Davison
Chart Calculator

A Davison chart takes the midpoint of two births in time and in space. The date and time halfway between the two birth moments, and the latitude and longitude halfway between the two birthplaces, define a chart that is then cast in the ordinary way, as if someone had been born there and then.

It is named after Ronald C. Davison (1914–1985), a president of the Astrological Lodge of London, who set out his “Relationship Horoscope” in Synastry: Understanding Human Relations Through Astrology (ASI Publishers, 1977). The book came out a few years after the midpoint composite had caught on, and the two methods have been compared ever since.

The difference is practical. A composite averages each planet on its own, so its result matches no moment in the sky. A Davison chart is the sky at a real instant: its planets keep their true spacing, Mercury stays near the Sun, retrograde motion is real, and the houses are calculated for an actual latitude. That is why astrologers who time relationship events by transits or progressions tend to prefer it.

Sources

• Ronald C. Davison, Synastry: Understanding Human Relations Through Astrology, ASI Publishers, New York, 1977, the book in which he describes the Relationship Horoscope.
• Astrodienst, Astro-Databank entry for Ronald C. Davison, for his dates and his presidency of the Astrological Lodge.

The calculator sends both birth dates, times, time zones and places to the API. On the server, each local birth time becomes Universal Time (the IANA zone rules supply the offset, summer time included) and then a Julian Day number, the continuous day count astronomers use. The Davison moment lies halfway between the two, and the chart is cast for it to the second.

Two details are easy to miss. Birth times are entered to the minute, so two births are always a whole number of minutes apart, and the halfway moment falls either on a whole minute or 30 seconds past one; the API keeps those 30 seconds. And the time is Universal Time, which is how the results page labels it; no time zone is attached to the midpoint place, so convert it yourself if you want the local clock time.

For the place, the API adds the two latitudes and halves the sum, counting north as positive. Longitudes are averaged along the shorter way round the globe: two places 40° apart meet 20° from each, even when the 180th meridian lies between them. The chart is cast at that place with Placidus houses, which the results page requests, and aspects between its planets use the same orbs as a birth chart (see the methodology page). Placidus cusps do not exist beyond about 66.5° north or south, where part of the zodiac never rises or sets, so the API declines a midpoint place that far from the equator rather than draw other cusps under the Placidus name. Positions are geocentric, so the midpoint place moves only the Ascendant, Midheaven, house cusps, Vertex and Part of Fortune, and decides whether the chart counts as a day or night chart.

Birthplaces across the date line

Averaging longitudes as plain numbers works until two birthplaces lie on opposite sides of the 180th meridian. Take births in Tokyo (35.69° N, 139.69° E) and Los Angeles (34.05° N, 118.24° W). The plain average of the two numbers is 34.87° N, 10.72° E, in eastern Tunisia, a long way from both cities. The calculator goes the short way, across the Pacific, and returns 34.87° N, 169.28° W. The great-circle midpoint, halfway along the shortest route over the globe, is 47.93° N, 168.57° W: farther north, because that route between two cities at similar latitudes bows toward the pole. Programs differ on which of these they use; this one keeps the plain average for latitude and takes the short way for longitude.

The choice leaves the planets alone, since they depend only on the moment. The angles and houses change completely: the degree on the horizon depends on local sidereal time, which shifts by four minutes for every degree of longitude. For this pair, at 03:00 UT on March 1, 1990, the Pacific point has 13°56′ Leo rising and the Tunisian one 16°16′ Capricorn.

Prince William was born at 20:03 UT on June 21, 1982, and Prince Harry at 15:20 UT on September 15, 1984, both in London. On local clocks that was 9:03 PM and 4:20 PM BST, the times given in the official birth announcements. The calculation runs in four steps:

1. Julian Days. The two births fall on Julian Days 2445142.335417 and 2445959.138889, 816.8 days apart.
2. Average. Half the interval is 408.4 days, which puts the midpoint at Julian Day 2445550.737153: August 4, 1983, 05:41:30 UT.
3. Clock time. The API casts the chart for exactly that moment. In London the clocks read 6:41:30 AM BST.
4. Place. Both princes were born at the same hospital, so the averaged coordinates are simply London's, 51.52° N, 0.18° W. This example therefore shows the time half of the method; the Tokyo and Los Angeles case above shows the place half.

Read it as you would a birth chart. With 23°06′ Leo rising, the Davison Sun (11°17′ Leo) occupies the 12th house, the sector just above the eastern horizon, as expected a little after sunrise. Venus, Uranus and Neptune were retrograde at that moment: a fact about the sky that day, not an average. The tightest aspects between the Davison planets:

To follow transits to this chart, enter August 4, 1983, 6:41 AM and London in the birth chart calculator, which recreates it with any house system. The form takes whole minutes, so that chart is cast 30 seconds early, and its Ascendant comes out at 23°01′ Leo instead of 23°06′ Leo. As on the other relationship pages, the example is here for the arithmetic and carries no claims about the two brothers.

The same two birth charts, run through both methods. The composite column is what the composite calculator returns; the Davison column is the chart above.

The Sun and the five slow planets agree to within 0.9°, and for a reason. Between the two births the Sun made 2 full circles plus the 82°50′ that separates the two Suns, and it moves at a nearly steady pace, so at the halfway moment it had covered about half that distance: close to the composite Sun. With an odd number of full circles it would have landed near the opposite point instead. Jupiter to Pluto move slowly and fairly steadily over a couple of years, so here the two methods give nearly the same answer for them as well.

The Moon and the inner planets don't follow that rule. The Moon goes round the zodiac about thirteen times a year, Mercury and Venus swing either side of the Sun, and Mars turns retrograde, so their Davison positions have no fixed relation to the composite ones. Here Mars differs most, by 109.2°. The angles differ too: the composite Ascendant and Midheaven are averages of the two people's angles, while the Davison ones come from a real place and time.

A Davison chart is a real moment, so transiting planets reach its positions exactly as they reach a birth chart. Astrologers who use it watch the slow planets crossing its angles, Sun and Moon, and read the timing for the relationship as a whole.

The readings below are the usual ones in the literature, not measured effects. The cycle lengths are astronomy and apply to any chart.