Checked against NASA JPL Horizons

For scale: one arcsecond is 1/3600 of a degree. A chart shown to two decimal places of a degree rounds at 36 arcseconds, so every difference on this page is far below what any chart displays. The Moon, the fastest body, moves about 0.55 arcseconds per second of time.

Method

Asterwise side. The tropical longitudes below are what POST /v1/western/natal returns for each instant: the engine at commit 2c25ab9, the build serving production when the page was generated, run with the same Swiss Ephemeris data files. The API returns longitude as an unrounded float; nothing here is rounded before the comparison. Location was set to latitude 0, longitude 0, which does not affect geocentric planetary positions.

Reference side. The Horizons API, observer table, centre 500@399 (geocentre), quantity 31, apparent airless positions, times given as Julian Day UT. That returns the observer-centred ecliptic-of-date longitude, the same quantity the Swiss Ephemeris returns by default. Horizons draws each body from its own JPL solution, listed in the per-body table.

What is not on this page. Horizons has no ayanamsa, so sidereal (Vedic) positions cannot be checked against it directly. Asterwise derives them by subtracting the Lahiri ayanamsa, in the Swiss Ephemeris definition, from the tropical longitude verified here; the ayanamsa is a convention rather than an observable. House cusps and the ascendant are also outside Horizons' scope. Both are covered on the reference-chart page, which compares the full Vedic and Western output against a direct Swiss Ephemeris calculation. Rahu is a mean lunar node, not a body Horizons tabulates, and is omitted.

Per body

The one outlier

All 80 comparisons

Longitudes in degrees along the ecliptic of date, 0 to 360. The difference is Asterwise minus Horizons, in arcseconds.

Reproduce it

The script queries Horizons live, runs the Swiss Ephemeris directly, and, if you set an Asterwise API key, calls the public endpoint so you can confirm the same numbers arrive over HTTP. A free key is enough. The Asterwise and Swiss Ephemeris columns agree to 0.0020 arcseconds, which is floating-point noise; the Horizons column is the independent check.

pip install pyswisseph requests
curl -O https://asterwise.com/public/accuracy/compare.py
SE_EPHE_PATH=/path/to/swisseph/ephe ASTERWISE_API_KEY=aw_... python3 compare.py

The Swiss Ephemeris data files are not bundled with pyswisseph; download sepl_18.se1, semo_18.se1 and sepl_24.se1 from the ephe directory of the Swiss Ephemeris repository (the location astro.com's own download page points to) and point SE_EPHE_PATH at them. Without them the library falls back to its built-in Moshier ephemeris, which is less precise than the files Asterwise runs on, and the Swiss Ephemeris column will drift from this page.

Downloads: raw comparison data (JSON) · compare.py. The Horizons responses are fetched live, so your run can differ from this page by hundredths of an arcsecond if JPL revises an ephemeris.