in geometry. And the number pi is not the circle itself, but the number that makes a circle
a circle in the Universe, in the whole Universe.

This stole my attention, and I decided to dig into it. And, of course, I began with
the key thing — what the Primum Mobile is. Then, further, I decided to search
on the internet for what the armillary sphere is and what the Sphaera Mundi is.
And here’s what I want to put the emphasis on now. The Primum Mobile is the
first moved. Here’s what I found on the internet:

This is the first moved, the prime mover, the first movable sphere. A key element of
medieval geocentric cosmology, based on the ideas of Aristotle and Ptolemy and
supplemented by Christian theology. The main idea: in the ancient and medieval
model of the Universe, the Earth is motionless and located at the centre. Around it
revolve transparent crystalline spheres, celestial orbs, on which the heavenly bodies are
fixed: the spheres of the Moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn, and the
sphere of the fixed stars — the stellatum, or the firmament. The Primum Mobile is the
outermost moving sphere, which does not bear visible luminaries upon itself. It revolves
faster than all the rest and transmits its movement to all the inner spheres. It’s precisely
thanks to it that the daily rotation of the heavens occurs, the sunrise and sunset of
the sun, the stars, and the planets from east to west every twenty-four hours. Ptolemy
introduced this sphere in order to explain the visible daily movement of the whole sky.
The concept reached Western Europe through the Arab philosophers, in particular through
Avicenna — Ibn Sina.