Synastry compares two birth charts while keeping each person's placements separate. A midpoint composite combines corresponding placements into a new, symbolic chart. Both belong to astrological interpretation; neither can establish another person's feelings or determine whether a relationship will succeed.
Start with a question you can discuss together: how do you prefer to make plans, show appreciation or handle a disagreement? A chart may offer language for that conversation. The answers people give, their consent and their actual behavior matter more than a matching score.
Try the free synastry preview if you have confirmed birth times for both people. Read the limitations below before interpreting its result.
Synastry versus composite: keep the construction clear
| Method | What is calculated | How astrologers use it |
|---|---|---|
| Synastry | Angles between a placement in A's chart and a placement in B's chart; house overlays when the needed birth information is reliable | Separate symbolic themes for each person's part in the comparison |
| Midpoint composite | A midpoint between each matching pair: Sun with Sun, Moon with Moon, and so on | One symbolic pattern interpreted as a relationship chart |
A composite Sun is not either person's natal Sun. It is also not the sky at a shared birth moment or the date you first met. Do not use a composite placement as if it were a fact about one partner's birth chart.
Astrodienst's chart-type reference describes the distinction between two-chart comparisons and the midpoint method. These are definitions of astrological techniques, not evidence that the techniques predict relationship outcomes.
A worked two-person example
These are synthetic coordinates, invented to make the arithmetic easy to check. They are not a real couple, a customer's data or an ephemeris calculation for a specified date. Count longitude from 0° Aries, around the zodiac to just below 360°.
| Person | Placement | Zodiac longitude | Sign position |
|---|---|---|---|
| A | Sun | 350° | 20° Pisces |
| A | Moon | 80° | 20° Gemini |
| B | Sun | 10° | 10° Aries |
| B | Moon | 112° | 22° Cancer |
Compare A's Sun with B's Moon. The raw difference is 350° − 112° = 238°. The shorter separation around the circle is 360° − 238° = 122°. A trine's reference angle is 120°, so the deviation is 2°. With a chosen maximum orb of 3°, this example qualifies as a trine. With a 1° allowance, it does not. The coordinates have not changed.
Now reverse the planet names while keeping the people identified: A's Moon with B's Sun is 80° − 10° = 70°. This is 10° from a sextile's 60°, so it falls outside our example's 3° allowance. “Sun–Moon” and “Moon–Sun” are two different cross-chart comparisons; one cannot stand in for the other.
A's Sun and A's Moon have a shorter separation of 90°. That is a square within A’s own chart, not a synastry contact with B. Mixing natal and cross-chart aspects would change the meaning of the list.
The 3° allowance is a teaching choice, not a promise about this site's default orb table. Read the aspects and orbs guide for the distinction between an allowed tolerance and the displayed deviation. Comparing longitudes also differs from measuring the full three-dimensional distance between two objects in the sky; the Swiss Ephemeris manual documents the coordinate conventions.
Make a composite from the same example
Combine matching planets instead of making cross-chart aspects. The Suns at 350° and 10° are 20° apart across the zodiac boundary. Their shorter-arc midpoint is 0° Aries, not 180° longitude (0° Libra). The Moons at 80° and 112° have midpoint 96°, or 6° Cancer.
Those are composite positions. A's Sun trine B's Moon was a synastry aspect. Keeping the labels clear prevents you from reading one construction as the other. For a pair exactly 180° apart, the two arcs are equal; a midpoint method needs a convention to resolve that special case.
Turn an interpretation into a question
In astrological tradition, a Sun–Moon contact is often discussed through identity, habits and comfort. For our hypothetical trine, you might ask: “When do we feel understood, and what actually helped?” A close trine does not prove that understanding is automatic.
| Traditional focus | A question to discuss | Evidence to notice |
|---|---|---|
| Mercury and communication | Do you prefer time to think or an immediate conversation? | Whether each person gets a chance to speak and be heard |
| Venus and enjoyment | Which shared activity do you both want to do? | Expressed preferences, including the freedom to decline |
| Mars and assertion | How should we pause a disagreement and return to it? | Respect for the agreed pause and subsequent follow-through |
| Saturn and responsibility | What have we each agreed to take on? | Whether the agreement is realistic and consistently honored |
These are reflection prompts, not diagnoses of a partner. A square does not prove hostility; a trine does not prove safety. No contact can establish consent, prove attraction, excuse controlling behavior or certify relationship destiny. A person can disagree with an interpretation without being “less evolved.”
House overlays need reliable birth information
An overlay asks where a planet from one chart falls within the other person's houses. A's Venus in B's seventh house and B's Venus in A's seventh house are separate claims. They need separate checks.
The receiving chart's house boundaries depend on birth time, location and house method. A planet close to a cusp may change house when an uncertain time or a different method is used. The moving planet's position also depends on its owner's birth time, particularly for the Moon. Review the house-method guide before treating a house label as certain.
An overlay in a traditionally partnership-related house does not establish that someone wants a relationship with you. Ask them directly; do not infer their intentions from a sector of the chart.
What if one birth time is unknown?
The current synastry calculator requires confirmed birth times for both people. Its unknown-time option identifies a limitation; it does not calculate a noon-based relationship result. Do not turn a default or guessed noon into a recorded time simply to obtain a score.
Without a reliable time, Ascendant, Midheaven and houses are uncertain. The Moon may move enough to change its sign or an aspect over the possible interval. Even a planet that stays in one sign can change degree and orb. A confident-looking decimal does not make an uncertain input reliable.
You can explore each person's birth chart separately, using its unknown-time option where needed. That tool's representative-time planetary positions do not resolve the missing time or establish a full synastry result. Mark uncertainty before combining interpretations, and share someone else's birth information only with their permission.
What the current tools provide
The free synastry preview uses the tropical zodiac and preset aspect allowances. It shows an editorial score, the total number of included cross-chart aspects, and up to five selected aspects with their deviations. It does not include house overlays or the full chart metadata. The preview has no custom-orb or house-system controls. The shorter list is not proof that the other aspects are absent.
Enter each person's local birth date, confirmed time and birthplace carefully. Review prefilled details. If the form asks which of two repeated clock times applies during a daylight-saving change, choose from the birth record rather than guessing. A nonexistent local time needs correction.
The full synastry calculation in the main application's Synastry tab requires Pro access or a higher eligible plan. Its current form uses tropical positions and the default Placidus house method. That result includes cross-chart aspects and house overlays. Record the inputs and displayed settings when comparing it with another service.
The composite tool also requires Pro access for its full calculation. A free account alone does not unlock the full composite. A free synastry preview shown alongside access options is not a computed composite chart. Check the current access options before purchasing; this guide does not promise that a separate report purchase unlocks the calculator.
A score is not a probability
The displayed compatibility score is an editorial aspect-weighting heuristic, not a measured probability of relationship success. A score of 80/100 does not mean an 80% chance of a happy partnership, and a lower score does not mean you should leave. Different aspect selections and weighting rules can produce different summaries of the same positions.
Read the actual aspect names and orbs before the summary number. Note which chart each body belongs to and whether the birth information is reliable. Use the result for a question you can test through conversation, not to rank people's worth or decide for them.
Save a comparison and return to it
Sign in before calculating if you want to save the free preview. After calculation, choose “Save this synastry to my dashboard” and wait for the confirmation. Saving requires an account and is subject to its saved-chart allowance. The guest registration link does not guarantee that an unsaved comparison will survive registration; be prepared to return and calculate again.
Open your dashboard and choose “View comparison” on the saved synastry record. The view shows the stored result; merely opening it does not run a fresh calculation or silently update the original. A saved free preview remains a preview. The free form’s saved input format is not supported by the full-result recalculation action; enter both people again in the full tool if you need that calculation. An older record may not contain a calculation version or all settings, and missing information should stay visible as missing.
“Recalculate” is a separate action using the saved two-person inputs where they can be validated, and it follows the current Pro access rules. Review the new result before choosing “Save updated copy.” That saves a separate record and leaves the original intact. If a save's outcome is uncertain, check the dashboard before making another copy. None of these steps requires changing your individual birth profile.
The current Advisor uses one individual natal chart; this two-person comparison is not automatically sent to it as a relationship reading. Keep that distinction in mind when choosing a next step.
A manageable first reading
If two placements look close but have different sign names, the cross-sign conjunction walkthrough shows how to check their separation before interpreting the contact.
- Confirm both people's permission and birth details, including uncertainty.
- Identify one cross-chart contact and check its angle and orb.
- Add a house overlay only when its inputs support it.
- Write down one observation that fits the interpretation and one that does not.
- Ask one open question, then listen to the actual answer.
Return to the relationship learning path for the tools, or revisit aspects and houses when a result needs explanation. You do not need a verdict about the whole relationship to have one useful conversation.