A single chart for two people, computed from the midpoint between their births.
You take the date halfway between the two births and the geographic point halfway between the two places, and cast a chart as if someone had been born there. It describes the relationship as an entity in itself, rather than the meeting of two individuals — which is why computing transits over it makes sense.
How it differs from the composite chart
There is another widely used relationship chart, the composite, which is built quite differently: you take the midpoint between one person's Sun and the other's, between the two Moons, the two Marses, and so on, planet by planet. The result is a mathematical figure that never existed in the sky.
The Davison chart, named after the British astrologer Ronald Davison, makes the opposite choice. It looks for a real moment and a real place, and reads the sky as it actually was. That is why it has everything a birth chart has: a genuine Ascendant, genuine houses, aspects that really formed. And that is why you can follow it over time with transits, just as you would a person's chart.
How the midpoint is found
Time is halved in Universal Time, not local time: both births are first set to the same clock, then the instant exactly halfway between them is taken. If one of you was born in March 1990 and the other in March 1994, the chart falls around March 1992, at whatever hour the calculation gives.
For the place, latitudes and longitudes are averaged. Between London and New York the midpoint lands in the middle of the Atlantic, and that is fine: what matters is a position on Earth, not whether there is a town there. For longitude SideraMea follows the shorter arc, so two births either side of the 180th meridian are not sent round to the opposite side of the globe.
Frequently asked questions
What if one partner doesn't know their birth time?
The midpoint can still be calculated, but the chart's time inherits the uncertainty. One unknown birth time out of two is enough to make the Ascendant and houses unreliable, since the Ascendant moves through the whole zodiac in a day. Planets in signs and the aspects between them can still be read, with some caution over the Moon, which moves about half a degree an hour.
Davison chart or synastry: which is better?
They answer different questions. Synastry compares the two birth charts and shows where two people spark or clash: it is about each of you within the relationship. The Davison chart describes the relationship as a third party with a character of its own. Many astrologers read both together: synastry to understand the people, the Davison chart to understand the bond.
Can you use a Davison chart for friends or business partners?
Yes. The calculation has no idea what kind of relationship it is looking at: it uses only dates, times and places of birth. Astrologers apply it to any bond with a life of its own, including business partners, friends and family. What changes is the interpretation: the seventh house or the chart's Venus reads differently in a friendship than in a romance.
See it in your own chart
Calculating your birth chart is free: wheel, planets, houses, aspects and a first reading. All you need is your date, time and place of birth.