Quick answer: Co-Star reads your chart in one tradition, Western astrology on the tropical zodiac, using the Porphyry house system and NASA planetary data. Starwell reads the same birth data across three traditions, Western, Vedic, and Hellenistic, computed on the Swiss Ephemeris, and marks where the traditions agree and where they diverge. Co-Star is a free app with in-app purchases; Starwell delivers one complete report, $9 for the mini or $39 for the full, with an optional companion at $6.99/mo or $49/yr that adds unlimited chat with your chart, a personalized read every Sunday, and your weekly archive.
3 vs 1
Traditions read from the same birth data (Starwell vs Co-Star)
The context
Co-Star and Starwell both begin with real astronomy: your exact birth time and place, and planetary positions from precise ephemeris data. They differ in how many traditions they read that chart through, which house system they use, and what they hand you at the end.
Co-Star is a Western astrology app. It reads the tropical zodiac, the same zodiac behind most daily horoscopes, and presents algorithmically generated placements and daily updates. Its house system is Porphyry. Co-Star has said it chose Porphyry because the first house lines up with the Ascendant and the tenth with the Midheaven, which is quick to read at a glance.
Starwell reads the same birth data three ways. Western on the tropical zodiac. Vedic on the sidereal zodiac, which is offset from the tropical by about 24 degrees and often moves a planet into a different sign. And Hellenistic, the ancient Greek system built on sect and essential dignities. Every placement is computed on the Swiss Ephemeris, delivered as one complete report, with the points where the three traditions agree and disagree marked explicitly.
Side by side
The scorecard.
Feature
Co-Star
Starwell
Traditions read
1 (Western)
3 (Western, Vedic, Hellenistic)
Zodiac
Tropical
Tropical + sidereal
House system
Porphyry
Whole-sign + Placidus
Planetary data
NASA data
Swiss Ephemeris (JPL-derived)
Shows where traditions disagree
No (single tradition)
Yes, explicitly
Format
App: daily updates + placements
One complete report, same day
Ongoing access
Free app, ongoing daily updates
Optional companion: unlimited chart chat, a read every Sunday, weekly archive
Pricing model
Free app, in-app purchases
$9 mini / $39 full one-time; optional companion $6.99/mo or $49/yr
For raw planetary positions, both rely on precise astronomical data. The difference is scope: Co-Star reads one tradition and returns a running feed; Starwell reads three from the same chart and returns one complete report that marks where the traditions agree and disagree.
What Co-Star does
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Reads your chart in the Western tropical tradition, the zodiac behind most daily horoscopes.
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Computes placements from NASA planetary data, not a personality quiz.
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Sends daily updates and lets you compare charts with friends.
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Is free to use, with in-app purchases for additional features.
What reading three traditions adds
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A Vedic (sidereal) read of the same chart, offset about 24 degrees from the tropical zodiac, which often lands a planet in a different sign.
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A Hellenistic read using sect and essential dignities, which can rate a placement strong or weak on grounds Western interpretation does not use.
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An explicit map of where the three traditions agree, which is higher-confidence, and where they diverge, which is usually the most revealing part.
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One complete report you keep, delivered the same day, rather than a running feed.
How we compare
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This comparison is published by Starwell (Hermes Horizon Pty Ltd). It is our own comparison, not an independent review.
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Co-Star details are drawn from Co-Star public app listings, Co-Star statements, and published reviews, verified as of July 2026. Co-Star is free with in-app purchases; we do not list a single price because Co-Star does not publish a fixed price.
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Starwell details describe our own product: three traditions computed on the Swiss Ephemeris, delivered the same day, $9 for the mini or $39 for the full, plus an optional companion subscription at $6.99/mo or $49/yr that adds unlimited chat with your chart, a personalized read every Sunday, and your weekly archive in the member area. Cancel anytime.
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If any detail here is out of date or inaccurate, contact us at starwell-hh.com and we will correct it.
Frequently asked
Compare, answered.
Is Co-Star accurate?
Co-Star computes placements from NASA planetary data, so its raw positions come from real astronomy. Its house placements use the Porphyry system, which can differ from the whole-sign or Placidus systems other tools use, so house-based readings may not match. It reads the Western tropical tradition only.
Does Co-Star use Vedic astrology?
No. Co-Star reads the Western tropical zodiac only. It does not compute a Vedic (sidereal) or Hellenistic chart, so it does not show the sign differences that appear when the same birth data is read in the sidereal zodiac, which is offset from the tropical zodiac by about 24 degrees.
What is a good Co-Star alternative for a full birth chart reading?
If you want more than one tradition, Starwell reads the same birth data across Western, Vedic, and Hellenistic astrology on the Swiss Ephemeris and delivers one complete report the same day. The mini is $9, the full report $39. If you want something ongoing rather than one-time, the optional companion is $6.99/mo or $49/yr and adds unlimited chat with your chart, a personalized read every Sunday, and your weekly archive.
Why is my Co-Star sign different from another app?
Different tools can use different zodiacs and house systems. Co-Star uses the tropical zodiac and Porphyry houses. A Vedic tool uses the sidereal zodiac, offset about 24 degrees, which can place a planet in a different sign. Starwell shows both the tropical and sidereal reads so you can see the difference.