← Reference · 07 of 07

Making one chart out of two

Synastry compares two charts side by side. These two techniques go further and construct a single chart for the relationship itself — by two entirely different routes.

A composite chart is not a chart of any moment at all. Each body's position is the midpoint of its two natal positions — the composite Sun sits exactly halfway between the two natal Suns, on the shorter arc, the same convention used everywhere a midpoint is taken on this site. Nothing was ever in that degree; the composite Sun is an average, and reading it as anything else overstates what it is. The composite Ascendant is built the same way, from a Whole Sign house circle anchored to the midpoint Ascendant, because a genuine Placidus-style house division needs a real latitude and a real sidereal time, and a composite chart has neither.

A Davison chart is the opposite kind of construction: a real chart, for a real instant and a real place. The instant is the midpoint in time between the two births; the place is the geographic midpoint between the two birthplaces, found as the average of the two positions as vectors on a sphere rather than as a naive average of latitude and longitude, which stays correct even for two birthplaces on opposite sides of the antimeridian. Everything in a Davison chart genuinely happened — the Sun really was at that degree, at that place, at that moment — which makes it the more defensible of the two techniques, and by most accounts the less commonly used.

The two rarely agree closely. A composite Sun sitting exactly between two natal Suns can land tens of degrees from a Davison Sun, which is wherever the Sun genuinely stood at the real midpoint moment — the two techniques are answering different questions, not approximating the same one. A chart with widely separated birth years or birthplaces will generally show the largest gap between them, since both the time-midpoint and the geography-midpoint move further from either individual chart.

Both differ in kind from synastry itself, which never constructs a new chart — it draws lines between two existing ones and lets each chart's angles reach the other's directly. A composite or Davison chart instead behaves as a single new object: it gets its own wheel, its own houses, its own aspect grid, and it can in principle be read the way any other chart on this site is read, with the same caveat any constructed chart carries here — it describes a relationship, not either person in it.