What the midpoint method calculates
A progressed composite starts with two separate people and a chosen comparison date. Each birth chart is advanced by the secondary-progression convention: one ephemeris day after birth represents one tropical year of life. The progressed planets are the tropical positions at those two symbolic instants. Only after progressing the two charts do we construct the composite. Progressing an already constructed natal composite would perform the operations in a different order and would not be this method.
Each person also needs progressed angles and houses. This calculator directs each natal sidereal orientation, or ARMC, by the Naibod mean solar key of 0.98564733° for each tropical year elapsed. It then converts the directed orientation to a Midheaven using obliquity at that person's symbolic date and computes that person's Ascendant and twelve house cusps at their natal latitude. The local clock used by the house solver only recovers a matching sidereal orientation; its planets do not enter the result.
Midpoint houses and reference-place houses
The midpoint version takes the shorter zodiac-arc midpoint of each corresponding progressed planetary pair. It does exactly the same with the two progressed Ascendants, the two progressed Midheavens, and each of the twelve matching house cusps. No common reference latitude is entered. The resulting house boundaries may differ from those of the reference-place progressed composite, which first takes the composite MC and then makes an entirely new house frame at one selected latitude. The planetary midpoints should agree between these two site tools given the same birth records and comparison date.
Suppose a pair of cusp longitudes is exactly 180° apart. There are two equally short midpoint choices. The tool shows both candidates instead of picking one silently. Even when every pair has one clear midpoint, the twelve independently averaged cusps can fail to make an ordered house grid. In that case the individual cusp degrees still appear, while planet house numbers are withheld. This avoids presenting a house assignment that the calculated boundaries cannot support.
Three charts on one sheet
The three-chart sheet places Person A's progressed chart, the midpoint composite, and Person B's progressed chart side by side using one zodiac orientation. Each circle has its own house cusps. The accompanying tables identify the exact progressed and composite longitudes, because glyphs close together on a small wheel are easier to distinguish numerically. The two free tools use the same input rules and calculation engine; the three-chart option changes the drawing, not the midpoint formula.
Records and interpretation
Use the historical UTC offset actually in force at each birthplace on the birth date, including daylight saving time. This calculator cannot infer every historical legal time rule. Both recorded birth times matter greatly for angles and house cusps. The target date is read at noon UTC; a different target date changes each person's symbolic ephemeris age. Near exact oppositions, tiny input changes can flip a shorter-arc midpoint. Difficult latitudes may trigger an explicitly labeled Porphyry fallback for a selected quadrant house system.
Different ephemerides, obliquity models, progression keys, and house algorithms can produce different degrees. This page states its choices so the result can be understood on its own terms. A progressed composite is a symbolic diagram, not a measurement of compatibility, consent, relationship safety, or future events. Its best use here is to make the construction and its assumptions inspectable.