The Astrolium composite chart calculator takes two birth records and returns the midpoint composite, the Davison chart, or both side by side. Robert Hand's midpoint method (average the longitudes of every planet across the two natals, place each composite planet at the midpoint, derive composite angles from the two natal angles) is the modern default. The math runs on the Swiss Ephemeris in your browser. No signup.
The full chart renders in under 60 seconds, with the composite Ascendant, the 10 planetary positions, whole-sign houses, and the 5 tightest internal aspects. The optional Davison chart (a real chart cast for the midpoint date, time, and place of the two births) runs side by side for comparison.
This is the free preview. The full composite & Davison feature attaches the timing layer (profections and transits on the composite Ascendant) and ships both composites on one screen with the inter-aspect grid. For the technique comparison, read the composite vs Davison guide. For the synastry layer that pairs with it, see synastry charts in Astrolium and the free synastry calculator. For the $29 per month Pro pricing, see pricing.
What the calculator returns
A composite chart describes the relationship as its own entity. Robert Hand's 1975 Planets in Composite made the midpoint composite the modern default: average the longitudes of every planet, place each composite planet at the midpoint, derive the composite angles from the 2 natal angles. Ronald Davison proposed an alternative in 1977: instead of averaging positions, average the birth moments themselves, and cast a real chart for the midpoint date and place. Both approaches answer the same question (what is the relationship as a third entity?) with different mathematics. For a clear explanation of which to use when, read the composite vs Davison guide.
The calculator outputs:
- The composite Sun, Moon, and Ascendant (the headline placements)
- 10 planetary positions in sign and degree
- Whole-sign house cusps from the composite Ascendant
- The 5 tightest internal aspects (orb under 5 degrees)
- A toggle between midpoint, Davison, and side-by-side mode
When midpoint reads differently from Davison
Midpoint and Davison agree most of the time on the broad strokes. Sun in Cancer, Moon in Aries, Saturn in the 7th house: both methods will usually land in the same sign for the planets that move slowly relative to the gap between the 2 birth dates. Where they diverge is on the faster bodies and the angles, and that divergence is where the practitioner reading earns its keep.
A worked example. Partner A is born in March 1985, partner B in September 1987. Two and a half years apart. The midpoint composite Sun lands in June 1986: a Gemini Sun. The Davison Sun is calculated for the actual midpoint date between the 2 births, which falls in mid-June 1986. The Davison Sun also lands in Gemini, around 24°. Same sign, slightly different degree, broadly consistent.
A second example. Partner A is born in March 1980, partner B in September 1984. Four and a half years apart. The midpoint Sun is at the average longitude (around 6° Sagittarius). The Davison Sun is calculated for early December 1981, and lands at 12° Sagittarius. Still consistent.
A third example. Partner A is born in March, partner B is born the following March 11 months later. The midpoint Sun lands in September (the average longitude); the Davison Sun lands in September of the actual midpoint year. Both methods agree, the longitudes are within 1°, and the reading is stable.
Disagreement appears most often when the gap between births is small but spans a sign boundary, or when one of the 2 partners is born close to a Sun-sign cusp. In those cases the midpoint Sun may sit on one side of the cusp and the Davison Sun on the other, and the reading splits.
The composite Ascendant matters most
The composite Ascendant is the line you build the chart around, because it determines the house placement of every other body. Midpoint and Davison disagree on the Ascendant much more often than they disagree on the Sun, because the Ascendant moves 1° every 4 minutes — small differences in birth time get amplified.
For couples with reliable birth times for both partners, the calculator returns both Ascendants and notes the discrepancy. For couples with 1 missing time, midpoint composite still computes (with a warning on the Ascendant) but Davison flags the entire angles row as unreliable.
The 5 tightest internal aspects
A composite chart has aspects between its own planets, the same way a natal chart does. The calculator surfaces the 5 tightest internal aspects of the composite or Davison: composite Sun trine composite Moon, composite Venus square composite Saturn, and so on. These describe the inner architecture of the relationship as a third entity.
These are different from the synastry inter-aspects between the 2 natal charts. Both readings matter, and they answer different questions. Inter-aspects: how do the 2 partners contact each other? Composite internal aspects: what does the relationship itself look like internally? The free synastry calculator handles the first; this calculator handles the second.
What the score does not tell you
The calculator returns a chart, not a verdict. A composite Sun-Saturn conjunction is heavy regardless of method, but heavy is not the same as bad. Saturn aspects in the composite are the architecture of long relationships: they ask both partners to do work, and they often produce the most durable connections. Liz Greene's Relating and Lois Sargent's How to Handle Your Human Relations both make this argument at length.
The reading also depends on timing. A composite with a heavy Saturn that is currently in a 5th-house profection year ruled by Jupiter under transit Venus reads very differently from the same composite in a 12th-house profection year under transit Saturn. The static chart and the timing layer answer different questions. For the timing layer attached to the composite, see the full feature.
After the calculator
Once you have a composite, the natural next moves are:
- Run the same 2 charts through the free synastry calculator to see the inter-aspects between the 2 natals
- Read the composite vs Davison guide for the technique comparison and when to prefer each
- Read the how to read synastry chart guide for the full 4-layer reading method
- Run profections on the composite Ascendant to find the relationship's current year-lord
For practitioners running composite charts across a roster (couples-therapy practices, casting, hiring), the $29 per month Pro tier on the pricing page ships the roster scanner: "which couples in my book are in a heavy composite Saturn year?" returns a sorted list.
Cross-link
For the full feature with the timing layer, see composite & Davison in Astrolium. For the synastry layer that pairs with it, see synastry charts. For the related techniques, run the synastry calculator, the Saturn return calculator, the profections calculator, or zodiacal releasing. For the reading method, read the synastry guide.