Synastry: how astrologers compare two birth charts

Synastry is the way astrologers compare two birth charts. They place one chart over the other and look at how one person's planets touch the other person's, above all Venus, Mars and the Moon. The result describes how two people affect each other: where things feel easy and where friction comes from. It does not decide whether a relationship works. That part belongs to the two people in it.

What synastry is

A birth chart is a map of the sky for the minute and place you were born. If you are new to it, start with the Sun, the Moon and the rising sign. Synastry takes two of these maps and reads them together.

The word is modern, but the idea is old. The astronomer Ptolemy, who worked in Alexandria in the second century, wrote about comparing two people's charts for marriage in his book Tetrabiblos. He looked mainly at the Sun and the Moon in both charts and asked whether they stood at harmonious angles to each other.

Today astrologers look at every planet. They ask two main questions:

• Which planets in one chart form an aspect with planets in the other chart?
• Where do one person's planets fall in the other person's houses, the 12 areas of life in a chart?

Aspects between two charts

An aspect is an angle between two points on the zodiac circle. In synastry, one point comes from your chart and the other from the other person's chart. Astrologers allow a few degrees either side of the exact angle. This margin is called the orb. The smaller the orb, the stronger the contact.

Aspect	Angle	What it often describes between two people
Conjunction	0°	The two planets merge; strong and hard to ignore
Sextile	60°	Easy cooperation that grows when you put effort in
Square	90°	Friction that asks both of you to adjust
Trine	120°	Ease and natural understanding
Opposition	180°	Attraction and tension at once; you mirror each other

An example: your Venus is at 12° Libra and the other person's Mars is at 14° Libra. The two planets are 2° apart. That is a conjunction with an orb of 2°, which astrologers read as a classic sign of attraction.

Do not read "square" as "bad". A square often shows where you rub against each other. It also shows where you push each other to grow. A comparison full of trines is not a guarantee, and one full of squares is not a warning.

Venus, Mars and the Moon: the core contacts

Every planet counts in synastry, but three say the most about closeness.

Venus: what feels like love

Venus describes what you find beautiful and how you show affection. When one person's Venus touches the other person's Sun, Moon or Venus, astrologers expect warmth and a sense of liking each other as people. Tense Venus contacts can point to different ideas of what care looks like. One of you wants kind words; the other shows love through practical help.

Mars: desire, drive and conflict

Mars describes energy, desire and how you act when you want something. In astrology, contacts between one person's Venus and the other person's Mars are the best-known sign of attraction. Mars also shows how you argue. A square between two Mars placements can mean that you both push at the same moment, and that you need a fair way to take turns.

The Moon: feeling at home

The Moon describes emotional needs: what soothes you and what makes you feel safe. When one person's Moon sits well with the other person's Sun or Moon, daily life together tends to feel easy. You understand each other's moods without long explanations. Tense Moon contacts rule nothing out. They mean that each of you finds comfort in different things, and that it helps to say so out loud.

Two more planets are worth a look. Mercury shows how each of you thinks and talks. Saturn contacts often add a sense of commitment and duty, and sometimes a feeling of weight.

Houses: where you land in each other's life

If both birth times are known, astrologers also look at house overlays. Your planets land in the houses of the other person's chart, and theirs land in yours. If your Sun falls in their 7th house, the house of partnership, they may see you as a natural partner. If your Moon falls in their 4th house, the house of home, you may feel like family to them.

House overlays need both exact birth times. Without them the houses cannot be set, and a careful report says so.

The composite chart, briefly

Synastry compares two charts. The composite chart does something different: it builds one new chart for the relationship itself. For each pair of planets, it takes the midpoint between your position and the other person's position. Your Sun and their Sun give the composite Sun, your Moon and their Moon give the composite Moon, and so on.

Astrologers read the composite chart as the character of the relationship: its purpose (the composite Sun), its emotional climate (the composite Moon) and what it asks of you both. The astrologer John Townley published the first book on the method, The Composite Chart, in 1973.

What compatibility really means

No chart decides whether a relationship works. There is no score that says yes or no.

Research points the same way. In 2007, David Voas, a researcher at the University of Manchester, studied census records for about 10 million married couples in England and Wales. Couples did not pair up by zodiac sign more often than chance would predict. That study looked only at Sun signs, not at full charts. Still, it supports a simple point: one factor in a chart cannot tell you whether two people belong together.

Astrology is a language of symbols and a tool for self-reflection, not a proven prediction. Used that way, synastry is useful. It gives you words for patterns you may already feel. It shows where each of you needs something different. And it can turn "you never listen" into a calmer conversation about two different styles.

Compatibility, in the end, is something two people do. It lives in how you talk, how you repair things after an argument and how you respect what is different in the other person.

What helps. Read the comparison together. Each of you picks one contact that feels true and one that does not. Talk about both. The disagreement is often more useful than the agreement.

Couple versions in other systems

Astrology is not the only system with a way to look at two people. Skyspoken also offers couple reports in three other systems:

• Human Design. Human Design for couples lays both charts together into one connection chart. It shows channels that only appear when you are together, where one of you is consistent and the other open, and which compromises make daily life easier. If you are new to the system, read about the five Human Design types first.
• Numerology. Numerology compatibility sets both people's life path, expression and soul urge numbers side by side and adds a shared couple number. It needs only full birth names and dates of birth. Here is how the life path number is calculated.
• Destiny Matrix. The Couple's Destiny Matrix combines both dates of birth into one matrix of the 22 major arcana. It describes the purpose of the relationship, a shared money line and the lessons that repeat between you.

Each system uses different data and a different language. Where several of them say the same thing about you as a pair, it is worth paying attention.

How to start

1. Calculate both charts with the free birth chart calculator. Use the date, time and place of birth for each of you.
2. Note each person's Sun, Moon, rising sign, Venus and Mars.
3. Compare the signs. Are your Moons in the same element (fire, earth, air or water)? Do the signs of Venus and Mars sit well together?

That gives you a first impression. For the full picture, the Relationship compatibility (synastry) report compares the two charts point by point: attraction, communication, feelings, typical conflicts and the composite chart. It needs both people's date, time and place of birth. If one birth time is unknown, the report marks the Moon, the houses and the composite angles as approximate.