Astrolium's Natal Compare puts two charts in one workspace. Enter both birth dates, click Compare, and switch between synastry cross-aspects, composite midpoint, and Davison chart without leaving the page or re-entering data.
Three views, one submit
Most tools force you to open three separate calculators. Natal Compare runs all three computations from a single pair of birth data inputs and presents them as switchable tabs.
Synastry shows the cross-aspects between the two charts: which of Person A's planets aspect Person B's, at what angle, and with what interpretation. The aspect table filters to hard (square, opposition), soft (trine, sextile), or all aspects. A dynamics card shows the harmony-to-tension ratio and flags the five strongest contacts — the ones practitioners lead with in a reading.
Composite produces the midpoint chart: a third horoscope generated by averaging each planetary longitude across both birth charts. It describes the relationship as a separate entity with its own tone and direction. A Sun in the composite 10th house describes a partnership oriented toward public work. A composite Moon in Scorpio describes emotional depth and privacy as shared needs.
Davison finds the midpoint in time between the two births, places a chart for that exact astronomical moment at the geographic midpoint, and erects a horoscope. It is anchored to a real sky — which is why practitioners prefer it for timing work. When a transit or progression activates the Davison Ascendant, something moves in the relationship.
What the cross-aspect grid tells you
The Synastry tab shows every major aspect one person's planets make to the other's. The grid leads with the tightest orbs because tight aspects operate the most consistently. A 0.3° conjunction between A's Mars and B's Saturn will structure their dynamic — and their conflict — reliably over years.
Hard aspects are not problems; they are activations. A Mars–Saturn square demands that the two people work out different approaches to effort, time, and ambition. When they do, it becomes a source of mutual discipline. When they don't, it becomes a recurring argument about the same underlying pattern.
Soft aspects carry natural affinity. A Venus–Jupiter trine indicates that one person's aesthetic and relational sensibility expands in the presence of the other's generosity and philosophical reach. These contacts feel easy. They are often the reason people stay.
Filter the table to hard-only when you want to understand where the growth edges are. Filter to soft-only when you want to see where support flows. Read both together for a complete picture.
Composite vs Davison
Composite midpoint works by arithmetic: for each planet, add the two longitudes and divide by two. The result is a horoscope that never occurred in the real sky — it is a mathematical construction.
Davison works differently. The midpoint in time between two birthdays is a real moment. The chart is cast for that moment at the geographic midpoint. Every planet in a Davison chart was actually in that position in the sky at that time.
Both are valid. Practitioners who work with progressions or solar arcs to relationship charts tend to prefer Davison because the progressions unfold from a real baseline. For the descriptive read of relationship character, either method produces recognizable results.
When the two charts agree on a theme — say, both show Saturn prominent, or both show the 7th house loaded — that theme is reliable.
How practitioners use this
The Natal Compare workspace is built for the five-minute window before a client session. Enter both charts, scan the tight aspects, note the dominant dynamic from the harmony percentage, and switch to composite to check whether the relationship's own chart confirms what the synastry shows. Save the reading to history and it restores instantly the next time.
The cross-aspect table sorts by tightness by default. The strongest contacts are at the top. For most readings, the first five rows contain the information that matters.
For the consumer compatibility question ("are we compatible?"), see the synastry calculator. For the composite chart as a standalone tool, see the composite chart calculator. For the predictive timing layer — transits to the composite, progressions to the synastry — see predictive timing.