Opposition — 180°, orb 8°

There are six undirected pairs of signs at 180°: Aries–Libra, Taurus–Scorpio, Gemini–Sagittarius, Cancer–Capricorn, Leo–Aquarius, and Virgo–Pisces. They are listed below. A planet pair whose gap is 172° to 188° also matches. A gap of 150°, sometimes called a quincunx, does not: 180 − 150 = 30, and 30 is greater than 8.

The Descendant of a chart is the point opposite the Ascendant, which is an opposition of horizon angles, not automatically the Libra point. Libra is simply the sign whose window contains ecliptic longitude 180°. Keep those meanings apart when you read a sign page.

Each sign page leads its geometry table with its single opposition partner. That one name changes every page. The angle, 180°, and the orb, 8°, do not.

Worked numbers: 15° and 195° are the Aries and Libra centers, exactly 180°. 15° and 187° are 172° apart, inside the 8° orb. 15° and 165° are 150° apart, the Virgo center versus Aries, and 180 minus 150 is 30, far outside 8°. That 150° gap is why the geometry tables say no major aspect instead of inventing a quincunx. The Descendant opposes the Ascendant on the horizon. It is not automatically longitude 180°, and it is not automatically Libra.