About Stars.expert — A Free Astronomical Calculator

What the Calculator Covers

Stars.expert bundles more than twenty-five calculators into one toolkit, grouped into four families and free at the core. Every calculator runs on the same astronomical engine and the same JPL DE431 dataset, so a position you read in the birth-chart tool matches the position in the transit tool to the arcsecond. The interface is built for clarity: enter a date, a time and a place, and read the result.

Popular calculators: Birth chart · Synastry · Daily horoscope · Transits · Moon sign · Rising sign · Saturn return · Solar return · Lunar calendar · Composite chart

Technology & Accuracy

Every position on Stars.expert is computed with the Swiss Ephemeris, the C library published by Astrodienst and derived directly from NASA's JPL DE431 ephemeris. Calculations follow the IAU 2006 precession model and the IAU 2000B nutation series, with a stated precision of ±0.001 arcsec across roughly 30,000 years. House cusps, aspects, transits and eclipses are resolved to the degree, minute and second — never approximated from sun-sign tables. You can hover any planetary position in the tools to inspect the underlying ephemeris coordinates and check the arithmetic yourself. Our content follows a two-layer model. The first is the calculation layer: deterministic astronomical positions computed from the Swiss Ephemeris with ±0.001 arcsec precision, returning identical results for any user who queries the same birth data. The second is the interpretation layer: traditional astrological associations drawn from Hellenistic, Western and Vedic systems — what astrology has historically said about Mars in Aries, Venus in Libra, or a Saturn return near age 29. The calculation layer is deterministic and falsifiable; the interpretation layer is presented as traditional astrology, not as predictive guidance about specific life events.

Optional AI Interpretations

Beyond the calculators, Stars.expert offers an optional text layer that turns a computed chart into plain-language notes. It is built on Anthropic's Claude models and is kept entirely separate from the astronomy: the numbers come from the Swiss Ephemeris, while the wording is generated text. Every visitor can try one interpretation for free; the feature then unlocks with the optional PRO plan from €2.99 per month. The calculators themselves never require it.

• Chart notes — Claude turns a computed birth chart into plain-language text (PRO 30/month, ASTRO 60/month).
• Synastry notes — a written summary of the aspects between two charts (PRO 15/month, ASTRO unlimited).
• Transit notes — a description, in words, of the current planetary positions.
• Daily AI briefing — a written summary of the day's sky for a saved chart (ASTRO).
• Astronomy chat — up to 30 messages a day with the AI assistant (ASTRO).

Free vs Premium

All twenty-five-plus calculators are free and need no account. Optional subscriptions add the AI text layer, more saved birth profiles, an ad-free interface and ASTRO-only extras such as the daily briefing and the interactive 3D sky view.

Payments are processed by Stripe. A 7-day money-back guarantee applies to a first purchase. Yearly billing saves 20%, and a subscription can be cancelled anytime from the Stripe customer portal — access continues until the end of the billing period.

See full pricing details →

Fifteen Languages

Stars.expert is written natively in fifteen languages: English, Italian, Spanish, Portuguese, French, German, Russian, Polish, Dutch, Turkish, Hindi, Japanese, Romanian, Czech and Greek. Each version is a native rewrite reviewed by the Stars.expert Editorial Team, not a machine translation, so astronomical terminology stays accurate. URL slugs are localised (/it/tema-natale, /de/geburtshoroskop, /fr/theme-natal), hreflang tags are reciprocal across every page, and the sitemap declares the cross-language relationships server-side.

Privacy, Accounts & Hosting

Your birth data stays private and is never sold. Accounts are handled by Supabase Auth — sign in with Google OAuth or with an email and password. Every database table enforces Row Level Security, so only you can read your own profiles and saved charts. The platform runs on European infrastructure, is GDPR-compliant by design, and encrypts data in transit and at rest. You can export or delete an account at any time from the settings page.

How We Work

• Precision first. We publish computed astronomical positions and leave their meaning to the reader.
• Traceable data over hype. Every figure can be traced back to the ephemeris that produced it.
• Free at the core. The calculators never cost money; the optional AI layer funds the engineering.
• Privacy over tracking. We never sell birth data and never use it to target advertising.

How It Is Built

The interface runs on React, CRACO and Tailwind CSS. The backend is FastAPI in Python, with Supabase providing Postgres, authentication and Row Level Security. Stripe handles EU-compliant payments, Anthropic's Claude powers the optional AI text layer, and Cloudflare sits at the edge as a CDN and DDoS shield. The Swiss Ephemeris C library, compiled for production, computes every planetary position. Reference and explanatory copy across the site is drafted with AI assistance and reviewed by the Stars.expert Editorial Team before it is published.

Frequently Asked Questions

Contact

Questions, corrections or partnership ideas? Write to the Stars.expert Editorial Team at [email protected]

About WAYT

WAYT is an Italian technology studio that builds focused utility tools for specific verticals. Stars.expert is one of its projects. The WAYT Expert Network also includes vat.expert (an international VAT calculator), codicefiscale.expert (Italian tax-code tools), avif.expert (a modern image-format toolkit) and qrgen.expert (a QR-code generator) — one studio, several single-purpose calculators.