In brief: For tropical longitude L, the antiscion is (180° − L) modulo 360°; the contra-antiscion is 180° opposite that mirror point.
The mirror construction
Antiscia is a traditional degree symmetry. Place a point at a tropical longitude and reflect it across the solstitial line joining 0° Cancer and 0° Capricorn. A planet at 25° Gemini, or 85° absolute longitude, reflects to 95°: 5° Cancer. The contra-antiscion is the opposition to that reflected degree, here 5° Capricorn. These are mathematical locations on the zodiac circle, not additional physical objects.
Why the solstice axis
The reflection pairs degrees that lie equally far from a solstice point. This is a geometric analogy to the Sun’s changing seasonal path, although the same printed longitudes do not imply identical local daylight at two birthplaces. The construction belongs to the tropical zodiac because its reference line uses equinox and solstice points. A sidereal chart would need its convention clearly stated rather than silently borrowing these tropical degrees.
Reading a contact
Some traditional readers inspect whether another planet or angle lies near the antiscion degree. A close contact can be described as a symbolic hidden connection in that tradition, but the term “hidden” does not establish a measurable influence. Give the original position, calculated mirror degree, the other body’s degree, and an explicit orb. Consider normal aspects and house context as well; a mirrored longitude by itself is a very narrow claim.
Do not confuse with declination
A contra-antiscion is not a contra-parallel. Antiscia reflects ecliptic longitude around a solstitial axis. Declination parallels compare north or south distances from the celestial equator; those coordinates vary with latitude and the reference frame. A pair of bodies may satisfy one relationship without satisfying the other. The site’s declination tool answers that separate question and provides its own numerical method.