Orbs in astrology: applying and separating aspects
An orb is the margin of error allowed around an exact aspect. A trine is exactly 120 degrees; if two planets sit 124 degrees apart, astrologers still count it, using a 4-degree orb. Applying means the faster planet is still closing on exactness. Separating means it has already passed and the distance is widening.
What is an orb in astrology?
An orb is the number of degrees either side of an exact aspect within which the aspect is still held to work. Aspects are specific angular distances between planets — 0, 60, 90, 120 and 180 degrees are the main five — and planets almost never sit at those distances precisely. The orb is what makes the system usable at all.
Two planets 92 degrees apart are 2 degrees from an exact square. With a 6-degree orb on squares, that counts. At 99 degrees apart, most astrologers would say the square has gone out of orb and is not a square any more, just two planets in roughly awkward signs.
The word is older than the usage. In medieval and Renaissance astrology, the orb belonged to the planet rather than to the aspect: each planet was held to have a moiety, a half-orb, a body of light extending around it, and two planets aspected each other when their moieties touched. Sun and Moon had the largest, Mercury and Mars the smallest. William Lilly's Christian Astrology, published in London in 1647, gives a table of these — 17 degrees for the Sun, 12 for the Moon, 7 for Mercury and Mars, 9 for Venus, 12 for Jupiter, 10 for Saturn. Add two moieties and halve the sum, and you get the working orb for that particular pair. Sun and Saturn: 13 degrees and 30 minutes. That is a very wide square by modern standards.
Modern practice mostly abandoned planetary moieties in favour of orbs attached to the aspect type, which is simpler and less generous. Both approaches are in use. Neither is provable.
How many degrees is a normal orb?
There is no agreed figure, and anyone who gives you one without saying whose it is has hidden a choice from you. What follows is a common modern working set, close to what most software defaults to.
| Aspect | Exact angle | Typical modern orb | Notes |
|---|---|---|---|
| Conjunction | 0° | 8–10° | Widest allowance of any aspect |
| Opposition | 180° | 7–8° | Nearly as wide as the conjunction |
| Trine | 120° | 6–8° | |
| Square | 90° | 6–8° | |
| Sextile | 60° | 4–6° | Narrower, being the weaker of the majors |
| Semi-sextile | 30° | 1–2° | Minor aspect, tight orb |
| Semi-square | 45° | 1–2° | Minor |
| Sesquiquadrate | 135° | 1–2° | Minor |
| Quincunx / inconjunct | 150° | 2–3° | Disputed status; see below |
| Quintile | 72° | 1–2° | Kepler's, from 1619 |
Two adjustments are near-universal even among astrologers who disagree about everything else.
Luminaries get more room. Aspects involving the Sun or the Moon are usually given an extra degree or two, which is a thin modern survival of Lilly's moiety table, where those two bodies had by far the largest orbs.
Minor aspects get much less. The 30, 45, 135 and 150-degree aspects are worked within a degree or two, on the reasoning that a weak aspect stretched wide is indistinguishable from nothing at all.
Johannes Kepler, writing in Harmonices Mundi in 1619, introduced the quintile and biquintile and argued the aspect series should be derived from musical harmonic ratios rather than inherited from Ptolemy. The minor aspects in the table below the majors are largely his legacy and that of twentieth-century harmonic astrologers, chiefly John Addey in the 1970s.
What is the difference between an applying and a separating aspect?
An applying aspect is one still forming: the faster of the two planets is moving toward exactness, and the gap is closing. A separating aspect has already been exact and is now dissolving, the gap widening with every hour.
Which planet is faster decides the direction. The Moon moves roughly 13 degrees a day, Mercury and Venus around a degree, Saturn a few minutes, Pluto barely at all. The Moon at 88 degrees from Saturn is applying to the square. The Moon at 92 degrees is separating from it. Same orb, opposite condition.
Retrograde motion reverses the arithmetic. A planet moving backward through the zodiac can apply to an aspect it has already separated from, which is why transits from the outer planets often hit three times: direct, retrograde, direct again.
The distinction is old and it is structural rather than decorative. Traditional astrology treats applying and separating as different things, not as degrees of the same thing. In horary — the branch that answers a specific question from a chart cast for the moment the question was asked — an applying aspect between the significator of the querent and the significator of the thing asked about is the standard indication that the matter comes together. A separating aspect between the same two indicates it already happened, or was missed. Lilly is explicit about this, and so is the Arabic-language tradition he drew on through Latin translation, Abu Ma'shar in the ninth century among them.
Modern natal astrology inherited the distinction but uses it more loosely. The common modern reading holds that an applying aspect in a birth chart describes something still working itself out across a life, while a separating aspect describes something more settled, more like an inheritance than a task. That reading is twentieth century and interpretive. The mechanics of which planet is closing on which are simply astronomy.
Does a tighter orb mean a stronger aspect?
Yes, by near-universal agreement, and it is the one point on which traditional and modern astrologers do not argue. An aspect within a degree of exact is held to be loud. An aspect at the far edge of its permitted orb is held to be faint, and some astrologers would not read it at all.
A partile aspect — one exact to the same degree, so 14 degrees of Aries square 14 degrees of Cancer — is treated as the strongest form. The term is traditional and appears throughout the Hellenistic and medieval material.
Where this matters most is in deciding what to read. A chart drawn with 8-degree orbs on everything produces a web of lines in which no planet is unaspected and nothing stands out. Tighten to 3 degrees and a handful of aspects remain, and those are usually the ones an astrologer will actually talk about. Practitioners differ on where to draw the line; the useful habit is to notice the line has been drawn and by whom.
What is a void-of-course Moon and what does it have to do with orbs?
A void-of-course Moon is a Moon that will make no further major aspect to any planet before it leaves the sign it is currently in. It is an orb question turned into a time question: not how wide is the gap, but how much of the sign is left and is there anything in it.
The condition is entirely calculable and the times are in any ephemeris. What it is held to mean is interpretation, and the traditional interpretation is narrow and specific. Lilly's line is that nothing will come of a matter begun under a void Moon, which in a horary context is a statement about whether a question has an answer, not a prohibition on doing anything. Modern electional astrology — the branch concerned with choosing a time to begin something — generally avoids void periods for launches and signings, and generally does not care about them for routine work.
The definitional disagreement is real. Some astrologers count only the five Ptolemaic aspects when deciding whether the Moon is void. Others include minor aspects, which makes void periods much rarer. Others again restrict the test to the traditional seven planets, excluding Uranus, Neptune and Pluto, which changes the answer on a fair number of days. Two ephemerides can disagree about whether the Moon is void this afternoon, and both are correctly applying their own rule.
Do orbs differ between natal charts and transits?
Yes. Transit orbs are conventionally much tighter than natal ones, often 1 to 3 degrees for the majors and under a degree for anything minor.
The reasoning is about duration rather than strength. A natal aspect is fixed for life, so a wide one still sits in the chart permanently. A transit is a moving planet crossing a natal point, and a wide orb turns a moment into a season. Pluto moves so slowly that an 8-degree orb on a Pluto transit would cover years, which is no use to anyone asking when.
Electional work runs tighter still. Choosing a moment means choosing the hour when an aspect is closest to exact and still applying, because applying is the condition traditional astrology reads as bringing something about rather than reporting it. The width of the window is the whole question, and a chart that only offers a wide, separating aspect is a chart that is not offering much.