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