Synastry: how two birth charts get on

It goes far deeper than comparing sun signs. Two people with clashing Suns can share a Moon link that makes them feel instantly at home together. You can check your zodiac compatibility with just two birth dates, then read on to understand what you see.

Every planet in your chart sits at a precise degree of the zodiac, worked out from real astronomical data for the day you were born. Synastry measures the distance between your planets and theirs. Certain distances, called aspects, mean the two planets are talking to each other.

Our compatibility reading uses the five personal planets for both people, because these are the ones that shape everyday closeness:

• The Sun. Your core self, what you want from life.
• The Moon. Your feelings, your needs, how you want to be looked after.
• Mercury. How you talk, think and argue.
• Venus. What you find beautiful, how you give and receive affection.
• Mars. Desire, drive, temper and attraction.

Most compatibility readings boil a relationship down to one number. That hides the thing you most want to know. A pair who feel calm and easy together and a pair who cannot stop thinking about each other might score the same, yet they are having completely different relationships.

So we show both halves.

Harmony comes from the flowing aspects. These are the links that feel like recognition: you laugh at the same things, you want the same kind of Sunday, you rarely have to explain yourself.

Spark comes from the tense aspects. These are the links that create friction and heat: you push each other, you disagree in ways that keep things interesting, and the attraction has an edge. Spark is not a warning. Most memorable relationships have plenty of it.

• Conjunction. Two planets at the same point. The energies merge. A Venus conjunct someone's Sun is classic adoration. Counts towards harmony.
• Trine. Planets a third of the zodiac apart, usually in signs of the same element. The easiest link there is, a natural understanding. Counts towards harmony.
• Sextile. Planets a sixth of the zodiac apart. Friendly and supportive, an opening you both have to step through. Counts towards harmony.
• Square. Planets a quarter of the zodiac apart. Friction, pressure and growth. A Mars square Venus is the attraction that keeps you up late talking, and occasionally arguing. Counts towards spark.
• Opposition. Planets directly across the zodiac. Fascination and tension in equal measure, the pull of someone who has what you lack. Counts towards spark.

The closer an aspect is to exact, the more it counts. Each aspect has an orb, a margin either side of exact: eight degrees for links involving the Sun and Moon, six for the other planets. A tight aspect, one or two degrees from exact, is felt strongly; one near the edge of its orb is a softer hum.

Your result splits harmony and spark into four pairs of bars, so you can see where each one lives.

• Core. Sun to Sun, and Sun to Moon. Whether your basic natures fit, and whether one person's way of being meets the other's emotional needs. A Sun-Moon link is one of the classic signatures of a lasting couple.
• Emotional. Moon to Moon, and Moon to Venus. How you feel around each other day to day. Harmony here feels like home; spark here means moods catch between you quickly.
• Attraction. Venus to Mars, and Mars to Mars. Physical chemistry and desire. Spark in this row is often where the heat comes from.
• Everyday. Mercury to Mercury, and Mercury to Sun. Talking, planning and running a life together. Harmony here makes the logistics easy; spark makes for lively debates over the route.

Instead of a single score, you get one word that describes the shape of your chemistry.

• Easy. Plenty of harmony, little friction. You understand each other without effort. The thing to watch is complacency: plan surprises, because nothing will force them.
• Electric. Strong spark with solid harmony underneath. Intense, magnetic and rarely dull. You will argue, and you will make up quickly.
• Grounded. Harmony leads without heavy spark. Steady, warm and reliable. The kind of bond that grows deeper rather than louder.
• Stormy. Spark leads and harmony is lighter. Exciting, demanding, sometimes exhausting. It works when you both treat disagreements as part of the fun and say kind things out loud.
• Slow burn. Fewer tight aspects either way. The chemistry is quiet at first and builds with time and shared experience. Many long friendships and late-blooming romances look exactly like this.

The planets move slowly, so a birth date is enough for most aspects. The exception is the Moon, which travels about thirteen degrees a day. Without a birth time, a Moon aspect that sits near the edge of its orb is marked uncertain, so you know which links are firm and which might shift once you add a time. Our guide to charts without a birth time explains more.

Read the bars as a map, not a verdict. Harmony shows where you can relax. Spark shows where you will grow, and where you need patience with each other. If you have one strong row, lean into it; if one row is quiet, give that part of the relationship some deliberate attention.

Sun signs still add colour. Look up your pair on the sign pages, such as Leo and Scorpio, for the broad strokes, then use your full reading for the detail.

For a conversation that goes beyond the stars, the couples compatibility test compares how you both think about lifestyle, priorities and conflict. And when you are ready, check your zodiac compatibility to see your word.