Equinox axis

Meaning

The equinox axis is the line across the zodiac from 0° Aries to 0° Libra. These are the two points where the ecliptic, the Sun's yearly path, crosses the celestial equator.

The astronomy. At 0° Aries, around 20 March, the Sun crosses the equator going north; at 0° Libra, around 22–23 September, it crosses going south. On those days its declination is zero and day and night are roughly equal everywhere on Earth, which is what equinox ("equal night") means. From 0° Aries the Sun's declination grows northward to its peak at 0° Cancer, falls back to zero at 0° Libra, reaches its southern extreme at 0° Capricorn and returns.

In the tropical zodiac 0° Aries is defined as the spring (northern) equinox and is the starting point from which all longitudes are counted. It is the Aries Point.

Why it is a mirror. Degrees the same distance either side of an equinox point, such as 10° Aries and 20° Pisces, have declinations of the same size but opposite direction, one north and one south of the equator. So the length of day the Sun gives at one equals the length of night it gives at the other, and the stretches of zodiac on either side of an equinox take the same time to rise at any latitude. Reflecting a degree across this axis gives its contra-antiscion, 360° − λ.

Origin and history

Ptolemy names Aries and Libra the equinoctial signs: at the beginning of each, the Sun makes the nights exactly equal to the days [1]. He then calls parts of the zodiac equally distant from the same equinoctial sign "commanding" and "obeying", because they rise in equal times and lie on equal parallels; those in the summer half command, those in the winter half obey, because the Sun there makes the day longer or shorter than the night [1]. (See commanding and obeying signs.) Robbins lists the pairs as whole signs balanced around Aries and Libra, Taurus with Pisces, Gemini with Aquarius and so on, leaving Aries and Libra without partners [1].

Al-Bīrūnī calls signs equidistant from an equinox point "equipollent", because the day hours of one equal the night hours of the other and their ascensions are equal everywhere; his examples are Aries with Pisces and Taurus with Aquarius, with the degrees running in reverse [2]. Note that "equipollent" means "of equal power", which in Robbins's Ptolemy is the name of the solstice pairs; al-Bīrūnī himself remarks that the names for these relations vary from book to book and none is fixed [2].

Lilly locates the contra-antiscion as the point opposite the antiscion, which is the same as the reflection across this axis [3].

How it is used

• Contra-antiscia. The equinox axis is the mirror for contra-antiscia: fold the chart along it and each planet lands on its contra-antiscion.
• Contra-parallels. For points on the ecliptic, a contra-antiscion contact is also a contra-parallel of declination, equal declinations on opposite sides of the equator.
• The Aries Point. In midpoint astrology 0° Aries, together with 0° Libra and the solstice points, is used as a point of contact with the wider world. A planet near 0° Aries or 0° Libra sits on the equinox axis itself, so its contra-antiscion falls on or very near its own degree.
• On this site. On the Antiscia panel's wheel, with 0° Cancer at the top, the equinox axis runs straight across from side to side. Each contra-antiscion is the top–bottom reflection of its point, and every equinox contact is drawn as an upright line. The axis can be switched on and off in the panel.

Examples

• 10° Aries and 20° Pisces are each 10° from 0° Aries, so each is the other's contra-antiscion. The Sun at 10° Aries is about 4° north of the equator, at 20° Pisces about 4° south.
• 0° Aries and 0° Libra are their own contra-antiscia.
• 0° Cancer and 0° Capricorn swap places across this axis: each is the other's contra-antiscion.