Calculation Accuracy

AstroAPI reads NASA's planetary ephemeris files directly to deliver highly accurate positions. This page explains our data sources, supported house systems, and why your results may differ from other astrology software.

Ephemeris Data: NASA JPL DE442

At the core of every calculation is an ephemeris — a pre-computed table of celestial body positions over time. AstroAPI reads NASA's JPL DE442 ephemeris directly in its native Binary SPK (BSP) format. DE442 is the latest long-term planetary ephemeris published by NASA's Jet Propulsion Laboratory (2023).

• Date range: 1550 AD to 2650 AD
• Precision: Sub-milliarcsecond accuracy for inner planets
• Source: Fitted to the latest radar, VLBI, and spacecraft tracking observations
• Coverage: Sun, Moon, Mercury through Pluto, True Node, Mean Apogee (Lilith), and Chiron
• Fallback: For dates outside the DE442 range, AstroAPI falls back to JPL DE441, which covers 13,200 BC to 17,191 AD

Our engine applies standard astronomical corrections including iterative light-time, aberration, gravitational light deflection, topocentric parallax, IAU 2000A nutation, and FK5 precession.

What is a JPL Development Ephemeris?

NASA's Jet Propulsion Laboratory maintains a series of planetary ephemerides (DE-series) that are the most accurate models of solar system body positions available. They are used for spacecraft navigation, astronomical research, and as the reference standard for professional astronomy. Each new release (DE440, DE441, DE442) incorporates additional observations and improved dynamical models.

Supported House Systems

AstroAPI supports 7 house systems:

House System	Key	Description
Placidus	placidus	Time-based (default)
Koch	koch	Birthplace system
Equal	equal	Equal 30° divisions from ASC
Whole Sign	wholesign	Signs as houses
Campanus	campanus	Space-based, prime vertical
Regiomontanus	regiomontanus	Equatorial projection
Porphyry	porphyry	Quadrant trisection

Why Do My Results Differ From Other Software?

If you compare AstroAPI's output with tools like Astro-Seek, Solar Fire, Astro.com, or other astrology apps, you may notice small differences. Here's why:

Older or Less Precise Ephemeris Data

Many astrology applications use older or simplified ephemeris data. Some use the Moshier semi-analytical ephemeris (a mathematical approximation) rather than reading full-precision JPL data. Others use the Swiss Ephemeris, which repackages older JPL data (DE431/DE441) into a compressed format. While both are adequate for most purposes, they cannot match the precision of reading DE442 directly.

Ephemeris Source	Typical Accuracy	Used By
JPL DE442 (BSP)	< 0.001 arcsecond	AstroAPI
Swiss Ephemeris (SE1)	< 0.01 arcsecond	Astro.com, Solar Fire
Moshier	~1 arcsecond	Many free online tools
Older DE versions (DE406, DE200)	~0.01-0.1 arcsecond	Legacy software

Different Delta-T Values

Delta-T (the difference between Terrestrial Time and Universal Time) is crucial for historical dates. Different software may use different Delta-T tables or extrapolation methods, leading to discrepancies especially for dates before 1900 or after 2050.

Ayanamsa Differences (Sidereal Astrology)

For sidereal calculations, the choice of ayanamsa (the offset between tropical and sidereal zodiacs) varies between programs and directly affects all positions.

Summary

AstroAPI provides calculations that are at least as accurate as — and often more accurate than — mainstream astrology software, because we:

• Read NASA JPL DE442 directly — the latest and most accurate planetary ephemeris available
• Fall back to JPL DE441 for dates outside the DE442 range
• Apply all standard astronomical corrections (light-time, aberration, parallax, nutation, precession, relativistic deflection)

For the vast majority of charts, the differences between AstroAPI and other professional tools will be negligible (well under 1 arcminute). However, for precision-critical work — exact timing of sign ingresses, stations, or return charts — every arcsecond matters, and AstroAPI delivers.