Synastry Principles: Comparing Two Natal Charts

“Are we compatible?” is one of the most common questions in astrology consultations. Before learning synastry, set aside one familiar habit: answering it with a Sun sign compatibility table. Real synastry is far more detailed than “Aries with Libra.”

This lesson turns that intuitive question into a repeatable method. You should already be able to read a natal chart, the prerequisite for this course. Synastry literally means looking at stars together. Its central action is simple: overlay two natal charts and examine the cross-chart aspects between one person’s planets and the other’s.

What synastry examines

Each person’s natal chart is a unique map of energy. When two people enter each other’s lives, those maps overlap: your Moon might contact their Saturn, or your Venus might fall in their 7th house. These intersections describe real patterns of interaction between the two people.

Synastry does not describe “whether you will get together,” but “how energy flows when you are together.” It can explore where attraction comes from, what underlies conflict, where understanding comes naturally, and where conscious effort is needed.

It also extends beyond romance. Parent–child, friendship, and workplace synastry can reveal both ease and undercurrents. You may talk effortlessly with one person yet struggle to connect with another; synastry aspects often describe those experiences.

Why Sun sign compatibility is not enough

Sun sign compatibility considers only one dimension. A chart contains ten planets, twelve houses, and dozens of aspects. Overlay two charts, and the possible combinations multiply dramatically.

That is why one Aries–Libra couple is affectionate while another falls into cold wars. Two couples with the same Sun signs can have completely different Moon, Venus, and Mars placements, as well as different Saturn cross-aspects. Those details shape what being together feels like.

In the following two-wheel chart, the inner wheel is one person’s natal chart and the outer wheel is the other’s. First, see what Sun sign matching leaves out:

Three principles of synastry analysis

Keep these three principles firmly in mind before any reading. They apply throughout the course.

Principle one: understand each natal chart first. This essential step is easily skipped. Without understanding your Moon sign’s emotional needs, Venus’s relationship style, and Mars’s way of acting, synastry is just lines between symbols. Learn to interpret one chart before interpreting two charts together. This is why natal astrology is the prerequisite.

Principle two: use tighter orbs. In natal charts, astro.how uses aspect orbs of 7° for conjunctions and oppositions, 6° for trines and squares, and 4° for sextiles. In synastry, keep cross-chart aspects within 3° (2° for sextiles). More exact aspects tend to be felt more clearly; those wider than 3° are usually background influences. The next lesson explores this further.

Principle three: consider the whole chart; no single aspect gives a verdict. Declaring incompatibility from Saturn square Moon, or a perfect match from a Venus conjunction, oversimplifies. Supportive aspects may ease difficult ones, while other tensions may offset a pleasant link. Synastry is a complete picture, never a single brushstroke.

Cross-chart aspects: the basic unit of synastry

The smallest practical unit that brings these principles together is the cross-chart aspect.

Measure the angle from one of your planets to one of the other person’s planets. For example, your Venus in Aries at 10° and their Mars in Leo at 12° are about 120° apart, forming a trine. “Your Venus trine their Mars” describes an easy flow between how you experience attraction (Venus) and how they act and pursue (Mars).

The entire analysis consists of finding meaningful cross-chart aspects and combining them into a coherent relationship story. Later lessons examine different planet groups (Moon, Venus–Mars, Sun–Moon, Saturn, and the outer planets), but each uses this same action.

Synastry’s limits: charts do not decide for you

One point matters more than any technique: synastry shows energy patterns, not a verdict on fate.

Two people with many difficult aspects may build a better relationship through honest communication than a couple with beautiful aspects who make little effort. Synastry cannot tell you whether to be together; it can suggest what needs attention if you are. It cannot justify unhealthy relationships, either. “Our Pluto aspects mean we are destined to torment each other” is avoidance, not astrology.

Astrology is a mirror that helps you notice existing relationship patterns. What you do with that awareness remains your choice.

Use the whole-chart approach with a combination of difficult and supportive aspects:

Now that you understand the basic logic, the next lesson examines orbs: how close an aspect needs to be, and how to decide which contacts deserve attention.