Building a 100% Bhāratīya ephemeris from classical texts — and benchmarking it against NASA/JPL Horizons

The Unholy Indian Magician — Mon, 22 Jun 2026 14:35:13 +0000

Most "Indian" astrology apps quietly run on the Swiss Ephemeris — a Western-built library under AGPL/commercial licensing. It's excellent (sub-arcsecond, and more precise than what I'll describe below), but it means the trust chain of a supposedly Bhāratīya product ends at a foreign binary. We wanted to know: what does sovereignty actually cost in accuracy if you compute the sky from classical Indian texts instead?

This is the honest write-up of building Bharat Ephemeris from scratch and benchmarking it against NASA/JPL Horizons — including where we lose.

Method

The pipeline is three classical layers, no foreign ephemeris in the chain:

1. Mean longitudes — Sūrya-Siddhānta. Each body's mean position is propagated from the Siddhāntic mean motions (revolutions per Mahāyuga → daily motion). This is the deterministic backbone.

2. Corrections — Kerala-school Tantrasaṅgraha. On top of the mean position we apply the manda correction (the equation of centre — orbital eccentricity) and the śīghra correction (the heliocentric→geocentric transformation that produces retrograde loops). The Kerala school's formulations are remarkably close to the modern two-body solution.

3. Sidereal frame — Lahiri ayanāṃśa. Finally we subtract the Lahiri ayanāṃśa (the Government-of-India Rāṣṭrīya Pañcāṅga standard) to land in the sidereal zodiac Vedic practitioners actually use.

No Swiss Ephemeris, no JPL kernel at runtime — just the classical math, implemented and tested.

The bake-off: validation against NASA/JPL Horizons

A self-built engine is worthless if you can't say how wrong it is. So we validate against NASA/JPL Horizons — the modern ground truth — over a date grid, and report the p95 error per body (95th-percentile, not a cherry-picked best case):

Sun: within ~0.1 arcminutes (p95)
Moon: within ~1 arcminute (p95)

Acknowledging the tradeoff (where we lose)

Let's be direct: Swiss Ephemeris is more precise than we are. It's sub-arcsecond; our Moon sits around an arcminute. If raw precision were the only axis, you'd pick Swiss Ephemeris every time.

What ~1 arcminute buys instead is a fully auditable, fully self-built stack — every number traceable to a classical text and our own code, with no third-party binary you have to trust on faith. For a product whose entire claim is Bhāratīya provenance, an arcminute that you can fully audit beats a hidden arcsecond you can't. That's the honest tradeoff, stated up front rather than buried.

Why publish error bars at all?

Because a number you can audit is worth more than a marketing adjective. "Most accurate" is unfalsifiable; "Sun ~0.1′, Moon ~1′ vs NASA-JPL, p95" is a claim you can check. Publishing the error bar is the difference between asking for trust and earning it — and it's the only honest way to compete with a library that genuinely is more precise than you.

Closing

If you want to see it, the live bake-off and a free chart are here: bharatephemeris.com/accuracy.