What is Swiss Ephemeris data?

An ephemeris (plural: ephemerides) is a table of the exact positions of the Sun, Moon and planets at any given moment in time. Astronomers and navigators have kept ephemerides for thousands of years — but the most precise ones today come from NASA's Jet Propulsion Laboratory (JPL), the same institution that calculates trajectories to send spacecraft to other planets.

The Swiss Ephemeris is a high-precision astronomical calculation engine, built by the astrology software company Astrodienst. Crucially, it is derived directly from NASA JPL's DE431 ephemeris. It can place every celestial body to a fraction of a degree, for any date across a span of roughly 30,000 years.

The data is hard astronomy. The reading is astrology. Most apps quietly skip the first part.

Why this matters for astrology

Here is the uncomfortable truth about most astrology apps: they don't calculate anything. They ask for your birth date, sort you into one of twelve sun signs, and serve a generic horoscope written for a twelfth of the population. There's no astronomy involved at all.

Real astrology has always depended on real planetary positions. A traditional astrologer worked from printed ephemeris tables and did the maths by hand. The signs, houses, and aspects in a chart are all derived from where the planets actually were at a precise moment and place.

When an app is built on Swiss Ephemeris data, every statement it makes is anchored to a real astronomical event. If it says Mars is conjunct your natal Venus, that is a specific alignment with a specific date and a measurable orb — not a vibe. The interpretation is still astrology, and you can take that as seriously or as lightly as you like. But the foundation is genuine science.

How Stars Lab uses it

Stars Lab calculates every reading — daily, weekly, monthly, yearly — against Swiss Ephemeris data, for your exact birth moment and your exact location. Every transit it describes names the precise degree, sign and house. That's the difference between "Geminis will have a busy week" and "Mercury is crossing your natal Midheaven on Thursday."