What is an orb in astrology?

An orb is the tolerance allowed around an exact aspect. Two planets almost never sit at a clean 90 or 120 degrees, so astrologers permit a margin, and inside that margin the aspect is still counted. The size of the margin is a judgment call rather than a fixed law, which is why two programs can look at the same chart and disagree about whether an aspect exists. Moon Shine AI reports each contact with its actual degree gap, so you can see how tight the aspect really is instead of taking the line on the wheel at face value.

The orb once belonged to the planet, not to the aspect

Ask a modern astrologer for an orb and you get an answer about the aspect: eight degrees for a conjunction, six for a trine, three for a sextile. That arrangement is recent. In the older tradition the orb was a property of the planet itself, a notional sphere of light that each body carried around with it, and an aspect formed when those two spheres touched.

William Lilly printed the numbers in Christian Astrology in 1647. The Sun carries fifteen degrees, the Moon twelve, Jupiter and Saturn nine each, and Mercury, Venus and Mars seven. Sources from that period vary by a degree here and there, but the structure is stable: the two lights get generous room and the smaller bodies get very little. The working rule was the moiety, meaning half of each planet's orb added to half of the other's. A Sun and Saturn pairing gets seven and a half plus four and a half, so twelve degrees of allowance. Mercury and Venus together get three and a half each, so seven.

Notice what that produces. The allowance changed with which two bodies were involved and stayed the same across every aspect they made. A Sun and Saturn square held together at eleven degrees while a Mercury and Venus square had already dissolved at eight. Modern tables inverted the logic and hung the number on the aspect instead, which is easier to tabulate and loses the sense that a slow heavy planet reaches further than a quick light one.

What the modern tables actually say

Common practice now gives the conjunction and the opposition the widest room, somewhere between eight and ten degrees. Squares and trines usually get six to eight. The sextile drops to about four, and the minor aspects such as the semisextile or the sesquiquadrate are usually held to two or three, since a wide minor aspect is indistinguishable from nothing at all.

On top of that most astrologers add a bonus when the Sun or the Moon is involved, typically a degree or two, which is the one surviving trace of the old planetary orb system. A Sun square Mars at nine degrees is often kept. A Mercury square Mars at nine degrees is usually dropped. Some practitioners also tighten everything by a degree or two for the outer planets on the grounds that Uranus, Neptune and Pluto move so slowly that a wide orb would keep the same aspect live for years.

None of this is settled, and it is worth saying so plainly rather than pretending there is a correct table hidden somewhere. Traditional horary astrologers still use moieties. Cosmobiologists work with orbs under one degree and throw out most of what other schools keep. Both approaches produce readings their practitioners find useful. What matters far more than the number you choose is that you apply it consistently, because a chart read with shifting standards will always confirm whatever you already suspected.

Applying, separating, and the transit exception

An orb has a direction as well as a size. When the faster planet is still moving toward exactness the aspect is applying, and when it has passed the exact degree and is pulling away it is separating. The tradition weighted these very differently. An applying aspect described something still gathering, a separating one something already spent, and in horary work that distinction often decided the whole judgment.

In a natal chart the difference is subtler but still readable. An applying Venus and Saturn contact at two degrees tends to describe a theme that keeps arriving, while the same pair separating by two degrees often reads as a theme the person feels they have already been through. Neither version says what happens. It describes where the tension sits and how present it feels.

Transits run on a much tighter budget. Most astrologers watching current sky use one degree, sometimes two, because the point of a transit is to locate a window rather than a season. Even at one degree an outer planet transit is not brief, since a retrograde loop can carry the same planet back across the same degree three times across the better part of a year. Synastry sits in between, usually read at four or five degrees, tighter for the contacts between two people's inner planets.

Choosing a number that keeps the chart legible

Generous orbs have a cost that is easy to miss. Open every aspect to ten degrees and a chart of ten bodies will show forty or fifty contacts. At that density every planet touches every other planet, nothing stands out, and the reading turns into a list. The orb is not only a measurement of strength. It is the filter that decides what a chart is about.

A practical habit is to run the chart twice. First pass at three degrees or less, which usually leaves five or six aspects standing. Those are the spine, the contacts most likely to describe something the person recognizes immediately. Second pass at the full width, where the extra aspects fill in color and secondary themes rather than defining anything.

The rough correlation, and it is a tendency rather than a rule, is that a tight orb tends to show up in ordinary weeks while a wide one surfaces only when a transit lights it up. A Mars and Pluto square at zero degrees is usually something a person can describe without prompting. The same square at seven degrees may need pointing out. Same two planets, same geometry, very different volume.