Solstice axis

Meaning

The solstice axis is the line across the zodiac from 0° Cancer to 0° Capricorn. The two ends are the solstice points, where the Sun turns back in its yearly swing north and south of the celestial equator.

The astronomy. The Sun's path, the ecliptic, is tilted to the celestial equator by about 23.4° (the obliquity of the ecliptic, which changes very slowly). Through the year the Sun's declination, its distance north or south of the equator, rises and falls. At 0° Cancer, around 21 June, it reaches its greatest northern declination, about +23.4°: the longest day in the northern hemisphere. At 0° Capricorn, around 21 December, it reaches its greatest southern declination, about −23.4°: the shortest northern day. Near these points the Sun's declination hardly changes from day to day, which is the origin of the word solstice, the Sun "standing still". The Tropics of Cancer and Capricorn on Earth are named after them.

In the tropical zodiac 0° Cancer and 0° Capricorn are defined by the solstices, so the axis stays tied to the seasons whatever the constellations behind it.

Why it is a mirror. Declination rises and falls symmetrically about each solstice. Degrees the same distance either side of 0° Cancer, such as 20° Gemini and 10° Cancer, have the same declination; so do degrees either side of 0° Capricorn. When the Sun is on either degree of such a pair, the day is the same length and the Sun rises at the same point of the horizon. Reflecting a degree across this axis gives its antiscion, 180° − λ.

Origin and history

Ptolemy opens his account of the signs with the two he calls tropika, "having to do with turning", translated by Robbins as "solstitial": Cancer and Capricorn, so named because at the start of each the Sun turns back, bringing summer in Cancer and winter in Capricorn [1]. Robbins notes that astronomers now usually say "solstitial" rather than "tropical" for them [1].

Ptolemy then describes parts of the zodiac equally removed from the same tropical sign as being of equal power: the Sun in either gives equal days, equal nights and equal hours, and they rise and set at the same part of the horizon, so they are also said to behold each other [1]. (See signs of equal power.) In Robbins's reading the pairs are counted by whole signs around Cancer and Capricorn, so that those two signs are left without partners [1]. Counted degree by degree around the solstice points, the same rule gives the antiscion pairs used by Firmicus and later authors [4].

Al-Bīrūnī describes signs equidistant from a solstice as "corresponding in course": they revolve on the same parallel, their day hours and their night hours are equal, and their ascensions are the same at the equator [2]. He also uses the solstices to divide the zodiac into an ascending half, Capricorn to Gemini, and a descending half, Cancer to Sagittarius (§ 378) [2].

Lilly defines antiscion signs as those equally distant from the first degree of "the two Tropick Signs, Cancer, Capricorn" [3].

How it is used

• Antiscia. The solstice axis is the mirror for antiscia: fold the chart along it and each planet lands on its antiscion.
• Equal declination. For points on the ecliptic, an antiscion contact is also a parallel of declination. The match is exact only when both points lie on the ecliptic.
• Cardinal points. With the equinox points, 0° Cancer and 0° Capricorn make up the cardinal axis that midpoint astrologers treat together with the Aries Point.
• On this site. On the Antiscia panel's wheel 0° Cancer is at the top and 0° Capricorn at the bottom, so the solstice axis runs straight up and down. Each antiscion is the left–right reflection of its point, and every solstice contact is drawn as a level line across the wheel. The axis can be switched on and off in the panel.

Examples

• 20° Gemini and 10° Cancer are both 10° from 0° Cancer, so each is the other's antiscion, and the Sun at either has the same northern declination (about +23.1°).
• 0° Cancer and 0° Capricorn are their own antiscia: a planet exactly on a solstice point mirrors onto itself.
• 0° Aries and 0° Libra swap places: each is the other's antiscion, because both lie 90° from the solstice points, on the equator.