If you're using Co-Star, The Pattern, or most popular astrology apps, there's a good chance your birth chart isn't as accurate as you think. Not because astrology itself is flawed, but because most apps cut corners on the astronomical calculations that underpin every chart reading.
Here's what most astrology apps won't tell you about how they actually calculate your planetary positions.
The Problem: Simplified Calculations vs Real Astronomy
When you generate a birth chart, you're asking an app to calculate where every planet was in the sky at your exact time and location of birth. This requires serious astronomical maths - the kind NASA uses for space missions.
Most astrology apps take one of three approaches:
- License pre-calculated data from services like AstroDatabank or Astrodienst
- Use simplified ephemeris tables that approximate planetary positions
- Outsource to third-party APIs that may or may not be using accurate methods
The result? Your Sun sign might be correct (that's easy to calculate), but your Moon, rising sign, and house placements could be off by degrees - which in astrology, changes everything.
What "Real" Astronomical Accuracy Looks Like
There are basically two gold standards for astronomical calculations:
Swiss Ephemeris
The most famous is Swiss Ephemeris, developed by Astrodienst. It's incredibly accurate and widely used in professional astrology software. Many apps claim to use it, but licensing Swiss Ephemeris properly requires either paying for a commercial licence or open-sourcing your entire codebase (GPL licence).
Most apps don't do either. They either use a different library and say it's Swiss Ephemeris equivalent, or they use an older, less accurate ephemeris.
VSOP87 / NOVAS
The alternative is to use the same planetary theories that observatories use. VSOP87 (Variations Seculaires des Orbites Planetaires) is the French Bureau des Longitudes' planetary theory - the same maths used by space agencies.
NOVAS (Naval Observatory Vector Astrometry Software) is the US Naval Observatory's implementation. When combined with modern JavaScript libraries like astronomy-engine, you get Swiss Ephemeris-level accuracy without licensing restrictions.
This is what Lunary uses. Not because we're trying to build a telescope, but because if you're making life decisions based on planetary transits, you deserve calculations accurate to the arcminute.
Where Accuracy Actually Matters
"Does a degree or two really matter?" Yes. Here's where it shows up:
House Cusps
If your Ascendant (rising sign) is calculated even slightly wrong, every house cusp shifts. This affects which planets fall in which houses - which completely changes the interpretation of your chart.
Moon Sign
The Moon moves about 13 degrees per day. If an app uses yesterday's ephemeris position or rounds to the nearest degree, someone born at 11:58 PM might get the wrong Moon sign entirely.
Transits
Daily horoscopes and transit predictions depend on knowing exactly when a planet crosses a specific degree. If the app's planetary positions are off, the timing of "Mercury conjunct your Venus" could be wrong by days.
Progressions and Solar Returns
Advanced techniques like secondary progressions and solar return charts require precision down to the minute. Simplified calculations make these features essentially useless.
How Apps Hide This
Most apps don't lie outright. They just don't tell you which ephemeris they're using. Look for these red flags:
- No mention of data sources in their FAQ or About page
- "Proprietary calculations" (translation: we made it up or licensed something cheap)
- Only offering Sun sign horoscopes (because those are easy to fake)
- Birth time "not required" for features that absolutely require it
If an app is using real astronomical data, they'll brag about it. Swiss Ephemeris, JPL Horizons, VSOP87 - these are badges of honour. If you don't see them mentioned, assume the worst.
Why This Matters for Learning Astrology
If you're trying to actually learn astrology - not just read your daily horoscope - you need accurate data to spot patterns.
When you track transits and notice "every time Saturn crosses my Midheaven, my career shifts," that pattern only holds if Saturn's position is calculated correctly. Use an app with bad maths, and you're looking for patterns in noise.
This is why Lunary was built with education in mind. The app doesn't just give you a birth chart - it teaches you to read the ephemeris yourself, track transits in real-time, and understand why a planetary position matters. But none of that works if the underlying calculations are wrong.
How to Verify Your App's Accuracy
Want to test if your astrology app is legit? Here's how:
- Compare planetary positions with a known-accurate source like Astro.com or NASA's JPL Horizons
- Check house cusps between two apps - if they differ by more than 1-2 degrees, one of them is using bad maths
- Look for the fine print - does the app mention Swiss Ephemeris, VSOP87, or JPL data anywhere?
- Test edge cases - enter a birth time right at the cusp of two signs (like 11:59 PM) and see if the app gets the Moon sign right
Or just use Lunary and compare. If your current app's chart looks different, one of you is wrong - and it's probably not the app using Naval Observatory maths.
The Takeaway
Astrology is already asking you to trust a lot. The least your app can do is get the astronomy right.
If you're serious about learning astrology, demand transparency. Ask which ephemeris the app uses. If they won't tell you, that's your answer.
Want a birth chart you can actually trust? Generate yours at lunary.app - real astronomy, no shortcuts, completely free to use. We show you the maths, teach you to read the ephemeris, and never simplify the calculations just to make our lives easier.
Because if you're going to base your self-understanding on planetary positions, those positions should at least be correct.
I'm Sammii, founder of Lunary and indie developer building tools I actually want to use. I write about shipping products solo, the technical decisions behind them, and figuring it all out in public.