How to Update a Synastry Comparison After Correcting Birth Data

A corrected chart deserves a new audit, not a patched paragraph

When one person's birth data changes, update the synastry comparison by recalculating the chart factors and cross-chart contacts that depend on that input. Keep interpretations that still hold, revise those that changed, remove claims the new comparison no longer supports, and mark unresolved findings as conditional. Then rewrite the reading around the corrected pattern. The practical question isn't whether the old reading was wholly right or wrong. It's which parts remain supported, and what useful conversation follows from them. Synastry compares two natal charts and reads the contacts between them. Cafe Astrology's interchart-aspect guide treats each contact as a relationship between factors in two charts, with each person's planet retaining its role. That makes an input audit stronger than editing only the sentence that mentioned the correction: one changed factor can affect several contacts, while other parts may remain intact. A changed date or unresolved time conversion can call for a broader rebuild than a correction that leaves the calculated moment unchanged. Record what changed before deciding what to preserve. Compare the old and corrected versions, sort each finding as retained, changed, removed, or conditional, and let the revised evidence set the reading's emphasis.

Sources: Aspects Between Charts

First identify which input changed

Before changing the interpretation, write down exactly what was corrected: the calendar date, the recorded local clock time, the birthplace, or the conversion from local time to universal time. Note the old value, the corrected value,. Keep the chart settings fixed while you compare versions, including the zodiac, selected aspect set, and house method wherever houses enter the reading, where relevant. These inputs have different jobs. A date correction calls for recalculating the planetary placements for the corrected date and reviewing every dependent contact. It does not mean every planet changes sign; check the new positions. A local clock-time correction changes the moment used for calculation once the local time has been converted correctly. Recheck planetary degrees, then review time-sensitive angles and houses. A birthplace correction changes the geographic coordinates used for location-dependent calculations. If the corrected place lies in a different civil-time region, it may also change the conversion to universal time. Place coordinates and time-zone rules are related to location, but they are separate inputs. | Correction | First items to recalculate or verify | | --- | --- | | Date | Planetary placements and contacts that depend on them | | Local clock time | Time conversion, planetary degrees, angles, houses, and dependent contacts | | Birthplace | Coordinates for angles and houses; time-zone conversion if the place changes the applicable rule | | Time-zone or daylight-saving conversion | Universal moment, planetary positions, angles, houses, and dependent contacts | The table is a dependency map, not a promise that each item will change. Swiss Ephemeris documents house calculations as taking Universal Time, geographic latitude and longitude, and a selected house method. when time or place changes, recalculate the location-sensitive chart factors from the corrected inputs instead of estimating the result from the size of the correction. That size alone tells you little. A small clock adjustment can matter if a degree-based contact sits close to the report’s chosen orb boundary, or if a time-sensitive angle or house cusp shifts across a relevant point. A larger adjustment need not erase every planetary sign description. Without the corrected data and a recalculation, no one can say which specific contact changes. Also separate a local-time correction from a time-zone correction. The first changes the recorded clock value; the second changes how that clock value maps to the universal moment. IANA’s time-zone documentation describes civil-clock histories by location and warns that its pre-1970 coverage is incomplete for many places. For an older birth, an unresolved historical offset should remain an open input rather than being replaced with a modern offset by guesswork. which chart factors depend on that input, and which old claims still survive after those factors are recalculated?

Sources: Swiss Ephemeris Programmer's Manual; Theory and pragmatics of the tz code and data

Resolve local time before comparing charts

Convert the corrected recorded clock time using the civil-time rule for that date and place, then compare charts from the resulting universal moment. A birth record gives local time; a chart calculation needs a moment on a shared time scale. Don’t apply today’s UTC offset to an old date, and don’t add or subtract daylight saving by habit. First establish what the recorded time meant where and when the birth occurred.

The sequence is simple: read the local clock entry, determine the applicable historical offset and any daylight-saving rule, convert that time to Universal Time, then calculate the chart. Swiss Ephemeris documents planetary position functions that take Universal Time and house functions that take Universal Time together with geographic latitude and longitude. Its manual also states that computing Universal Time from local time, including determining the relevant time zone, falls outside the ephemeris module’s own job. The time conversion is therefore a separate step, and an error there carries into the chart that follows.

For a recent date with a clear record, the location’s time-zone history may settle the conversion. The IANA time-zone database describes location-based clock histories and transitions, including daylight-saving changes. But it warns that its historical coverage before 1970 is incomplete: local practices varied, records can be sparse, and some entries are based on uncertain evidence. For an older birth, treat a database result as one piece of evidence. Compare it with reliable records tied to the place and period, such as civil rules or contemporary local documentation, when those are available. If sources leave two plausible conversions, keep both in view rather than quietly choosing the one that produces the tidier chart.

A repeated hour during a clock change creates a particular problem: the same written local time can correspond to two different universal moments. A registry may also have used a local convention that its record does not explain. Don’t invent a resolution from the city name alone. Record the ambiguity, identify the possible conversions, and calculate each version under the same chart settings. A close contact that appears in only one version is conditional on the time conversion; it isn’t yet a stable basis for rewriting the reading. If the relevant placements and contacts remain the same across both, those findings can stay in the comparison while the uncertain layer remains open.

Separate a corrected place label from a corrected time-zone conversion. If the birthplace name or coordinates were wrong but the recorded moment in Universal Time is independently verified, update the location-dependent calculation while holding that moment fixed. If correcting the place changes which historical civil-time rule applies, revisit the conversion too. The two changes can travel together, but they are not the same input. Write down the recorded local time, date, place, conversion source, resulting Universal Time, and any unresolved alternatives. Then compare the resulting charts. That record makes clear which changed contact follows from the correction and which conclusion still needs better evidence.

Sources: Theory and pragmatics of the tz code and data; Swiss Ephemeris Programmer's Manual

Which parts of the corrected chart need a fresh calculation?

Recalculate the corrected person's placements and angles first, then rerun every cross-chart contact that uses a changed degree or point. A same-date time correction may leave many sign themes intact, but verify that against both chart versions. A sign describes a broad placement; its degree helps judge exact aspects. The sign can stay the same while the degree shifts enough to change whether a close contact falls within the chosen orb. Near a sign boundary, even the sign label may change. So don't preserve a degree-based interpretation just because its sign-level description still sounds familiar. Swiss Ephemeris's programming interface documents separate calculations for planetary positions and house cusps: its planetary functions use a date and time, while its house functions take Universal Time and geographic coordinates. First verify the corrected moment and location. Then calculate the planet positions and the time-sensitive angles or cusps that the correction could affect. Finally, check each synastry contact that depends on a changed factor. This checks dependencies; it doesn't claim every birth-time correction moves every planet meaningfully. The actual before-and-after positions decide. Use a small worksheet for the corrected person's relevant factors. Record the input that changed; the old position; the corrected position; whether the sign stayed the same; whether the degree changed; whether an angle or house assignment is affected; and which cross-chart contacts depend on that factor. Leave a field unresolved when the underlying birth information doesn't settle it. The worksheet keeps a chain from source data to chart factor to interpretation, so you can see why a line in the old reading survives or needs revision. For example, if a planet keeps its sign but shifts in degree, retain the sign-based description only as a provisional layer. Recheck its angular distance from the other person's relevant planets or points before keeping any aspect claim. If the recalculated separation still meets the report's declared orb, that contact remains available for interpretation under the same method. If it no longer does, remove it from the active contact list, even if the old description once felt central. If the sign changes, revisit both the sign interpretation and the dependent contacts. No example degree is needed; the rule follows from comparing the two calculated positions. Cafe Astrology's Free Natal Chart Report: Equal Houses asks for birth date, exact birth time, birth city, and UTC offset. It notes that the Ascendant anchors its equal-house cusps, and that an unknown birth time excludes several time-sensitive points and house positions from its report. This shows why a correction's reach depends on what changed: a time or place correction calls for renewed attention to angles and houses, while a verified moment feeds planetary calculations too. Keep those layers distinct. A changed house placement doesn't, by itself, erase an aspect between two unchanged planetary degrees; likewise, a stable sign label doesn't prove a close aspect survived. The useful result is a map of what remains supported, not an assumption that the old chart was wholly right or wholly wrong. Preserve sign themes that the corrected calculation confirms. Recalculate every degree-based contact involving a changed position, and revisit angles and houses when their time or location inputs changed.

Sources: Swiss Ephemeris Programmer's Manual; Free Natal Chart Report: Equal Houses

Rebuild the two-chart comparison, not just the corrected natal chart

Keep the unchanged person’s chart fixed, replace the corrected person’s verified chart factors, and rebuild the cross-chart comparison. Updating only the corrected natal chart leaves the central question unanswered: which contacts between the two charts still exist? Synastry reads one person’s planets and points in relation to the other person’s, so every contact has two owners. In “Aspects Between Charts,” Cafe Astrology makes that distinction explicit and advises readers to consider the roles of both people. Preserve that two-person structure as you revise. A new chart for one person is an input to the comparison, not the finished comparison. For each contact that depended on a corrected factor, follow the same sequence. First, name the two chart factors and their owners: for example, Person A’s Venus with Person B’s Mars. Next, use the corrected degree to find their angular separation, then identify whether that separation falls within the aspect and orb rules you have chosen. Only after that should you interpret the pair in sign context. This order separates three questions that can otherwise blur together: what the chart positions are, what geometric contact they make, and what that contact means in the reading. The “Synastry” guide likewise discusses planetary contacts alongside signs and other chart context; it does not treat an aspect name as the whole interpretation. Keep the same method for the old and revised versions so the comparison shows the effect of the corrected input. One changed position can affect several contacts. If a corrected Venus degree is compared with several planets in the other chart, each separation involving that Venus needs a fresh check. A contact may remain, move closer to exact, move farther away, fall outside the selected orb, or qualify as a different aspect. Those are distinct outcomes, and the ledger should record them separately. By contrast, a contact formed only by two unchanged factors remains mathematically untouched by this particular correction. Keep it in the revised list, while still checking that the old report used the same settings. This dependency view prevents two opposite mistakes: carrying forward every old contact as if nothing changed, or discarding the entire comparison because one person’s data changed. Then revise the interpretation, not just the contact inventory. Cafe Astrology’s “Aspects Between Charts” recommends considering what each person contributes through the planet involved and how sign and house placements shape the interaction. That gives the reader a useful way to preserve the relationship between the geometry and its meaning: “A’s Venus contacts B’s Mars” is a chart statement; the interpretation describes how those Venus and Mars themes may meet, with the signs adding style. If the corrected factor removes a contact that anchored the earlier reading, the emphasis may need to shift even if other contacts survive. Do not force the old headline onto the new pattern, and do not replace it with a verdict or score. A composite or Davison chart is a separate relationship-chart technique, not a substitute for recalculating these two-chart contacts. The clean revision is simple: retain independent contacts, recheck every contact dependent on the changed factor, and write the synthesis from the resulting pattern.

Sources: Aspects Between Charts; Synastry

Hold the comparison method constant

Use the same zodiac, selected aspect set, house system where relevant, and orb ceiling in both versions of a synastry comparison. Changing them with the birth data obscures which change altered the reading. Keep the method steady first. Then assess what the corrected chart actually changed. An orb is the distance between the exact angle of an aspect and the measured angle between two chart factors. For example, a conjunction is exact when the factors occupy the same zodiacal degree; a separation from that exact alignment is its orb. A report that accepts contacts only up to six degrees draws a boundary: a contact inside it qualifies for that report’s selected major-aspect list, while one beyond it does not. That boundary is a working convention, not a law shared by every astrologer or chart service. The Synastry Comparison Report uses a six-degree maximum for selected major aspects. It is this comparison’s rule, not a universal standard. This matters when a corrected birth input shifts a planet’s degree. The aspect’s measured separation may move closer to exactness or farther away. If it crosses the report’s six-degree limit, the contact can disappear from or enter the list even though the aspect type and both planets remain the same. That is a data-driven change. But if the first chart used a wider orb and the revised chart used six degrees, a contact may vanish because the rule changed, not because the corrected data changed its geometry. Record both the measured separation and the chosen limit when explaining a changed label. Keep other settings visible too. The zodiac determines the sign framework and the positions being compared; switching zodiac systems can relabel placements and alter the comparison. The aspect set determines which angular relationships count as candidates. A list limited to conjunctions, sextiles, squares, trines, and oppositions will not match one that includes additional aspects. These choices define the list, not a verdict on the relationship. Cafe Astrology’s “Aspects Between Charts” guide likewise treats interchart contacts in context: it asks the reader to consider each planet’s sign and house, and which person’s planet supplies each role. Its “Synastry” guide discusses multiple layers of comparison, rather than making one aspect label carry the whole reading. House systems matter when you compare house placements or overlays, because the house framework is part of that calculation. They don’t explain a changed planet-to-planet angular separation. Keep house settings fixed when auditing overlays; they don’t account for an aspect-list difference. Separate the two questions: did the corrected inputs change the contact’s geometry, or did a changed setting change what the report displays? When two calculators disagree, compare their settings before choosing between their interpretations. Match the zodiac, aspect types, orb limit, and, for house claims, the house system. Then check whether the same contact appears under those shared rules. A label alone isn’t the interpretation. The useful conclusion is the one that states its method, names what changed, and gives the reader a fair basis for comparing the old and corrected readings.

Sources: Aspects Between Charts; Synastry

Recalculate overlays only when the corrected chart supports them

A house-overlay claim depends on the chart whose houses receive the other person's planets. If a correction changes that receiving chart's recorded time or place, recalculate its angles and house cusps before keeping the overlay interpretation. If the corrected time remains uncertain, mark the claim conditional or leave it out. That doesn't require discarding every other part of the synastry comparison. A planet-to-planet aspect and a planet-in-house overlay answer different questions, so each can have a different level of support. House overlays have direction. In the method described by “Romantic Compatibility Synastry House Overlays,” one person's planets are placed against the other person's houses, showing where the first person's chart factors fall in the second person's chart. Reverse the charts and you ask a separate question: where do the second person's planets fall in the first person's houses? The source recommends reading both directions because the two placements need not be symmetrical. Name both people in every note: whose planet and whose seventh house. This distinction makes the correction easier to contain. Suppose the birth time being corrected belongs to the person whose houses receive a partner's planets. The overlay assignments may change if recalculation changes that person's house cusps. The overlay source itself cautions that uncertain or unknown birth times limit this layer. By contrast, if the corrected time belongs to the person supplying the planets, recalculate those planetary positions from the corrected moment; a changed degree can affect where a planet falls relative to the other chart's houses, and a boundary crossing may change its sign. A time correction alone does not prove a particular placement changed; compare the recalculated factors. Swiss Ephemeris documentation makes the technical reason clear: its house calculation takes Universal Time and geographic latitude and longitude as inputs. The result belongs to a specified moment and location. So first resolve which time and place the correction establishes, then regenerate the relevant house structure and overlay assignments. A place correction at the same verified moment still calls for recomputing location-dependent houses and angles. If the place also changes the time conversion, check the moment too. The calculated cusps and placements decide which overlays remain. Keep the aspect ledger separate while doing this. If the corrected chart still supports a planet-to-planet aspect under the comparison's chosen method, uncertainty about a receiving house does not erase that contact. Record the aspect as supported, and record the overlay as conditional or unresolved. The reverse also applies: a stable house overlay does not establish that a close aspect exists. The layers can inform one another, but neither substitutes for the other. A useful revision note names the planet owner, the house owner, the corrected input, and the recalculated house placement. Then state whether the placement holds for the verified input or only for one plausible time. Interpret the overlay as a symbolic area of emphasis, not a promise about how either person will behave. The source offers an interpretive approach, not a guaranteed outcome. If house data remain uncertain, keep any independently supported aspect reading and withhold the house-specific language. That leaves the revised comparison more precise without pretending it contains more certainty than the birth record allows.

Sources: Romantic Compatibility Synastry House Overlays; Swiss Ephemeris Programmer's Manual

What if the corrected time is still a range?

Calculate or verify the plausible endpoint charts, then classify each interpretation as stable across the range, conditional on part of it, or unsupported. Don’t choose a single convenient time and present its chart as certain. A range shows which parts of the comparison can be read now and which depend on a more precise birth time. Record the earliest and latest times supported by the available source. Keep the date and birthplace attached, and note how each local time was converted to Universal Time. If the source supports only a broad interval, preserve it; don’t narrow it to a minute for a cleaner chart. The Swiss Ephemeris programming documentation identifies Universal Time as an input for planetary calculations and pairs it with geographic coordinates for house calculations. Change the uncertain input across its supported range while holding other inputs and chart settings constant. At each endpoint, compare the corrected person’s planetary signs and degrees, angles, and house cusps. Recalculate relevant contacts with the other person’s unchanged chart. Record what stays the same and what changes. A contact inside the chosen orb at both endpoints is stable across them. If it falls inside at one end and outside at the other, mark it conditional. The same applies to a house overlay: the House Overlays guide describes planets falling into another person’s houses, so if the receiving chart’s cusp changes across the range, that overlay isn’t settled. Endpoints show boundaries, not every value between. If the interval is wide, or a planet, angle, cusp, or orb is near a boundary, compare intermediate times too. A sign can remain fixed while a degree-based contact changes; a cusp crossing can change an overlay while the planet’s sign stays put. State a dividing point only if the chart has actually been recalculated there. Otherwise, say the result varies somewhere within the supported interval. Historical clock conversion may add uncertainty. The IANA time-zone database’s “Theory and pragmatics of the tz code and data” explains that its records are location-based, but warns that pre-1970 coverage is incomplete and may not represent every local practice. For an earlier birth or uncertain clock history, treat competing conversions as separate plausible inputs. Don’t select one because it produces a more compelling interpretation. If independent records resolve the conversion, remove that branch and rerun the comparison. Use three labels in the revised reading. **Stable** means the interpretation holds across every supported time and conversion. **Conditional** means it applies only to part of the range, with the condition named. **Unsupported** means the available inputs no longer establish the claim. This doesn’t mean the symbolic reading is false; it means this comparison cannot carry it now. Keep supported sign themes and contacts, but set aside uncertain angles, houses, or close aspects. The result can still guide reflection. Keep the durable pattern, name the conditional part, and identify what record or narrower time range would settle it. If no better time source exists, preserve the uncertainty instead of rectifying a chart from the relationship story.

Sources: Theory and pragmatics of the tz code and data; Swiss Ephemeris Programmer's Manual; Romantic Compatibility Synastry House Overlays

Rank surviving contacts by exactness and interpretive relevance

Once both charts have been recalculated, rank the contacts that remain by exactness, the planets or points involved, and the role each plays in the larger pattern. Then give equal care to what disappeared. A removed contact may have carried the old reading's central theme; if it no longer fits the corrected inputs, that theme needs to be reconsidered even when no new aspect takes its place. The aim is a faithful revision, not a defense of the first interpretation. Start with a four-status ledger: retained, changed, removed, and added. For each entry, record the two factors, their owners, the aspect, and the orb under the same declared rules used for both chart versions. “Retained” means the contact still qualifies under those rules. “Changed” means the contact remains but its closeness or contextual reading shifted. “Removed” means it no longer meets the chosen aspect and orb criteria. “Added” means the corrected positions now form a qualifying contact. Next, sort the retained contacts by orb, from closest to wider. Orb describes how far the measured separation is from the exact angle of an aspect. Cafe Astrology’s “Aspects Between Charts” advises readers to consider the roles of both people’s planets and the signs and houses involved; its “Synastry” overview likewise presents interchart aspects within a broader comparison of placements. Read the pair from both sides. Ask what the first person’s planet symbolizes in the interaction, what the second person’s contacted planet brings, and how their signs shape the expression. A Venus contact and a Mars contact do not carry interchangeable themes, and the same aspect can be described carelessly if the reading drops the distinction between whose factor is whose. The source guide specifically cautions that each person can take up the role of their own planet in that exchange. Now identify which contacts previously anchored the interpretation. If one of them was removed, name the consequence plainly: perhaps the old reading leaned heavily on a particular tension or ease, and that emphasis is no longer supported by the corrected comparison. Ask whether the new contact changes the balance or only adds one note. A tight aspect is not automatically more important than every wider one. Traditions differ in how they weigh planets, aspects, signs, and houses. Treat exactness as a ranking aid, not a verdict. Stable sign themes can remain part of the reading when degrees shift, while a degree-based aspect near the chosen orb limit may need a conditional label. House interpretations also require reliable time and place inputs. The most useful synthesis tells the reader which layer is steady, which changed, and why. Finally, write the revised interpretation from the updated ledger. Lead with the contacts that both qualify and matter most under the stated approach; mention removed anchors where their absence changes the old emphasis. This is an interpretive ranking, not a compatibility percentage or a promise about the relationship.

Sources: Aspects Between Charts; Synastry

Finish with one revised reading and one useful conversation

Save the corrected birth details, the source of the correction, and the settings used for both comparisons. Give every claim from the earlier reading one status: retained, changed, removed, or conditional. Use a calculation method that accepts the corrected date, time, place, and applicable time conversion. Compare its output with the earlier chart under the same zodiac, aspect rules, and orb limit. Rewrite the synthesis from the contacts that remain, including any removed contact that used to carry the main emphasis. Cafe Astrology’s guide to interchart aspects treats a contact in context, including the planets’ roles and their signs; use that as an interpretive prompt, not a prediction about what either person will do. Choose one surviving theme and turn it into a question about lived experience: “When we need reassurance, what helps each of us feel heard?” The chart can suggest a topic. The answer belongs to the conversation. If you already know both people’s Moon, Venus, and Mars signs, with optional degrees, the Synastry Comparison Report can compare those supplied placements and selected major aspects within its six-degree limit. It doesn’t calculate natal charts from birth details or include houses. If you need corrected charts calculated, use a chart source that accepts those inputs; then decide whether a supplied-placement comparison covers what you want to revisit. The update is complete when you can name what changed, what still holds, and what you want to ask each other next.

Sources: Aspects Between Charts

A little more clarity