By Dorian Grimault, astrologer · Published · updated
The semi-square aspect links two bodies 45° apart, half a square. It is tension in a minor key: it does not structure a chart, but it can make itself felt when the orb is very tight. Ptolemy ignored it; the 17th century brought it into the calculations. Here is its history, its tolerance, its place in your report, and the small constellation of semi-squares in the birth chart of Camille Morel, AstralPro's sample chart (fictional data).
360° divided by eight gives 45°: the semi-square is the smallest division in the square family, alongside the square (90°) and the sesquiquadrate (135°). It is also found under the names semi-quadrate or octile. Like the sesquiquadrate, it is not counted in whole signs: 45° is a sign and a half, so two bodies in semi-square can fall in neighboring signs or in signs with one sign between them.
Ptolemy's aspects (Tetrabiblos, book I, chapter 13) all divide the circle into whole signs: opposition, trine, square, sextile. The semi-square is not among them.
William Lilly, in Christian Astrology (1647), brings it into the calculations. In book III, he gives the table of “new aspects” with their symbols: semi-sextile (30°), semi-quintile (36°), semi-square (45°), quintile (72°), sesquiquintile (108°), sesquiquadrate (135°) and biquintile (144°). He uses them in the directions of an English merchant's nativity, and closes his work by noting that the semi-square and the sesquiquadrate “participate of the nature of the square.”
The exact origin of the semi-square is less clear than that of the sesquiquadrate. David Plant's study on Skyscript credits Kepler with three aspects, the quintile, the biquintile and the sesquiquadrate, and notes that Lilly adds others, including the semi-square.
The semi-square has the character of the square in a softened form. The two functions do not agree on their own: they clash over a detail, keep up a small recurring irritation, or a particular watchfulness. The legend of the aspect table in your report groups it with the sesquiquadrate: minor frictions, background tension.
To spot it without calculating, add a sign and a half to a body's position. From Mars at 3°38' Sagittarius, one sign takes you to 3°38' Capricorn, and fifteen more degrees to 18°38' Capricorn: Neptune, at 18°56', is 0°18' from that point.
To read it, name the two functions and look for where they quietly get in each other's way. With the Sun semi-square the Moon, what you want to express and what you feel are slightly out of step. It is not a major contradiction, but a nuance worth noticing, especially if other aspects in the chart point the same way.
In your birth chart, the orb of the semi-square is 3°: two bodies 42° to 48° apart are in semi-square, whatever the body. Its name appears in the grid in the orange of the squares.
It is a minor aspect, and the report treats it as such. It appears in the complete grid, with its orb and direction, but the wheel does not draw it and the Book's table, reserved for majors, leaves it out. When the report chooses the aspects the chapters discuss, it gives a clear advantage to the five majors, then takes into account the bodies involved and the orb. A very tight semi-square can therefore open the grid without being discussed at length. It still keeps its entry in the Reading column as soon as it is among the lines at 3° of orb or less.
Camille Morel, AstralPro's sample chart (fictional data), has four semi-squares, and they form a pattern. Mars (3°38' Sagittarius) is conjunct the North Node (3°00' Sagittarius), and Uranus (19°10' Capricorn) conjunct Neptune (18°56' Capricorn). Between the two pairs lie about 45°: each member of one pair is in semi-square to each member of the other.
The first two are the third and fourth tightest aspects in the whole chart, right after Mercury conjunction Venus (0°07') and Uranus conjunction Neptune (0°15'). No square or trine in the chart is as exact. Mars, a fast body, gives its lines a direction; the last two carry a dash, because Neptune, Uranus and the North Node are all slow bodies.
The semi-square has a complement: the sesquiquadrate, since 45° and 135° add up to 180°. A body in semi-square to a point is in sesquiquadrate to the opposite point. In Camille's chart, this concerns the axis of the Nodes, of which the grid carries only the North Node. Neptune, semi-square the North Node at 0°55', is therefore also sesquiquadrate the South Node, in Gemini, at the same distance from exact. So is Uranus, at 1°10'.
These contacts have no line of their own: they are read through those the grid displays. It is the same principle as for the angles: the grid connects the Ascendant and the Midheaven, not their opposites, so that a single contact is not counted twice.
The semi-square between Mars and Neptune also shows the value of direction. Mars, at 3°38' Sagittarius, moves faster than Neptune and is closing in on the exact 45° angle: the line says Applying. A friction that is forming, in the modern reading, rather than a reflex already in place.
All four lines have a reading: the column covers the first twenty lines of Camille's grid, all at 3° of orb or less. None is drawn on the wheel. And none makes it into the twenty aspects discussed in chapter I, all of them majors, or into the thirteen sections of Life Domains.
This is the direct consequence of the selection: in Camille's chart, the Mars square to the Midheaven, at 3°23', weighs more in that choice than the Mars semi-square to Neptune, at 0°18'. The report hides nothing for all that. The grid keeps everything, from tightest to widest, and that is where you see Mars, already tied to the North Node, caught in a quiet friction with the Uranus-Neptune conjunction of her generation.
These entries describe astrology for everyone. Your birth chart, though, is unique: discover what your birth sky reveals about you.
Discover my birth chart — €69.90 →