Before you start: what you are reading
Synastry lays two birth charts over each other and reads two things: the aspects between one person's planets and the other's, and the houses of one chart that the other's planets fall into. This article is the how-to; for the wider picture of what synastry measures and what it means, read synastry and compatibility first and keep it open alongside this. The mindset that makes a reading useful is the one to fix now: synastry describes a dynamic, not a destiny. It shows where two people spark, soothe and snag, not whether they should be together, and a chart full of hard aspects is not a sentence any more than a chart full of easy ones is a guarantee. You are mapping the terrain of a bond, and the two people still do the walking. Hold that and the technique below stays a tool rather than a verdict.
Step 1: Cast both charts accurately
You cannot read what you have not built correctly, so start with clean data for both people: date, exact time and city for each. Both birth times matter as much here as in a single chart, because the Moon and the angles move fast, and so much of synastry turns on the Moon, the Ascendant and the Descendant. If one time is missing or shaky, note which contacts become unreliable rather than reading them as solid; see finding the ascendant without a birth time for what a missing time costs. With both charts cast, set them as a bi-wheel: one person's chart on the inside, the other's planets ringed around the outside, so you can see at a glance which of their planets touch yours and which of your houses theirs land in. Everything that follows is reading that double wheel in a deliberate order rather than letting your eye jump to whatever looks dramatic.
Step 2: Read the luminaries first
Always begin with the Sun and the Moon, because they set the baseline. Look for Sun to Moon contacts between the two charts: one person's Sun touching the other's Moon is the classic sign of a bond where identity and emotional need fit together, and a tight conjunction or trine there often feels like instant recognition, as if you already knew each other. Then check Moon to Moon, which shows whether two emotional rhythms naturally sync or grate, how it feels to simply live alongside this person day to day. Sun to Sun tells you how two core identities and life directions sit together, easily or at an angle. If the luminaries connect well, the relationship has a floor to stand on even when other things are rough; if they barely touch, a dazzling Venus or Mars link can still leave the two people feeling like strangers underneath the attraction. Read the luminaries before anything glamorous pulls your attention.
Step 3: Weigh Venus and Mars
Venus and Mars are the chemistry pair, so they come second. Venus is how you love and what you find lovely; Mars is desire and drive. One person's Venus in close aspect to the other's Mars is the single most cited marker of physical attraction in synastry, the conjunction especially, and it describes a pull that is felt in the body, not just the mind. Venus to Venus shows shared taste and values, how alike your pleasures are; Mars to Mars shows how your energies and tempers meet, whether you push in the same direction or strike sparks. Watch for a double-whammy here: your Venus to their Mars and their Venus to your Mars at once binds two people far more tightly than a one-way link, because each person feels the pull from their own side. Strong Venus-Mars with weak luminaries reads as heat without depth; with strong luminaries behind it, it reads as a bond that is both warm and alive.
Step 4: Check Saturn and the slow planets
Saturn is the ballast, so weigh it third. Saturn contacts to a partner's Sun, Moon or Venus bring seriousness, structure and staying power, they are the aspects behind marriages and partnerships that endure, but the same Saturn can feel heavy, dutiful or critical when a relationship sours, so it is the glue and the grind at once. A bond with no Saturn contact at all can struggle to find ballast to last. The outer planets, Jupiter aside, are read with more caution: Uranus, Neptune and Pluto move so slowly that two people of the same generation share many of the same placements, so an outer-planet link only becomes personal when it lands tightly on the other person's Sun, Moon, Venus, Mars or an angle. A Neptune on your Venus can feel dreamy or deceptive; a Pluto on your Moon, intense and transformative, but only when the orb is tight enough to be truly yours rather than your whole age group's.
Step 5: Read the angles
The Ascendant and Descendant, and the Midheaven and IC, are not planets but they are deeply personal, so read contacts to them with weight. A planet landing on your Descendant, the point opposite your Ascendant that governs partnership, is a strong partner signal: that person carries exactly what you look for in an other. A planet on your Ascendant shapes how you present and feel seen; their Sun there and you feel energised and visible around them, their Saturn there and you may feel watched or dimmed. Contacts to the Midheaven touch career and public life, the sense that this person is tied to your direction in the world. Because the angles depend entirely on an accurate birth time, these are the first contacts to discount if either time is uncertain. When the times are solid, angle contacts are among the most immediately felt links in the whole comparison, far more personal than a slow planet-to-planet aspect two strangers might share.
Orbs: tighter than in a natal chart
An orb is how far from exact an aspect can be and still count, and synastry uses tighter orbs than natal work, typically three degrees or less for the contacts you lean on. The reason is quality: a conjunction within one degree will usually outweigh a trine at seven, whatever planets are involved, so a reading is not about counting aspects but about weighing the few tight ones. Resist the urge to tally harmonious versus challenging links and declare a winner; a single exact Sun-Moon conjunction can give a relationship more foundation than a dozen wide, loose trines. When you scan the bi-wheel, mark the contacts inside about three degrees first and build the reading on those, then let the wider aspects add colour rather than structure. A page that lists forty aspects is mostly noise; the story lives in the handful that are nearly exact, and learning to ignore the loose ones is half the skill.
House overlays: where their planets land
Aspects are only half of synastry. The other half is the house overlay: which of your houses the other person's planets fall into, which tells you what area of your life they naturally light up. Their Sun in your 7th house and they feel like a natural partner; in your 5th, like romance and play; in your 10th, tied to your ambitions and standing; in your 4th, like home and family; in your 12th, fated but elusive, hard to pin down. Their Moon in your 4th can feel like coming home; their Mars in your 8th, charged and intense. The overlay works both ways at once, you are landing in their houses exactly as they land in yours, so read both directions. This layer is one of the most revealing and most overlooked in the whole comparison, because it answers not just how two people connect but where, in which rooms of each other's lives, the connection actually plays out.
A worked example
Take an illustrative couple, Mara and Leo. Running the order: first the luminaries, Mara's Sun at 14 degrees Cancer sits trine Leo's Moon at 16 degrees Scorpio, a close water trine, so identity and emotional need fit easily, the recognition floor is there. Moon to Moon is wider and less exact, so their daily rhythms take a little more adjusting. Second, Venus and Mars, Leo's Venus at 3 degrees Aries squares Mara's Mars at 5 degrees Cancer: real attraction with friction built in, a square that generates heat and occasional clashes rather than easy flow. Third, Saturn, Mara's Saturn sits conjunct Leo's Sun within two degrees, a strong binding contact that lends the relationship weight and endurance, and, on a bad day, a sense of duty or pressure. Fourth, the angles, Leo's Venus also falls on Mara's Descendant, marking Leo as partner material in Mara's eyes. Finally the overlays, Leo's Sun lands in Mara's 5th house of romance and creativity. The reading: a warm, lasting pull with a workable edge of tension, not a storybook guarantee and not a warning, a terrain the two of them navigate themselves.
What synastry cannot tell you, and how AstraBrain reads it
Synastry cannot tell you whether to stay or go, whether a relationship is good or doomed, or what will happen next. Hard aspects are not curses and easy ones are not promises; many durable bonds are built on squares that keep the current running, and many flat ones on trines that never had to work. It is a mirror for a dynamic, not a verdict on it. AstraBrain keeps the line clear: the mathematics, the inter-chart aspects, the orbs, the house overlays, runs on Swiss Ephemeris and is exact for the birth data you enter, while the interpretation is written by an AI language model, and astrology itself is a language of reflection, not a science and not a prediction. Enter both sets of birth data, including both times for the Moon and the angles to be trustworthy, and read the result as a thoughtful description of how two people meet, then decide for yourselves what to do with it. You can run two charts together with the compatibility reading.