By Dorian Grimault, astrologer · Published · updated
Two planets rarely form a perfect angle. An exact square is 90° on the dot; does a gap of 87° or 93° still count as a square? The orb answers this question: it is the gap allowed around the exact angle for the aspect to be retained. The smaller an aspect's orb, the tighter the aspect, and the more weight it carries in the reading.
The orb of an aspect is the difference between the actual gap between the two bodies and the exact angle of the aspect. If two planets are 91°24' apart, they form a square (90°) with an orb of 1°24'. Your report always writes the orb in degrees and minutes, never in decimals: 1°24' and not 1.4°.
Two things need to be distinguished. The orb of a given aspect, which you read in the Orb column of the aspects table; and the maximum orb, the tolerance beyond which the aspect is no longer retained. It is this second sense that is subject to each astrologer's choices, and it is what decides how many rows your grid has.
Medieval and classical astrology did not set the orb by aspect but by planet. Each body was thought to radiate a sphere of light around itself, wider for the luminaries: in William Lilly, in the 17th century, about fifteen degrees for the Sun, about twelve for the Moon, around 7° for Mercury, Venus and Mars. To know whether two planets were in aspect, you added half of each one's orb, what the English texts call the moiety.
Modern astrology has most often reversed the logic: the orb depends on the aspect, wide for the conjunction and the opposition, narrower for the sextile and for the minor aspects. Many authors widen the orbs of the Sun and the Moon further still. The values vary from one manual to another, and no table is unanimously accepted.
AstralPro sets the orb by aspect, with a single table for the natal chart and for the Book, so that the two never detect different aspects on the same chart:
The first five are the major aspects, the last four the minor ones, and no major aspect has a narrower orb than a minor one. These orbs are the same for all bodies: the Sun and the Moon get no widened tolerance, Pluto no reduced one. The quintile is not part of this table. In choosing which aspects the report discusses, the orb also counts: the tighter it is, the more the aspect weighs.
Other, narrower tolerances apply to specific uses. The celestial points (Eros, Chariklo, Pholus and the others) are retained only in a major aspect, at 2° at most, and only with the Sun, the Moon, Mercury, Venus, Mars, the Ascendant or the Midheaven. The fixed stars count only in conjunction: 1° on a planet and 1°30' on an angle, 1°30' and 2° for the four royal stars. The secondary progressions work at 1°30'.
The table in Chapter I, “The complete aspect grid”, lists all the aspects allowed by the table, ranked from tightest to widest. The chart wheel makes another choice, to stay legible: it draws only the five major aspects, and only the tightest, at 5° for the conjunction, the opposition and the trine, 4° for the square, 3° for the sextile. It connects only the ten planets, the Ascendant and the Midheaven, and leaves out aspects between two slow planets (Uranus, Neptune, Pluto). A legend under the wheel recalls this.
An aspect present in the table and absent from the wheel is therefore not an oversight: it is wider than the drawing's threshold, it is minor, or it involves a point the wheel does not connect.
For Camille Morel, AstralPro's sample chart (fictional data), the grid has 40 aspects. It opens with Mercury conjunction Venus, orb 0°07': the two planets are at 6°37' and 6°30' Scorpio. This is the tightest aspect in the chart; it heads the table, is drawn on the wheel and is discussed in Chapter I.
It ends with Pluto conjunction North Node, orb 7°43', very close to the 8° limit allowed for the conjunction. The aspect is retained and appears in the table, but it does not appear on the wheel, which does not connect the North Node, and it is not among the twenty aspects discussed in Chapter I.
Between the two, the conjunction of the Sun and the Moon, orb 5°15', shows the effect of the wheel's threshold: clearly present in the grid, it exceeds the drawing's 5° by a few minutes. The Sun-Saturn square, orb 1°58', is drawn, on the other hand. For a minor aspect, the margin is narrower: the semi-square from Mars to Neptune, orb 0°18', is retained because it sits well under its 3°.
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