Aspects: the lines on a birth chart.

1. What an aspect is

The zodiac is a circle of 360°, and every planet has a place on it at the moment of birth. An aspect is the angle between two of those places, measured the short way round. Astrology counts an angle as meaningful when it divides the circle evenly: by one, which puts the planets together; by two, which puts them opposite; by three, four and six. Those five divisions are the five aspects every reading uses, and the only ones StarMap draws.

A line on the wheel joins two planets that make one of those angles, near enough. Gold is a conjunction, blue a trine or a sextile, rose a square or an opposition, and the thicker the line, the closer the angle is to exact. The Sun and the Moon count as planets here, as they do everywhere in astrology; the lunar nodes do not take part.

2. The five aspects, one by one

In the order of the circle, from together to opposite. The orb is how far from exact each one may be and still count; section 3 says more about it.

☌Conjunction: 0°, gold line

The circle divided: 360° ÷ 1, the whole circle · Orb: 8° (9° with the Sun or Moon) · Asks: What in me comes as one thing?

Two planets in the same part of the sky act as one. Their meanings fuse into a single strong note that is hard to hear separately: a Moon conjunct Venus feels and loves in one motion, a Mercury conjunct Mars thinks by arguing. A conjunction is neither easy nor hard in itself; it takes its character from the two planets in it, and the closer they sit, the less either can be told apart from the other.

In the example: 5; the closest is Mercury conjunct Pluto, 0.8° from exact.

⚹Sextile: 60°, blue line

The circle divided: 360° ÷ 6, a sixth · Orb: 5° (6° with the Sun or Moon) · Asks: What is on offer, if I take it?

Two planets a sixth of the circle apart sit in signs that get on, fire with air, earth with water. The pair cooperate when asked: an opening, a knack, a door that is unlocked but not open. A sextile rewards a little deliberate effort and sits quietly until it gets it.

In the example: 5; the closest is Jupiter sextile Saturn, 0.5° from exact.

□Square: 90°, rose line

The circle divided: 360° ÷ 4, a quarter · Orb: 7° (8° with the Sun or Moon) · Asks: Where do I push against myself?

Two planets a quarter of the circle apart are usually in signs of the same mode but different elements, which agree on pace and disagree on everything else. The friction shows as a recurring tension that will not settle on its own, and most of the growth in a life comes from working at it. A square is the engine of a chart, not a flaw in it.

In the example: 2; the closest is Venus square Neptune, 1.7° from exact.

△Trine: 120°, blue line

The circle divided: 360° ÷ 3, a third · Orb: 7° (8° with the Sun or Moon) · Asks: What comes easily to me?

Two planets a third of the circle apart are nearly always in signs of the same element, fire with fire, water with water, so they speak the same language. What they stand for flows together without effort: a talent, an ease, a thing the person does well without noticing. The only risk is that it goes unused, because nothing pushes it.

In the example: 3; the closest is the Sun trine the Moon, 0.4° from exact.

☍Opposition: 180°, rose line

The circle divided: 360° ÷ 2, a half · Orb: 8° (9° with the Sun or Moon) · Asks: What in me takes turns?

Two planets face each other across the whole sky, as the full Moon faces the Sun. Each pulls the other way, and the person tends to live the pair by turns, or to meet one side of it in other people. The work of an opposition is balance: a life spacious enough for both ends. It is the aspect of awareness, because nothing shows a thing so clearly as its opposite.

In the example: 2; the closest is the Sun opposite Saturn, 3.3° from exact.

3. Orbs: how close is close enough

Two planets are almost never an exact 120° apart, so every aspect is given an allowance, the orb. StarMap uses 8° for a conjunction or an opposition, 7° for a trine or a square and 5° for a sextile, with one degree more whenever the Sun or the Moon is one of the pair, because the lights carry more weight. Other astrologers use slightly different numbers, which is why two apps can disagree about one faint line; the strong aspects are the same everywhere.

The orb is also a volume control. An aspect within a degree or two of exact is loud and will be felt in most days; one at the edge of its orb is quiet and shows mainly under a transit. The wheel draws the difference as the thickness of the line, and StarMap lists aspects tightest first for the same reason. In the example the Moon opposite Mars at 8.6° is on the page only because the Moon is a light; two slower planets that far apart would not be joined.

4. Applying and separating

The planets were moving at the moment of birth, so every aspect was either still forming or already past. When the faster planet is moving towards the exact angle the aspect is applying; when it has passed, separating. Astrologers read an applying aspect as something still building in a life and a separating one as something the person arrives with. In the example, 11 of the 17 are applying and 6 separating; the tightest, the Sun trine the Moon, is separating.

A retrograde planet, one that appears to move backwards against the stars, makes the same aspects; the direction only changes which pairs are closing and which opening. Jupiter and Saturn were retrograde in the example chart.

5. Aspects that cannot happen, and ones everyone shares

Seen from Earth, Mercury never strays more than about 28° from the Sun and Venus never more than about 48°. So the Sun can only ever be conjunct Mercury or Venus, never square or opposite them, and Mercury and Venus reach at most a sextile with each other. If an app shows you a Sun square Mercury, something is wrong with the chart.

At the other end, Uranus, Neptune and Pluto move so slowly that an aspect between two of them stays within orb for years, and everyone born in that stretch has it. Such a line describes a generation more than a person, and a reading gives it less weight than a line to the Sun, the Moon or a personal planet. The example has two: Neptune sextile Pluto and Uranus conjunct Pluto, shared with everyone born for some years around the same time. What makes a slow planet personal is an aspect to a fast one, or a place on an angle.

6. Patterns: when lines make shapes

• A stellium: three or more planets conjunct in a row. One part of the chart does most of the talking.
• A T-square: two planets in opposition, both square a third. The third planet is where the tension lands and where the person keeps returning to act.
• A grand trine: three planets each trine the others, a triangle of one element. Ease all round, which can run on its own without the person steering.
• An unaspected planet: one with no line at all. It works on its own terms, sometimes all or nothing, and is easy to overlook in a reading. Every planet in the example has at least one line.

Patterns are worth noticing, not worth hunting for. A chart with none of them is as complete as any other.

7. How to read your aspects in order

1. The tightest line. StarMap lists aspects tightest first; the top one is the loudest theme in the chart.
2. Every line to the Sun and to the Moon. They colour the core of the chart and the daily life of it.
3. The planet with the most lines, the hub the rest of the chart talks to. In the example it is the Sun, with 5.
4. The rose lines, read as the work of a life rather than its faults, and the blue ones, read as what comes easily enough to be taken for granted.
5. Only then the lines between the three slow planets, which the whole generation shares.

The reading of the example’s tightest line, as the chart page writes it: “With a Sun-Moon trine, purpose and emotional needs back each other up with very little effort. Whitney Houston is likely to know what they want and to feel good pursuing it, which gives them an unusual, steady confidence that other people can sense.” That line was drawn from the Sun at 16° Leo 41' and the Moon at 17° Aries 05', 0.4° from an exact 120°.

8. Without a birth time

The planets move slowly, so most aspects are the same at any hour of a day and a chart without a time keeps them. The Moon is the exception: it moves about 13° a day, so a line to the Moon can form or fade between morning and evening, and a chart cast for noon can show one that a birth at dawn would not. If all you know is the day, read the Moon’s lines as possible rather than certain, and the Moon sign calculator will say whether the Moon changed sign that day at all.