Zodiac / Compare
Composite Chart Calculator: Midpoint Relationship Chart for Two People
Calculate the shorter-arc midpoint composite of two birth charts, with a wheel, internal aspects, whole-sign houses when known and private invitations.
Composite Chart Calculator: Midpoint Relationship Chart for Two People: Creates one symbolic relationship chart from the shorter-arc midpoint of each matching planet longitude. For example, 350 degrees and 10 degrees give zero degrees. ASC and MC use midpoints when both times and places are available, with whole-sign houses from the composite ASC. See the wheel, planet table, internal aspects and neutral Sun, Moon and Venus themes. Invite someone using computed longitudes rather than birth data. Runs 100% locally in your browser with zero server file uploads.
- Category
- Astrology & fortune
- Runs
- In your browser
- Cost
- Free · no sign-up
- Availability
- Ready to use
Runs entirely in your browser
One symbolic relationship chart, made from the shorter-arc midpoints of two charts.
Birth time and city are optional. Blank times use local noon in the selected time zone. ASC, MC and houses appear only when both charts have times and places.
Create a private invitation
Invite links contain eleven computed longitudes rounded to 0.001°, plus ASC and MC when known. They contain no birth dates, times, coordinates or names. Anyone with the link can read these positions; planetary positions may help infer an approximate birth period.
Astrology is a cultural and entertainment practice, without predictions. City data from GeoNames is licensed under CC BY 4.0. GeoNames attribution
Midpoint and house method
Astronomy Engine (https://github.com/cosinekitty/astronomy) supplies tropical geocentric apparent positions under the MIT licence. GeoNames (https://www.geonames.org/) supplies city coordinates and time zones under CC BY 4.0. Calculations run in your browser.
Take the mean along the shorter arc of the two normalised longitudes. An exactly opposite pair uses the midpoint 90 degrees forward from the smaller longitude. Whole-sign houses start at the midpoint ASC, with the MC retained as an independent midpoint. Internal aspects use the published natal-chart orbs. Invite values preserve longitudes to 0.001 degrees with a letters-only encoding.
Checked against Swiss Ephemeris
Offline comparisons use Swiss Ephemeris 2.10.03 in Moshier mode (https://www.astro.com/swisseph/). Reference numbers are test data only; the calculator does not bundle or call Swiss Ephemeris.
The natal positions feeding the midpoint calculation reuse the existing Swiss Ephemeris reference checks, with longitudes within 0.02 degrees at the recorded instants. Midpoint tests separately verify wraparound, the exact-opposition convention, house placement and invite precision. A midpoint composite is not an ephemeris chart for a physical instant.
How to use it
- Enter two birth dates, with optional times and cities, or open a computed-position invite.
- Calculate the midpoint wheel, planet positions and aspects within the composite.
- Explore traditional themes, compare synastry or save a private card.
Privacy & limitations
Birth data stays in your browser. Share cards contain only computed results, never birth dates, birth times or birth places. Invite links carry computed planet longitudes and optional computed angles, encoded as letters. Exact planetary patterns may help infer an approximate birth period. Anyone with the link can decode the positions.
Related tools
Frequently asked questions
What is a midpoint composite chart?
Each point is the midpoint of the matching points in two charts along their shorter zodiac arc. It is a symbolic construction, not the sky at an actual birth moment.
How is it different from synastry?
Synastry compares contacts between two separate charts. A composite creates a single set of midpoint positions and finds aspects within that new chart. Use the synastry link to compare the two charts instead.
How do you handle zero degrees and exact opposition?
The shorter arc wraps around zero degrees, so 350 degrees and 10 degrees yield zero degrees. At an exact 180-degree separation, either midpoint is equally distant. This calculator chooses 90 degrees forward from the numerically smaller longitude, consistently regardless of person order.
Which houses does the composite use?
When both times and places are known, ASC and MC are shorter-arc midpoints too. Whole-sign houses start at the sign containing the midpoint ASC. The midpoint MC is displayed independently; it need not fall on the tenth-house cusp.
What does an invite link contain?
It contains eleven computed tropical longitudes rounded to 0.001 degrees, plus computed ASC and MC when known. No birth date, time, coordinates or name is included. The recipient adds their own birth data. Planetary patterns can suggest an approximate birth period, so share the link only with intended recipients.
Do unknown birth times or composite signs predict compatibility?
No. Blank times use local noon in the selected zone; the Moon and its aspects can change with the actual time. Houses and angles are omitted without both timed places. Traditional themes are symbolic and do not measure relationship quality.
Free tool · runs in your browser · no account required