Davison chart: the midpoint in time and space of two births

A Davison chart is a relationship chart cast for a real moment and a real place: the midpoint in time between two births and the midpoint between the two birthplaces. The method is named after the British astrologer Ronald Davison. Unlike the composite chart, which is assembled from midpoints of planets, a Davison chart places each planet where it actually stood in the sky at that moment.

The midpoint in time

Both birth moments are first converted to Universal Time (UT), taking into account the time zone and any daylight saving time in force on that date. Skip this step and an error of an hour or two easily turns into an error in the house cusps. Then you find the interval between the two moments and add half of it to the earlier one.

Software does this with Julian day numbers: it averages the two Julian dates and converts the result back to a calendar date and time. It does not matter which partner is older, the midpoint is the same.

The midpoint in space

For the place, the usual approach is to average the two geographic latitudes and, separately, the two longitudes. Some programs offer a different option, the midpoint along the great circle between the two cities. For cities in the same country the difference is small. For cities on different continents it can amount to hundreds of kilometres and shift the houses, so it is worth noting which method a given chart uses.

The midpoint sometimes falls in the ocean or somewhere nobody lives. That does not matter for the calculation, since the chart is cast for coordinates rather than for a town. One trap to watch: if the partners were born on opposite sides of the 180th meridian, a plain average of longitudes lands on the wrong side of the globe, and that case has to be worked out by hand.

A worked example

A made-up couple. Partner A was born on 14 March 1990 at 08:20 UT in Kyiv (50°27′ N, 30°31′ E). Partner B was born on 2 November 1993 at 22:50 UT in Lviv (49°50′ N, 24°02′ E). The times are already in UT to keep time zones out of the way.

The interval between the two moments is 1329 days 14 hours 30 minutes. Half of it is 664 days 19 hours 15 minutes. Adding that to A's birth gives 8 January 1992, 03:35 UT.

Latitude: (50°27′ + 49°50′) / 2 = 50°08′30″ N. Longitude: (30°31′ + 24°02′) / 2 = 27°16′30″ E. That point lies in western Ukraine, near the town of Shepetivka. An ordinary birth chart is cast for these coordinates at 03:35 UT on 8 January 1992.

Comparing it with the same couple's composite is instructive. The Sun is almost identical in both, around 17° Capricorn. The Davison Moon, however, is at 21°26′ Aquarius while the composite Moon is at 22°21′ Leo, nearly opposite. Davison Venus is at 9°04′ Sagittarius, composite Venus at 15°14′ Sagittarius. The faster planets diverge the most, because their position in the real sky depends on where they happened to be at the midpoint in time, not on the midpoint of the two natal positions.

How a Davison chart differs from a composite

The core difference is what kind of chart it is. A composite is a geometric construction with no date. A Davison chart is a real sky, and several things follow from that.

Possible positions. Mercury stays within about 28° of the Sun and Venus within about 47°. The aspects between planets are the kind that occur in real charts.
Consistent houses. The Ascendant, MC and cusps come from a single moment and place through an ordinary house system, not from midpoints.
Forecasting. A Davison chart can be progressed, given a solar return, and read for transits just like a birth chart. For a composite, progressions have to be worked out indirectly.
No two-midpoint problem. In a composite, two planets close to an opposition can yield a midpoint on either of two opposite sides of the wheel. A Davison chart involves no such choice.

Transits and progressions

Because a Davison chart has a date, time and place, every standard forecasting technique applies to it. Transits are read as for a birth chart: slow planets to the lights and angles, a tight orb of 1-2°, and repeated hits from retrograde motion treated as one period. Secondary progressions run from the moment of the Davison chart, with one day after it standing for one year after the chart moment. A solar return is cast for the moment the Sun comes back to its Davison position, in our example every year in early January.

There is a catch. The Davison chart is "born" between the partners' birth dates, and that moment need not coincide with the start of the relationship; it may even fall before the two people met. Progressions run from the chart moment, not from the day they met, so the first years of the progressed chart describe a time when there was no couple yet. Partly for this reason some astrologers trust transits to the Davison chart more than its progressions.

When one partner's birth time is unknown

Both relationship charts are sensitive to birth time, but the Davison chart responds to an error in a predictable way. If one partner's time is known only to within two hours, the midpoint in time shifts by half that, one hour. In an hour the Ascendant moves about 15° on average, the Moon about half a degree, and the other planets barely move.

In practice this means: with an error of a few hours, the planets of a Davison chart and the aspects between them remain usable, while the angles and houses do not. If only the date is known for one partner, the midpoint is calculated from noon on that day, and the error in the midpoint can reach six hours: the Ascendant could be anywhere and the Moon shifts by a few degrees. In that case read only the planets, and treat the Moon's aspects as approximate.

Choosing between them

Both charts describe the couple as a unit in its own right, and many astrologers look at both. The choice depends on the question.

The composite is handy when you want to see how the planets of two people combine into a shared structure: its points come directly from the natal positions, so the link to each partner's chart is plain to see. The Davison chart is more often chosen when the relationship needs forecasting work, meaning transits, progressions and solar returns, or when you want a chart that can be read by the ordinary rules without caveats about impossible positions.

A Davison chart is read like a birth chart: the angles and planets near them, aspects with the same orbs as in a natal chart, the distribution of planets across the houses. A comparison of a couple usually starts with synastry and the inter-aspects, with the Davison chart as a further layer.

Calculate a Davison chart