The Circle of God by Brian Hobley will set you back £105 but it appears to be a pretty comprehensive study which may attract SIS members to indulge themselves. Is it worth trying the public library system, I wonder?
There are 45 separate chapters divided into ten parts as well as a conclusion and index. Part One, the circle as a symbol (with three chapters, the nature of symbolism, symbolism in the Greco-Roman world, imaging the gods. Part Two, the Sun as a universal symbol has 5 chapters – solar symbols in the Middle East. in megalithic Europe, the sun and the Celts, sun symbols in Mycenae and Crete, etc. Part Three, Neolithic and Bronze Age circular structures (henges and round barrows). Parts 4 and 5, Circularity at Rome, and Part 6, Roman architecture, and Part 7, Roman celestial iconography (which includes the circular zodiac). Part 8, cosmic master builders (which includes Augustus Caeser and Hadrian's buildings in the light of circularity in the heavens). Part 9, circular symbolism in Roman everyday life – including death and the afterlife, solar discs and solar shields etc. Part 10, the solar cycle and the Christian year, circular symbolism in Christianity, the Cross as a symbol etc (and even discusses sacred yew trees, Irish round towers and the Albigensians).
The assumption, we may note, is that the circle of God = the Sun, or that is what is inferred. However, it may be that a variety of cosmic phenomena may have contributed to 'sun symbols' – from comets to electro magnetic phenomena. The author appears to be primarily a Greco-Roman specialist, or at least focusses on that period. At this time the Sun indeed appears to have become a dominant symbol of God. The sun was visible, we might say, whereas the old 'sun symbol' had become defunct – or was transient in nature. In spite of that the information and hypothesis involved appears to be worth exploring, if only to reinterpret some of it (under the spotlight of catastrophism).
What appears to be the thrust of the book (interpreted at face value and without reading the book) is that the idea of the Sun as a symbol of God is projected backwards into the past – to the Celts, megalith builders, Mycenaeans etc. The identity of pre Greco-Roman sun symbols is a matter open to question.