Circle, semicircle, cross
Symbols are powerful tools. Like parables, interpretation of them depends on the experience of the observer. Symbols can mean different things to different people depending on their level of development. For instance a circle could be just a line connected up so there is no beginning and no end. It could be an empty circle or an area full of potential. To non-astrologers it is the symbol of the full Moon seen in secular diaries. To astrologers it will immediately mean the Sun but the dot in the centre is missing. The same symbol says different things to different people. So it is with all symbols.
The three basic components that make up the symbols of the planets are shown in Figure 1. The circle represents spirit. It has no beginning and no ending. It is the spark of life that is in everyone. It is that immeasurable factor that is in all life, both animate and inanimate. The semicircle represents the soul, which is that non-material part of a person that is the sum of all experiences gained during life in a physical body. The cross represents matter, the physical form of the three-dimensional world. It is the physical body and outer life lived on earth.
The three components thus represent the three key factors of human beings: spirit, soul and the physical body. Using these interpretations of the components, and their interactions within the symbols of the planets, it is possible to gain a deeper understanding of the influence of each planet in life.
The Sun, 
, represents the source of all life. It is spirit and spirit is perfect. Human beings are first and foremost spirit but only a very small part of spirit incarnates into each person. Spirit can never be destroyed. As spirit is in everything, it is the oneness that unites all that is. This is a profound esoteric concept. Like spirit, the circle has no beginning and no end. The dot within the circle in the symbol of the Sun may be considered to be that part of the whole that is manifest in a physical body.
The Moon, 
, is two conjoined crescents. It represents the soul and psyche and consequently is associated with the personality that is expressed in this incarnation. This is the only symbol, other than the Sun, that is made up of a single component. The symbol of the Moon focuses attention on the non-spirit and non-physical part of life.
Source – extract from The Full Circle: Spirit in Matter, by Patricia Godden