Co–Star and AsterNym: same sky, opposite instincts

One of these is ours. The other has about two hundred thousand more App Store ratings than we do.

5 min read · Published 7 September 2026

Straight to it

Co–Star is a free iPhone app that pulls planetary positions from NASA data and turns them into short, blunt daily lines, with paid add-ons for deeper readings and questions. AsterNym is a free website that computes your chart across eight symbolic systems and prints the placements and conventions behind every claim. Co–Star is better at being a daily habit and at writing a sentence you remember. We are better if you want to see the working, read more than one tradition, or check what an app told you. We built AsterNym, which you should factor in.

Starting with the obvious

We make one of these. Co–Star has 4.8 stars from about 206,000 ratings on its US App Store listing, and we're new enough that a comparable number doesn't exist for us yet. Take the rest accordingly, and note that everything below about Co–Star comes from their own listing and site.

The data underneath is the same argument

Co–Star's listing says they "use NASA data to chart the exact location of the stars in real-time", then adds that it "is then interpreted by real human astrologers who collaborate with AI technology to produce the birth chart and daily horoscopes you read in the app".

That first half is the part we'd sign our name to. Planetary positions are astronomy and there's a right answer; we compute ours with the astronomy-engine library from the same class of source, and our birth sky page reconstructs the actual sky over your birthplace so you can check it against an observatory if you like.

The second half is where we go a different way. Our interpretations are written by people and stored as data; no model writes a reading here. The one place a model appears is Ask AsterNym, which answers questions over our own pages, never calculates a chart fact, and refuses to answer at all when nothing on the site matches the question.

What Co–Star is better at

Being a habit. A push notification at the right moment is a product decision we haven't matched. Ours arrives as a morning email, which is quieter and easier to ignore.
Sentences that stick. The blunt one-liner is Co–Star's signature and it's a real skill. Nobody screenshots a paragraph about ayanamsa.
Friends and compatibility, socially. Co–Star built comparison into the app early and it shows in how people use it together.
Reach. Two hundred thousand ratings means a lot of people have found it more helpful than not, and that isn't nothing when you're deciding what to try.

What we do that Co–Star doesn't

More than the Western chart. Your BaZi, Zi Wei Dou Shu, Jyotish, numerology and Human Design placements sit beside your Western chart and are computed by their own rules. When they disagree we print the disagreement.
The arithmetic, in public. Every page states its zodiac, house system and ayanamsa and lists the placements it read. You can check us, or take our conventions to an astrologer who'll dispute them.
Nothing to buy. No question packs, no per-report unlocks. Co–Star's listing shows in-app purchases from $2.99 for five questions up to $11.99 for a year-ahead reading, and $8.99 a month for Pro–Star. Our reading is free and the only paid thing on the site is an optional star dedication.
A stated refusal list. We don't predict death, illness or pregnancy, and we withhold house placements when you have no birth time. It's all written down at what we won't tell you.

The tone question, which is the real one

Co–Star's voice is the thing people either love or bounce off. It's terse, occasionally cutting, and it doesn't hedge. That's a deliberate register, it made the app famous, and it does something a careful paragraph can't: it gets read.

The cost of that register is that a short definite line can't carry a caveat. "Don't text him" reads as advice from someone who knows. Nothing in a chart knows.

We went the other way, and we pay for it. Our copy is longer, it names its sources, and it says what it can't tell you, which is less quotable and less fun. If you find our pages dry, that's the trade we made and we'd make it again.

Which to open

Want astrology in your day, in a voice with an attitude, on your phone? Co–Star, and it's free to find out.

Want to see what eight systems make of the same birth moment, with the calculation on show? Start here. It costs nothing and takes about a minute.

Using both is coherent. One is a daily nudge; the other is the reference you check it against.

Is Co–Star's astrology accurate?

Its planetary positions come from NASA data, so the astronomy is as accurate as anyone's. Accuracy of interpretation is a different question that nobody can answer, because no accepted measure of astrological accuracy exists. What you can check is whether an app states its conventions and shows the placements behind a claim.

Does Co–Star use AI to write horoscopes?

Its App Store listing says NASA data is interpreted by real human astrologers who collaborate with AI technology to produce the birth charts and daily horoscopes in the app. AsterNym takes a different line: interpretations here are written by people and stored as data, and the only place a model runs is the Ask page, which never calculates a chart fact.

Is AsterNym free compared with Co–Star?

Both are free to start. Co–Star's US App Store listing shows in-app purchases ranging from $2.99 for five questions to $11.99 for a year-ahead reading, plus a monthly Pro–Star subscription at $8.99, checked 7 September 2026. AsterNym's full multi-system reading is free with no subscription, and the only paid item on the site is an optional one-time star dedication.

Can I use both Co–Star and AsterNym?

Yes, and they answer different questions. Co–Star is built for a daily rhythm on your phone. AsterNym is a reference you consult when you want the whole chart, several traditions side by side, or the arithmetic behind a claim someone has made about your chart.