Horospire planet positions against NASA JPL DE440, 1900 to 2050
Version 1.0, measured on 19 October 2026
https://horospire.com/research/ephemeris-accuracy

What this is
  How far the planet positions computed by Horospire's ephemeris engine are
  from NASA JPL's DE440s ephemeris: the Sun, the Moon and Mercury to Pluto at
  one moment a day from 1900-01-01 to 2050-12-31 (55,152 moments, 551,520
  placements), as apparent geocentric ecliptic longitude on the true ecliptic
  and equinox of date. Two other engines are measured the same way:
  astronomy-engine 2.1.19 and the Swiss Ephemeris 2.10.03 in Moshier mode.
  It measures the arithmetic of the positions, not any claim of astrology.

Files
  summary.csv          Horospire's engine, per body: mean, median, 95th
                       percentile and largest error in arcseconds, when the
                       largest fell, the signed mean, and how many placements
                       fall in a different sign, decan or whole degree.
  daily-errors.csv.gz  Every placement: the civil time, its time scale (UT1
                       before 1972, UTC from 1972) and Horospire's error per
                       body, engine minus reference, in arcseconds.
  libraries.csv        The three engines, per body, measured the same way,
                       with the version and the exact call used.
  horizons-check.csv   The reference against JPL Horizons at 15 instants for
                       all ten bodies (150 values; within 0.25 arcseconds).
  ingress-summary.csv  The table of sign-change dates Horospire's readings use
                       for Mercury to Saturn, against JPL's crossing times,
                       per planet. The table itself is not published.
  scripts/             The code that produced the files (MIT).
  LICENSE-DATA.txt     The data's licence (CC BY 4.0).
  SHA256SUMS           A checksum for every file above.

Time
  A civil time before 1972 is read as UT1, because a clock time of that era
  was kept in GMT; from 1972 it is read as UTC. Horospire's engine receives the
  civil time as given and applies no Delta T; Skyfield, astronomy-engine and
  the Swiss Ephemeris each apply their own Delta T.

Recipe
  1. python3 -m venv venv
     venv/bin/pip install skyfield==1.55 jplephem==2.24 numpy==2.5.4 pyswisseph==2.10.3.2
  2. Download https://ssd.jpl.nasa.gov/ftp/eph/planets/bsp/de440s.bsp
     sha256 c1c7feeab882263fc493a9d5a5b2ddd71b54826cdf65d8d17a76126b260a49f2
  3. venv/bin/python scripts/reference.py de440s.bsp work
  4. venv/bin/python scripts/swiss_ephemeris.py work/moments.json work/swiss.json
  5. npm install astronomy-engine@2.1.19
     node scripts/astronomy_engine.mjs node_modules/astronomy-engine work/moments.json work/astronomy.json
  6. Horospire's engine is not published in this version, so rebuild its
     values from the published errors:
     venv/bin/python scripts/house_from_errors.py work daily-errors.csv.gz
     (Inside Horospire, scripts/house_engine.php calls the engine itself.)
  7. venv/bin/python scripts/summarise.py work out 2026-10-19
     out/summary.csv, out/libraries.csv and out/daily-errors.csv.gz then
     match this folder's files byte for byte.
  8. To repeat the Horizons check (ten read-only requests to JPL, two
     seconds apart), put the first column of horizons-check.csv in a JSON
     list and run:
     venv/bin/python scripts/horizons_check.py de440s.bsp tt-jds.json out/horizons-check.csv

What Horospire's engine is
  The Sun by Jean Meeus, Astronomical Algorithms (2nd edition), chapter 25.
  The Moon by chapter 47, truncated to its 13 largest longitude terms.
  Mercury to Saturn from Meeus's mean orbital elements (Table 31.A), Uranus,
  Neptune and Pluto from JPL's Keplerian Elements for Approximate Positions of
  the Major Planets (E. M. Standish); each orbit solved by Kepler's equation,
  rotated from the J2000 equinox to the equinox of date (IAU 1976 general
  precession), with the largest Jupiter-Saturn perturbation terms as Paul
  Schlyter gives them in How to Compute Planetary Positions. The engine uses
  the civil time as its dynamical time (no Delta T). Version 1.0 measures it
  as of commit dfb56ab71; its code is not published in this version.

Cite
  Horospire (2026). Horospire planet positions against NASA JPL DE440, 1900
  to 2050 (version 1.0) [Data set].
  https://horospire.com/research/ephemeris-accuracy