The full opposition
A Full Moon happens when the Moon swings to the opposite side of the sky from the Sun — a hundred and eighty degrees away, an angle called an opposition (two bodies facing each other across the chart). The whole lunar face is lit, and the geometry is visible to anyone: the Full Moon rises in the east just as the Sun sets in the west, crosses the sky all night, and sets at dawn. It is the only night of the month the Moon is up from dusk to dawn. Symbolically it is the cycle's peak — the moment what was begun in the dark becomes fully visible, and the turn toward release begins. The full arc lives in the lunation walkthrough.
Because a Full Moon is an opposition, the two lights always sit in opposite signs. That facing pair is what gives each Full Moon its particular charge — and it is the anchor here.
Self and other, lit from both ends
A Full Moon in Aries means the Moon is in Aries while the Sun is in Libra — early-to-mid autumn, near the September equinox. Aries is cardinal fire, the will to act, the I. Libra is cardinal air, the reach toward balance and the other person, the we. The Aries-Libra axis is the chart's relationship axis, and at this Full Moon both ends are lit at once. The impulse that has been deferred — the assertion not made, the boundary not drawn — arrives now as something that can no longer be held inside, while the Libra Sun keeps insisting that there is another person on the far side of the table.
The rulers frame the whole night. The Aries Moon answers to Mars, planet of war and the direct move; the Libra Sun answers to Venus, planet of love and harmony. Mars and Venus, jointly hosting the brightest Moon of the cycle — which is exactly why this lunation tends to bring the unsaid thing in a partnership to the surface. Falling near the equinox, this is in some years the Harvest Moon. And as at every Full Moon, if the opposition lands near the Moon's nodes it sharpens into a lunar eclipse — the same geometry raised to a higher power.
Inhabiting the polarity
The work of this Full Moon is not to pick a side. The Aries Moon is not wrong to want to act, and the Libra Sun is not wrong to want fairness; the tension between them is the productive material, not a problem to be solved. The failure mode at the Aries end is the blunt, self-only move that flattens the other person. The failure mode at the Libra end is the appeasement that erases yourself to keep the peace. Both are real, and a Full Moon makes both visible.
So say the true thing — Aries gives you the nerve to say it plainly — but say it to someone, in Libra's register: directness offered for the sake of the relationship rather than against it. What culminates here is the honest word that has been waiting for enough light to be spoken.