Antiscia are mirror points across the Cancer-Capricorn or Aries-Libra axis. William Lilly's Christian Astrology (1647) treats them as hidden contacts that act like a conjunction within roughly 1° of orb. The technique sits inside the older Hellenistic and Arabic traditions, formalized by Lilly and revived through the 1990s Project Hindsight translations. Astrologers use it when the visible aspect set leaves something unaccounted for.
How an antiscion is computed
Take a planet's longitude in degrees from 0° Aries (its absolute position, 0-360). For a solstitial antiscion, subtract that longitude from 180° if the planet is in the first half of the zodiac, or from 540° if in the second half, and reduce to the 0-360 range. The result is the mirror across the Cancer-Capricorn axis. Equinoctial antiscia mirror across the Aries-Libra axis: subtract from 360°.
A planet at 12° Leo (132°) has a solstitial antiscion at 18° Taurus (48°) and an equinoctial antiscion at 18° Aquarius (318°). Same planet, three positions, three possible sources of contact.
When the hidden contacts matter
The two cases where antiscia earn their place in a working practice. First, horary: Lilly checks antiscia routinely because the visible chart of the question often does not show the contact the answer requires. Second, natal: when a client describes a dynamic the visible aspects do not show, the antiscia scan often reveals a hidden Mars-Saturn or Venus-Mars contact that explains the texture.
A Sun antiscion conjunct natal Mars within 1° is read with the same weight as a visible conjunction. The signature is not less real for being hidden. It is less obvious to the practitioner who does not check.
Solstitial versus equinoctial: which to use
Tradition is split. Ptolemy and the medievals through Lilly used solstitial antiscia (Cancer-Capricorn axis). The equinoctial mirror (Aries-Libra axis) appears in the same Lilly source as a secondary tool. Modern Hellenistic revival via Schmidt and George leans toward solstitial only as the primary reading. Astrolium returns both by default so you can use either, or both, depending on tradition.
The contradictia is the antiscion's opposition point. Lilly treats it as a hidden opposition, marked when the contradictia falls on a natal planet within 1°. Astrolium does not compute contradictia separately but they are inferable: every antiscion has an implied opposition at the 180° point.
Reading antiscia aspects
The antiscia aspects table in the result shows the hidden contacts: which natal planet conjuncts which antiscion within the chosen orb. Read each row as a conjunction. Sun antiscion conjunct natal Saturn within 0.4° is a Sun-Saturn conjunction in disguise: the energy is hidden behind sign and house but the contact is exact.
For the visible aspect set this sits inside, see the natal chart feature. For the related horary technique that uses antiscia heavily, see the horary calculator. For the deeper tradition of hidden Hellenistic contacts, see the lot of Spirit calculator and the profections calculator.
The antiscia calculator is not a replacement for the natal scan. It is the second pass that catches what the first pass missed.