Zodiac compatibility: how a birth chart comparison really works
Zodiac compatibility in astrology compares two full birth charts, not just sun signs: the Moon, the Rising sign and the planets, plus the angles they form with each other. You need both birth dates, accurate birth times and birth places. It is entertainment and a conversation starter, not a scientific prediction about a relationship.
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How do you calculate zodiac compatibility by date of birth?
For each person, the positions of the Sun, Moon and planets along the zodiac are calculated from date, time and place of birth. That part is pure astronomy: ephemerides give positions to fractions of a degree. The local birth time is converted to Universal Time (UTC) using the time zone and any daylight saving time in force at the birth place.
Then the two charts are laid over each other. Astrologers call this synastry: they look at the angles between one person’s planets and the other’s. What those angles “mean” is astrological interpretation, not calculation.
- Note both birth dates.
- Find each birth time as precisely as possible, for example from a birth certificate.
- Add the birth places so time zone and location are right.
- Calculate Sun, Moon, Rising and planets for both and compare the angles between the charts.
What are the Big 3 in astrology?
The “Big 3” are your Sun sign, Moon sign and Rising sign (Ascendant). The Sun sign is what people usually call their star sign and depends only on the date. The Moon passes through a sign in about two and a half days, so you already need the date and a rough time. The Rising sign is the sign coming up on the eastern horizon at the moment of birth and changes on average about every two hours.
| Placement | Changes about | Data needed |
|---|---|---|
| Sun sign | every 30 days | date |
| Moon sign | every 2–2.5 days | date and rough time |
| Rising sign | every 2 hours on average | date, exact time and place |
Can you find your rising sign without a birth time?
Not reliably. Because the Ascendant turns through all twelve signs during a day, any sign is possible without a birth time. The Moon sign, by contrast, can often be found without a time, unless the Moon changed signs on that day.
If the birth time is close to a sign change, a few minutes can change the Rising sign. A careful calculation tells you how certain a result is instead of faking precision.
What do aspects mean in a synastry chart?
Aspects are specific angles between two bodies, measured along the zodiac. Astrology treats some as “harmonious” and others as “challenging”. That split is a tradition of interpretation, not a measurable effect.
- Popular in compatibility readings: one person’s Sun to the other’s Moon, Venus to Mars, Moon to Moon.
- An aspect need not be exact; a margin (orb) of a few degrees is usually allowed.
| Aspect | Angle | Usual reading |
|---|---|---|
| Conjunction | 0° | fusion, strong emphasis |
| Sextile | 60° | easy, stimulating link |
| Square | 90° | friction, tension |
| Trine | 120° | harmony, ease |
| Opposition | 180° | attraction and contrast |
Is zodiac compatibility scientifically proven?
No. In a well-known double-blind test published in Nature in 1985, astrologers tried to match personality profiles to the right birth charts and did no better than chance. Later studies have not found solid evidence that charts predict personality or how relationships turn out.
A compatibility reading can still be fun as a prompt to talk about traits, wishes and differences. It is not a basis for decisions about a relationship.
How Zodeo calculates zodiac compatibility
Zodeo is a zodiac compatibility app for Android that calculates the Big 3 and planet positions for two people right on the phone and turns them into a match score. The app is explicitly for entertainment and says so: astrology is not a science and cannot predict how a relationship will go.
- Big 3 (Sun, Moon, Rising) with a short reading from date, time and place; without a birth time, Zodeo marks an uncertain Moon sign as “probable”.
- Positions of Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn and the Ascendant calculated after Meeus and JPL; the app shows local time, UTC and time zone of the birth place so you can check them.
- A match with a score from 0 to 100, four areas (attraction, talk, feelings, long run), Big 3 side by side and the strongest links in plain words.
- A birth chart wheel with positions, degrees and houses, plus a share card.
- 34,000 built-in places (GeoNames), no account, birth data stays on your phone.
- With Pro: unlimited people, every link between the charts (synastry), a daily vibe for two, the next 7 days and a morning note at 9:00.
What the app cannot do
Zodeo is entertainment: the match score and all readings follow astrological conventions and say nothing verifiable about a relationship. Without an accurate birth time the Rising sign is not reliable, and the app is available for Android only.
Price
Big 3, one match with the three strongest links, the share card and the chart wheel are free; Zodeo Pro costs 4.99 USD/EUR per month or 29.99 USD/EUR per year, with no trial, and can be cancelled anytime in Google Play. The free version shows ads; Pro is ad-free. We build this app ourselves; on Google Play it is listed under the developer name InfinityAkhi.
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Frequently asked questions
How accurate does the birth time need to be?
For the Rising sign, ideally to within a few minutes. For the Sun sign and usually the Moon sign, the date or a rough time is enough.
Which zodiac signs are most compatible?
Astrology traditionally pairs signs of the same element, such as fire with fire. There is no scientific evidence for this; relationships depend on people, not signs.
What is the difference between sun sign and birth chart compatibility?
Sun sign compatibility only compares the two star signs. A birth chart comparison looks at the full charts, including Moon, Rising, planets and the angles between them.
Does Zodeo work offline?
Yes, the calculation and place search run on the phone. Buying Pro through Google Play needs a connection.
Sources
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