What the sources claim for it
Authority and visibility — the position everything else is arranged around.
History
Where does the Sun come from?
The dotted circle is attested as a sign for the sun far earlier than the rest of the series, in Egyptian and Mesopotamian use, which makes the whole planetary set look less unified than it is. Its metal is gold, its day Sunday, and its operations concern authority, visibility, glory and healing.
The centre point is the part that carried into other systems: a circle with a point is also the standard alchemical mark for gold, and the same figure is read in later esoteric writing as an emblem of a centre with a circumference — which is a philosophical claim the astronomers using the sign were not making.
Afterwards
What did it turn into?
Still the astronomical symbol for the Sun and the historical alchemical symbol for gold.
Which texts describe it?
Oldest first, with what each one does with the mark. A mark described in a Bronze Age incantation and a mark first drawn in a seventeenth-century manuscript are different kinds of claim, whatever the shape has in common.
Renaissance & Early-Modern Occult Angelology
Agrippa, Barrett, and their compilers · 16th-19th c. · Latin and English
Given with the solar square, the metal and the hours.
Read Renaissance & Early-Modern Occult Angelology in full ↗Cover: Public domain · Otto Rietmann · Wikimedia file page
The Key of Solomon (Clavicula Salomonis)
Anonymous; ed. S. L. MacGregor Mathers · 15th-16th c. · Latin and Italian
Pentacles of the Sun for glory, and for compelling what is hidden into the light.
Read The Key of Solomon (Clavicula Salomonis) in full ↗Cover: Public domain · Unknown author · Wikimedia file page
Nearby
Related marks
Saturn
The scythe. Slowest planet, heaviest metal, and the authority a grimoire invokes for work involving binding, decay and the dead.
Jupiter
A stylised Greek zeta for Zeus. The sign of increase, favour and judgment in one's favour — the grimoires' preferred authority for asking.
Mars
Shield and spear, iron, Tuesday — and, since Linnaeus, the biological sign for male, which is a much later and quite separate job.
Venus
Copper, Friday, and every operation to do with love and favour — the counterpart to Mars in the grimoires long before it was in biology.
Mercury
A caduceus drawn as a glyph — the one sign in the set that names a metal, a planet and a god that all behave the same way.
The Moon
A crescent, silver, Monday — the planetary authority for anything involving dreams, water, change and things done at night.