Four ways to place a Davison chart
A Davison relationship chart begins with two recorded birth instants. Convert both local clock times to UTC, then find the instant exactly halfway between them. Unlike a composite chart, which averages each pair of planetary longitudes, the Davison chart calculates the planets anew for one real moment. It also needs one geographical location, and that choice is where several named variants diverge. The four results on this page reveal the date, coordinates, angles, and planetary longitudes of each choice.
Uncorrected: arithmetic geographical midpoint
Do the same independently for the longitudes. The UTC instant remains the exact temporal midpoint. This is an arithmetic calculation on signed coordinate numbers, so two birthplaces at 179° east and 179° west produce longitude 0°, near Greenwich, even though the locations are geographically near each other across the date line. That result is a consequence of the convention, not an error in the input.
Spherical midpoint: shortest route across the globe
The spherical option keeps the exact temporal midpoint but chooses the place halfway along the shorter great-circle path. It treats each location as a point on a sphere, adds the two unit vectors, and normalizes their sum. For the 179° east and 179° west example, it places the chart around 180° longitude. For intercontinental pairs, a great-circle midpoint can fall near a pole where some house systems cannot produce usable cusps. Exactly antipodal places have infinitely many halfway routes; near antipodes, tiny coordinate changes can swing the midpoint sharply. This tool explicitly withholds the spherical column in that case.
Reference place: choose where to cast the chart
The reference-place option also preserves the exact temporal midpoint. You enter a location relevant to the relationship, such as a shared residence or meeting place, and the site casts the midpoint-time chart there. This option changes local sidereal time, Ascendant, Midheaven, and houses. It does not change the planetary positions because those are calculated geocentrically for the same UTC moment. Record the chosen reference place when comparing charts; changing it later changes the interpretation of houses.
Corrected Davison: align the Midheaven
The corrected time therefore may differ from the exact temporal midpoint. This page calculates both natal MC longitudes with its own tropical chart engine, takes the shorter zodiac arc between them, and solves nearby for the UTC instant whose MC matches that target at the arithmetic place. If the two natal MCs are exactly opposite, two equal short arcs exist and the page refuses to select one silently. The result is an independently calculated implementation of the published criterion, not a numerical clone of another service.
What changes in the table
The uncorrected, spherical, and reference-place columns have identical apparent geocentric planetary positions because all three use one UTC instant. Their Ascendants, Midheavens, and house cusps can differ substantially. The corrected column moves the instant, so a fast body such as the Moon can shift slightly as well as the angles. Results can also differ between calculation software because of ephemerides, obliquity, and house conventions. This calculator uses Astronomy Engine 2.1.19 for the positions and the site's own house calculations. It accepts dates from 1800 through 2100.
Time and interpretation cautions
Enter the UTC offset that actually applied to each birthplace on the recorded birth date, including daylight saving time. The tool does not infer historical legal time zones. A small birth-time error can move local angles and houses, especially near a cusp. Near polar latitudes, an indicated house method may fall back to Porphyry, or angles and houses may be unavailable. These charts are interpretive conventions, not measurements of a relationship's health, compatibility, consent, or future.