Cardinal

Aries, Cancer, Libra and Capricorn are the four signs the Sun enters at an equinox or a solstice. Aries and Libra begin at the equinoxes, Cancer and Capricorn at the solstices, and between them they mark the four moments at which the year changes direction.

Ptolemy calls them the tropical signs, from tropikos, a turning point, and the name is astronomical before it is symbolic. It records what happens: the Sun's declination reverses at the solstices and it crosses the celestial equator at the equinoxes, so a season begins in each of these four signs and in no others. The initiating character the tradition assigns them follows from that fact rather than from a separate doctrine laid on top of it.

The four are distributed one to each element and one to each polarity in strict alternation. Aries is fire, Cancer water, Libra air and Capricorn earth, and they alternate active, receptive, active, receptive round the circle. That is a general property of the zodiac's construction rather than a special feature of this group, but it does mean that emphasising cardinality in a reading is not thereby emphasising any one element.

The modern names for the mode come later than the Greek one and carry a slightly different sense. Tropical says the season turns here. Cardinal says the sign leads.

The same four signs are called several things in different centuries, and a reader will meet all of them.

Tropical is the Greek term and the oldest, from the Sun's turning at the solstices. Movable is the usual medieval Latin rendering, and it shifts the emphasis from the astronomy to the character: these are the signs that move. Cardinal is the modern English standard, from the Latin cardo, a hinge, and carries the sense of a pivot on which a thing turns. Initiating and creating are looser modern descriptions rather than technical names.

None of these is wrong and they are not quite synonyms. Tropical is a statement about the Sun's path, and it also names the tropical zodiac itself, the one measured from the equinox rather than from the fixed stars, which is a different and much larger distinction that this entry does not settle. Movable and cardinal are statements about how the signs behave in a reading.

The confusion worth flagging is a different one. The three sign qualities, cardinal, fixed and mutable, are called the modalities in almost all modern astrological writing, while the three house groups, angular, succedent and cadent, are called modalities in some of it. The atlas keeps them apart by using quality for the signs and modality for the houses. That is not universal usage, and it is stated here rather than left implied, because it is the only way to name both groups without ambiguity.

Sources TetrabiblosClaudius Ptolemy with Christian AstrologyWilliam Lilly