Synastry Basics: How to Read Compatibility Between Two Charts

Synastry is the core technique of relationship astrology: overlay two people's natal charts to see where one's planets fall in the other's chart and what aspects they form — revealing the attraction, fit, and tension between them.

This page is the map. Each contact below links to a full page on that specific placement.

What the overlay actually is

Nothing moves in either birth chart. A synastry chart is a way of asking two questions about two fixed charts:

• Where do your planets land in my life? That is the house overlay — your Venus falling in my seventh house, my Mars falling in your first.
• What angles do our planets make to each other? That is the aspect layer — your Saturn conjunct my Moon, regardless of houses.

These are different questions and they get confused constantly. House overlays tell you which area of life the other person activates. Aspects tell you how the two charts' energies combine. A pair can have strong overlays and almost no aspects, or the reverse, and the two relationships look nothing alike.

The contacts that carry the most weight

• Sun–Moon contacts: one person's Sun aspecting the other's Moon is among the most important signs of long-term fit — "your core" and "my emotions" echoing each other.
• Venus–Mars contacts: Venus (love and attraction) aspecting Mars (desire and action) brings romantic and sexual chemistry.
• Ascendant contacts: one person's planet on the other's Rising or Descendant often brings a strong "instant recognition."
• Saturn contacts: responsibility, commitment and testing — stabilising, but potentially heavy.

House overlays: which part of your life they activate

Seeing which of the other's houses one's planets fall into tells you which area of their life the relationship mainly activates. The seventh house is partnership itself, which is why it carries the most weight in relationship work:

• Sun in the 7th house — being cast as the partner, and how quickly certainty arrives
• Moon in the 7th house — the partner who reads as home
• Venus in the 7th house — the overlay most often called a marriage aspect, read honestly
• Mars in the 7th house — attraction and argument from a single source
• Saturn in the 7th house — weight, duration, and what both cost
• Pluto in the 7th house — intensity, power, and what actually changes

The eighth house handles depth and merging rather than partnership as a category, and the twelfth handles what stays submerged — see Mars in the 12th house for how differently a house one step away reads.

Other house overlays that matter

Beyond the seventh, four sectors carry most of the weight in relationship work:

• The 8th — depth, privacy and what gets shared. Venus in the 8th · Moon in the 8th · Sun in the 8th
• The 12th — what operates below awareness. Venus in the 12th · Moon in the 12th
• The 1st — the person themselves, not an area of their life. Mars in the 1st · Venus in the 1st
• The 5th — play, courtship and enjoyment. Venus in the 5th · Mars in the 5th

Contacts to the angles

The Ascendant is a point, not a house — the exact degree rising at birth. That makes contacts to it sharper than any overlay, and far more dependent on an accurate birth time:

• Venus conjunct Ascendant — the most reliable first-sight attraction contact there is
• Mars conjunct Ascendant — physical charge, and what it costs
• Moon conjunct Ascendant — instant familiarity, and its blind spot
• Sun conjunct Ascendant — visibility, which is not the same as comfort

Planet-to-planet contacts

These need no house system and — except where the Moon is involved — survive an unknown birth hour, which makes them the most robust part of a synastry reading:

• Pluto conjunct Venus — attraction with a gravity problem
• Mars conjunct Pluto — raw force, and where it gets pointed
• Saturn conjunct Moon — security or a cold room
• Venus conjunct Venus — the same taste, and a shared blind spot
• Moon conjunct Moon — one emotional clock between two people
• Moon conjunct Mars — feeling and force in the same place
• Moon conjunct Venus — the gentlest contact, and its ceiling
• Sun conjunct Venus — being liked for what you actually are
• Sun conjunct Mars — energising, or competing
• Jupiter conjunct Venus — generosity, and knowing where it stops
• Saturn conjunct Sun — taken seriously, or made smaller
• Venus square Mars and Sun conjunct Moon — the two classic chemistry contacts

How accurate do the birth times need to be?

This decides which half of a synastry reading you can trust, and it is usually skipped.

• Anything involving the Ascendant, Descendant or house cusps needs a real birth time. The Ascendant covers the whole zodiac in a day — an average of about a degree every four minutes, though the true rate varies with latitude and with the rising sign, and some signs rise far quicker. A time rounded to the hour therefore carries fifteen degrees of uncertainty at best, which is wider than any orb worth using.
• Moon contacts need a reasonable one. The Moon moves roughly half a degree an hour, so several hours of error shifts it by a few degrees.
• Everything else survives an unknown hour. Sun, Venus, Mars, Jupiter, Saturn and the outer planets barely move in a day.

If one birth time is unknown, the honest reading is the planet-to-planet layer only. Nothing is gained by computing houses from a guess.

Reading mindset

• Many soft aspects = easy rapport; many hard aspects = tension, not necessarily bad — challenge can bring growth and spark.
• Synastry reads "dynamics and tendencies," not a verdict on "whether you can be together"; the work of a relationship is always the final variable.
• Overlays are directional. Your Venus in my seventh house is not the same contact as my Venus in yours, and reading one as though it covered both is the most common beginner error.
• House overlays depend on house system. A planet deep in a house can sit in a different house under Placidus, whole sign or equal houses, and no method settles which is right. A reading resting on one borderline placement rests on a methodological choice.
• Advanced readers also consult the composite chart (the chart of the relationship itself), or compare it with Davison.

Deep Oracle does not publish a Western chart calculator, so you will need both charts cast elsewhere before using any of this. What we do publish is the sign-pair compatibility articles and the placements library, which covers all 110 planetary placements sign by sign.

(This is an interpretive framework for relationships — not romantic or psychological advice.)

Related reading

• Composite Chart vs Synastry: Which Relationship Analysis Is More Accurate?
• Your Big Three: Sun, Moon, and Rising Signs Explained
• Elements and Modalities: The Underlying Grid of the 12 Signs
• How Astrological Transits Work: When Planets Move Across Your Chart