Perhaps the most prominent arc of the natural world appears in the elusive form of the rainbow, which primitive man saw as a bridge between the Heavens and the Earth.
As part of a circle, the arc symbolizes potential spirit. The position of the arc is important. Upright, shaped like a cup or chalice, it implies the feminine principle, something that can contain the spirit. If the arc is inverted, then the opposite is true and it becomes a triumphal, victorious, masculine symbol. As such, the arc can take the form of an archway. The vaulted or arched shape of many holy buildings, from a great variety of different faiths, represents the vault of the Heavens.
From: Encyclopedia of Secret Signs and Symbols
… the power of the world always works in circles …
Effectively an expansion of the dot, the circle represents the spirit and the cosmos. Further, the circle itself is constructed from “some thing” (the unbroken line) and “no thing” (the space inside and outside this line). Therefore the circle unifies spirit and matter. The structure itself has great strength – think of the cylindrical shape of a lighthouse, built that way in order to withstand the fiercest attack by a stormy sea.
The physical and spiritual strength of this symbol are there because the perfect circle has no beginning and no end; it is unassailable. This power is why the circle is used in magickal practices such as spell-casting. The magick circle creates a fortress of psychic protection, a physical and spiritual safe haven where unwanted or uninvited entities cannot enter.
Hermes Trismegistus said of the circle:
God is a circle whose center is everywhere
and circumference is nowhere.
Where would ancient man have seen the most important circles? Obviously, in the Sun and the Moon. As the Sun, the circle is masculine, but when it is the Moon, it is feminine. Because the passage of time is marked by the journey of the Sun, Moon and stars in orbit around our Earth, the circle is a symbol of the passage of time. In this form, it commonly appears as the wheel.
Because the circle has no divisions and no sides, it is also a symbol of equality. King Arthur’s Round Table was the perfect piece of furniture for the fellowship of Knights who were each as important as each other. Similarly, the Dalai Lama has a “circular” Council.
Source: Encyclopedia of Secret Signs and Symbols