Buying a Synastry Reading? Check How It Builds Both Charts

A chart-calculation claim should be checkable

A promise to calculate both birth charts should lead to a few plain answers. Which birth information will be used for each person? What chart settings will shape the result? What will the reader compare, and what will they leave out if a detail is unknown? Those questions let you look past the polished wheel and see what the service actually offers.

Keep two jobs separate. Chart construction turns a date, time and place into positions, angles and houses under stated conventions. Interpretation selects contacts between the charts and gives them astrological meaning. One can be clear while the other remains unexplained. A service might identify its calculation engine and settings but never say why it emphasizes one aspect over another. Or an astrologer might give a thoughtful reading from placements you supplied without claiming to calculate the charts.

The Swiss Ephemeris programmer’s guide provides a useful technical illustration. It describes converting a birth date and time to a Julian day, then calculating planetary positions from that time value. Its documentation also describes separate functions for house and angle work. It shows that chart construction has steps someone can name. It does not show that every service uses this library, and its technical detail cannot tell you whether an interpretation is wise.

A synastry article by Philip Graves offers a working picture of the next stage: set the two charts against one another, compare their factors, and keep track of which person’s planet contacts which. The astrologer’s procedure is one practitioner’s approach, not a rule every tradition must follow. That is precisely why a provider should be able to describe its own approach.

The useful standard is not a particular brand of software or one supposedly universal method. It is visibility. You should be able to tell what went into the chart, what rules shaped the comparison, and where the reading moves from calculated placement to interpretation.

Sources: Swiss Ephemeris Programmer’s Guide: Programming Steps to Get a Planet’s Position; Introduction to Relationship Astrology 2: Synastry (subjective compatibility factors)

What birth details does the calculation need?

For each person, check that the service asks for a birth date, a local birth time and a birthplace. Then check whether it distinguishes a recorded time from an estimate. Those details do different work. The date anchors planetary positions. The clock time and location matter directly for the local sky, angles and houses. If a provider says it calculates a complete chart but does not ask for time or place, ask what it means by complete.

A birthplace is more than a label on a map. A calculation needs a geographic position for house and angle work, and the named town can help establish the relevant local civil time. Some systems use city coordinates or a nearby location when exact coordinates are not available. That can be a reasonable convention, but the service should be honest about its input. A birth in a large city is not necessarily at the city-centre coordinates used by a calculator. Usually the difference will matter less than a missing time, but it remains part of the chain.

Birth time deserves a direct question. Does the form ask for the time printed on a birth record, a family recollection, or a rounded estimate? Can you mark the time as uncertain? Does the report preserve that distinction? If you remember only that someone was born early in the morning, entering 8:00 as though it were recorded changes an estimate into false precision. A careful provider will let you state what you know and will explain what that level of knowledge supports.

The IANA time-zone database explains why a historical clock reading is not always converted by subtracting today’s familiar UTC offset. Civil clock rules vary by place and date, and governments have changed them. A birth at 8:00 in a given town decades ago may not map to the same offset that town uses now. A sound workflow needs the historical local rule for the date, not a current offset pasted backward.

You need not demand a software audit. Ask the provider to confirm the recorded local time, birthplace and time-zone conversion it will use, and how it treats a time whose source is uncertain. If the answer is that it uses an estimated time, the next question is which features depend on that estimate. That is more useful than a general assurance that the chart is accurate.

Sources: Swiss Ephemeris Programmer’s Guide: Programming Steps to Get a Planet’s Position; Swiss Ephemeris Documentation; Theory and pragmatics of the tz code and data

How does local birth time become a chart?

A local clock reading has to be placed on a common time scale before a chart program can calculate positions for that moment. Then the software calculates planetary positions and, when requested, the local sky structure used for angles and houses. The stages connect, but they are not interchangeable. Ask enough to learn whether the service handles each one deliberately.

The Swiss Ephemeris programmer’s guide documents one route. It describes computing a Julian day from the birth date and Universal Time, then using an ephemeris function to calculate a planet or other body. The same body of documentation treats house and angle calculations as distinct functions, with local sidereal time and geographic coordinates among the relevant inputs. This is a transparent example of technical scope, not proof that another provider uses Swiss Ephemeris or should use it.

That difference matters when a report discusses house overlays. A planet’s zodiacal position and the receiving person’s house cusps are not generated from the same information alone. House systems are named conventions for dividing the chart; the Swiss Ephemeris documentation lists multiple supported systems, including Placidus and Koch. A provider can reasonably choose one approach. But if it interprets one person’s planets in the other’s houses, it should say which house frame it calculated.

Do not treat a different house placement as automatic evidence of a bad calculation. Two readers can use the same birth moment and planetary positions yet select different house systems. A planet near a cusp may fall into different houses under those choices. The interpretive emphasis can shift with it. The buyer’s practical check is simple: can the provider name the system, apply it consistently and explain why it belongs in this reading?

Zodiac choice also changes the language of the placements. Tropical and sidereal zodiacs use different reference frames, so their sign labels can differ while describing the same birth moment through distinct astrological conventions. Swiss Ephemeris documentation covers several sidereal options and ayanamshas; that breadth shows that a calculation engine can implement choices without declaring one choice universally correct. Ask whether the service uses tropical or sidereal placements, and, if sidereal, which ayanamsha it names.

A chart wheel often hides these decisions. A useful report makes them findable, either on the page or in a method note: time basis, zodiac, location handling, house system when used, and the bodies or points included. You do not need every internal setting to judge a reading. You need the choices that affect the placements and techniques the provider is selling.

Sources: Swiss Ephemeris Programmer’s Guide: Programming Steps to Get a Planet’s Position; Swiss Ephemeris Documentation; Swiss Ephemeris General Documentation; Theory and pragmatics of the tz code and data

What if one birth time is missing or approximate?

A missing time narrows a synastry reading; it does not automatically make every planetary comparison unusable. Ask which factors remain available from the known date and which time-dependent claims the astrologer will omit, qualify or calculate across a range. The answer should separate the parts of the chart instead of declaring the whole comparison either complete or impossible.

Planetary signs and many planet-to-planet contacts can often still be considered from a known date, depending on whether a planet moves across a sign or aspect boundary during that day. The Moon needs closer care because it moves more quickly than the other planets commonly used in a basic comparison. If the birth date sits near a sign or exact-aspect boundary, its degree or even sign may depend on the clock. The Ascendant and house cusps depend directly on birth time and place. Without a supported time, house overlays and Ascendant contacts should not be presented as settled facts.

There are several honest ways to handle a gap. The reader can omit angles and houses and focus on supported planetary placements. They can mark a reported time as approximate and flag the factors it changes. Or they can calculate a range across plausible times and retain only statements that hold through that range. The right choice depends on how much is known and on what the buyer wants from the reading.

One provider’s methods page describes a range-based practice for public profiles whose birth date is documented but whose time is not: it samples planetary positions across possible times and time zones, shows alternatives when a range crosses a boundary, and withholds rising signs and houses without a supported time. That is a declared method from one service, not an industry standard or an independent audit. It does illustrate the difference between showing uncertainty and silently assigning noon as though noon were the birth time.

Noon charts can have a limited role as a convenient reference point for some date-based positions, but a convenience must remain labeled as such. It cannot supply a genuine Ascendant or reliable house frame. Likewise, an astrologer may have a reason to use a remembered family estimate or a rectified time, but should identify that choice and avoid presenting its time-sensitive output as if it came from a birth record.

If one person’s time is unknown, ask for a narrower reading of the placements both people can support. You might still compare the Moon, Venus and Mars where their signs and degrees are known, while setting aside house overlays. If the exact Moon or another placement could change during that date, ask the provider to show that uncertainty before interpreting it. A smaller reading can be more useful than a complete-looking wheel built on a guess.

Sources: How Synastry calculates and explains your chart; Introduction to Relationship Astrology 2: Synastry (subjective compatibility factors); Swiss Ephemeris Documentation

What calculation choices should a provider name?

Ask for the conventions that shape the chart the provider will actually read: tropical or sidereal zodiac, the planetary bodies and points included, the house system if houses matter, and whether the report can show placements and degrees as well as a wheel. These answers let you understand what ‘both charts’ means in that service.

A calculation engine and an interpretive tradition answer different questions. An ephemeris is used to calculate positions. A zodiac convention sets the frame used for sign labels. A house system divides the local chart into houses. The astrologer then decides which factors carry weight and what those contacts signify. A provider may use a familiar library, a different engine, or a custom method. Naming a library helps make the calculation traceable, but does not by itself establish the quality of the reading.

Look for scope as well as settings. Does the comparison include the Sun and Moon, personal planets, outer planets, lunar nodes, asteroids, angles or house rulers? There is no reason every reading must use every available point. A focused method can be coherent. The problem is a mismatch between promise and delivery: for example, marketing a full chart-to-chart analysis while showing only a few sign matches without saying that the comparison is selective.

Degrees matter when a report claims to calculate aspects. An aspect is an angular relationship between two chart factors. Its exact angle is the ideal geometry; the orb is the permitted distance from that exact angle. A report that lists ‘Moon trine Venus’ without degrees may be difficult to inspect, especially when the relationship is close to the service’s inclusion threshold. Asking for degrees or an orb rule helps you see why a contact made the cut.

The method may also distinguish a synastry comparison from a composite chart. Synastry keeps two natal charts in view and considers contacts between them, including house overlays when time and settings allow. A composite chart constructs a separate chart from points between the two charts. The contemporary explainer Synastry Explained presents these as different techniques with different questions. If a service blends them, ask which part of the deliverable uses which method.

A useful provider answer does not need to be a technical paper. It should name the conventions that materially affect the promised reading and make the output inspectable enough to compare like with like. That leaves room for different astrological traditions while giving you a fair basis for choosing among services.

Sources: Swiss Ephemeris Programmer’s Guide: Programming Steps to Get a Planet’s Position; Swiss Ephemeris Documentation; Swiss Ephemeris General Documentation

Which interpretive rules shape the result?

Once the charts are constructed, interpretation begins. Ask which contacts the astrologer prioritizes, what orb rules they use, whether they read house overlays in both directions, and how they handle contacts that seem to point in different directions. A calculation can produce a list. It cannot explain why one item deserves more attention than another.

An orb is not just a software setting. It is part of an astrologer’s working method: the allowed distance from an exact aspect before they treat the contact as active. Graves’s synastry article proposes tighter, aspect-specific allowances for several major and minor aspects, including a six-degree cap for conjunctions and smaller limits for other angles. Those numbers belong to his method. They should not be borrowed as a universal rule, but the article makes the practical point that a reader can declare different allowances for different aspects.

The number of contacts also depends on which bodies are compared. Put more planets, points and angles into two charts, and more possible pairings become available. A broad orb will include more of them than a narrow one. A useful reader explains how they reduce that field to a manageable set rather than treating every listed contact as equally important. A long report is not automatically a deep one.

Astrologers also differ in what they place first. Sira Nur Uysal’s account of classical relationship astrology gives priority to Venus and Mars, the Sun and Moon, and each chart’s seventh house and ruler. It contrasts that hierarchy with modern approaches that may weigh planets more evenly. This is one astrologer’s account of a tradition, not an exhaustive history or a mandatory ranking. Its value for a buyer is that it makes the interpretive frame explicit.

A contemporary explainer, Synastry Explained, highlights another familiar set of contacts, including Sun–Moon, Venus–Mars and Moon–Moon, and describes the role of cross-aspects and house overlays. It also cautions against reducing a comparison to a percentage. That source is an editorial explainer, not a survey of all practitioners. Taken beside the classical account and Graves’s narrower orb method, it shows why a disclosed approach is a sounder comparison point than a single score or an aspect count.

Look for a reading that can hold tension without forcing a verdict. One contact may suggest ease while another asks for adjustment. The astrologer can explain what each contributes, why one factor takes priority in the chosen tradition, and what context might change the emphasis. If the answer is simply ‘the chart says you are compatible,’ the method has been reduced to a conclusion without showing its path.

Sources: Introduction to Relationship Astrology 2: Synastry (subjective compatibility factors); What Is Synastry? Classical Relationship Astrology; Synastry explained: how two birth charts are compared

What should make you pause before paying?

Pause when a provider will not identify its birth inputs or conventions, treats a guessed time as certain, or mixes calculated placements with interpretation without showing where one ends and the other begins. These are not small presentation issues. They affect what chart is being read and how the conclusion was reached.

A second reason to pause is a compatibility score with no visible method. A percentage is not a planetary position; it is a summary created by selecting and weighting factors. Synastry Explained argues that scores vary with the contacts included and how they are weighted. That is an interpretive observation from one explainer, not a market-wide audit. Still, before relying on a number, ask what it counts and whether the service can explain how conflicting indications affect the result.

Be cautious of an aspect list that provides no orb rules or of house overlays with no named house system. An astrologer may use wider or narrower orbs than another astrologer and may choose a different house frame. Those differences do not prove an error. They do make the output hard to compare when the service will not tell you what it chose. Ask for clarification once. If the answer stays vague, you have learned something about the fit between the service and your expectations.

Polished graphics, a large page count and technical vocabulary do not answer those questions. A striking wheel might simply present the result well. It cannot tell you whether the service received a reliable time, how it handled the local clock, or which contacts it interpreted. Look for a method note or a direct answer from the reader instead of treating surface detail as evidence of scope.

The strongest exception is a deliberately narrow reading. A service that compares placements you provide may be worthwhile even though it does not calculate complete natal charts. It should say what placements it uses, whether degrees are needed, which aspects it selects, and what it leaves out. A limited offer that describes its boundaries plainly may suit you better than a full-chart promise built around uncertain data.

That distinction matters for this publication’s Synastry Comparison Report. It uses supplied Moon, Venus and Mars signs, optionally with degrees, and selected major aspects under a six-degree orb. It does not calculate natal charts from birth details or include houses. That scope can help someone compare known placements; it is not a substitute for a service that calculates two charts from birth information.

Sources: Synastry explained: how two birth charts are compared; How Synastry calculates and explains your chart; Swiss Ephemeris Programmer’s Guide: Programming Steps to Get a Planet’s Position

Ask for the reading your information can support

Before paying, send one practical question: ‘Which date, time and birthplace will you use for each chart, which zodiac and house system shape the reading, and what will you leave out if either birth time is uncertain?’ If the answer is specific, compare that scope with the reading you want. If it is not, wait or choose a narrower service.

Then ask how the astrologer selects and ranks contacts. A clear answer might name the planets or angles it emphasizes, its orb rules, and how it reads two-way house overlays when the birth times support them. You do not need the astrologer to use your preferred hierarchy. You need to know what hierarchy you are buying, and to see that it is applied consistently.

If both times are well supported, a wider chart comparison may suit your question. If one time is missing, ask whether a planet-to-planet reading can answer what you want while leaving houses and angles aside. If the Moon or another placement sits near a boundary, request a range or a clear note about the alternatives. The useful choice depends on the available information and your purpose, not on the size of the report.

Even a transparent calculation does not make every interpretive choice universal. Synastry offers a way to consider how two charts meet through a particular astrological tradition. It can help you frame questions about emotional style, affection, desire or friction. It cannot make the decision to continue a relationship for you. That decision belongs to the people living it.

If you already know your Moon, Venus and Mars placements, you can use the Synastry Comparison Report to compare those supplied signs and, optionally, degrees. It examines selected major aspects with a six-degree orb. It does not calculate either natal chart from birth details or read houses. Choose it for that defined comparison, not because it claims the broader service you are evaluating.

Ask the provider, get the method in plain words, and match the reading to the data you can stand behind. That is a better first step than asking a chart to decide what the two of you should do.

Sources: Swiss Ephemeris Programmer’s Guide: Programming Steps to Get a Planet’s Position; Introduction to Relationship Astrology 2: Synastry (subjective compatibility factors)