The night of full light
A Full Moon occurs when the Moon stands directly opposite the Sun — 180° apart — with the Earth between them. The lunar face receives the Sun's light completely; the Moon rises as the Sun sets and travels the sky all night. That 180° angle is an opposition, the aspect of two things held at maximum distance, each made visible by the other. In the cycle this is the peak — whatever was seeded in the dark of the New Moon is now lit enough to be seen whole. Moon phases traces the full arc; this is its brightest point.
An opposition does not choose a winner. It lights both ends of an axis and asks you to hold them at once.
Sagittarius Moon, Gemini Sun
When the Full Moon falls in Sagittarius, the Sun sits opposite in Gemini — the zodiac's belief-and-evidence axis. Sagittarius is mutable fire, the seeker of the organizing truth: the principle, the meaning, the single idea everything is supposed to add up to. Gemini is mutable air, the collector of particulars: facts, links, exceptions, the next interesting detail that complicates the tidy story.
The rulers exchange registers in a telling way. The Sagittarius Moon answers to Jupiter, the planet of synthesis; the Gemini Sun answers to Mercury, the planet of the footnote (planetary rulership explains why a planet's host sign sets its terms). So this Full Moon lights the gap between what you have concluded and all the facts that test it. That is the anchor — true of no other opposition. The Gemini Sun has been collecting information; the Sagittarius Moon is asking what it all amounts to. The night they face each other, the tradition's sympathy runs both ways: a truth too large to fit any fact is a fantasy, and a heap of facts with no organizing truth is just weather.
What the light reveals
Full Moon light is accurate rather than flattering, and here it tests the conviction in both directions. Against the Gemini Sun's particulars, a too-tidy Sagittarian belief can be exposed as a story kept alive only by looking away from the contradicting facts. And against the Sagittarius Moon's demand for meaning, much of the Gemini detail shows itself to be noise — true, but not load-bearing.
So the culmination is a verdict on a belief that has been on trial. This Full Moon closes a longer arc too: it harvests what was begun at the Sagittarius New Moon six months earlier — the course of study started, the journey committed to, the conviction put to the test. The question that has been pursued reaches a provisional answer; the belief tested against experience arrives at something it can honestly hold, or honestly let go. The Sagittarius shadow to avoid is the sermon: broadcasting the conclusion before the trial is finished. Conviction arrived at honestly carries its own volume.
The invitation
The Sagittarius Full Moon invites you to say what you actually believe now that you have the evidence — and to check whether it is what you wanted to believe or what the facts support. Make the conclusion portable, simple enough to carry to someone outside the research, and let the waning fortnight be the season you walk it.