Geocentric vs topocentric

Geocentric positions are referred to the center of Earth. Topocentric (or apparent) positions are referred to the observer’s location on Earth’s surface — the natural frame for horizon, rising, and setting.

What core.chart uses

AstroRust natal positions from the VSOP backend are geocentric ecliptic longitudes, consistent with most horoscope software and with core.chart calculate / calculate_sidereal body snapshots.

Parallax shifts for the Moon are largest (often arcminutes); for planets and the Sun they are usually well below typical chart-display precision. Lunar work at extreme precision may need topocentric correction; this stack does not advertise topocentric planet longitudes in core.chart.

Where topography still matters

Even with geocentric λ for planets, birth latitude and longitude enter house and angle math:

• Ascendant — eastern horizon ∩ ecliptic at the birthplace
• Midheaven — meridian ∩ ecliptic above the horizon
• House cusps — depend on local sidereal time and horizon geometry (houses, chart-points)

So “geocentric planets + topographic angles” is the normal natal pattern.

References

• Topocentric coordinate system — Wikipedia
• Parallax — Wikipedia (lunar parallax)

See also planet-positions, coordinate-systems, birth-input.