Swiss Ephemeris — the calculation engine

The public calculator and the sky slice call the same family of endpoints. Birth charts post an instant plus coordinates and receive planets, houses, and angles. The sky map posts the current time plus the Paris reference coordinates (48.8566° N, 2.3522° E) and receives a longitude per body. Topocentric effects at the level of a sign label are negligible next to the 30° bin, but the coordinates are still sent because houses and altitudes need them.

DE431 is a numerical ephemeris published by NASA’s Jet Propulsion Laboratory. It is the data source behind the longitudes, not a horoscope text. When a sign page cites a degree, the degree is this coordinate system. The prose around it explains how the engine bins and aspects that degree.

Retrograde flags, house cusps, and aspect orbs are applied after the ephemeris returns a position. The orbs (8°, and 6° for the sextile) are AstroStella constants. They are not a field inside DE431. Keeping that split visible is the point of the glossary: the ephemeris supplies longitudes, and the aspect table decides which gaps count.

The split of responsibility is strict in the code. DE431, via the Swiss Ephemeris, answers where Mars is in ecliptic longitude. The aspect table answers whether that longitude is within 8° of 90° from the natal Sun. Paris at 48.8566° N, 2.3522° E answers what is rising and what is above the horizon, which signs do not need and houses do. When a number on a sign page and a number in a chart disagree, compare the timestamps before you compare the prose.