Synastry is comparing two birth charts to each other, rather than comparing two sun signs. It looks at how one person's planets land on the other person's chart, which is why it can describe a specific relationship instead of a general pairing of signs. It works for any two people, not only for couples.

Which astrology app is best for compatibility?

Co-Star is the best known for comparing charts with friends because of its social layer. Undersky reads each relationship as the kind of relationship it is, so a parent and child are never read like a couple. The right one depends on whether you want a social feature or a private reading.

Is astrology compatibility accurate for family members?

Synastry works for any two charts, but the interpretation should change with the relationship. Romantic language applied to a parent and child reads badly and can be genuinely uncomfortable. Undersky asks what the relationship is and changes the reading accordingly, and keeps readings about children developmental.

The best astrology app for compatibility and synastry

Q: Can you check astrology compatibility without typing in someone else's birth details?

In Undersky, yes. One person sends an invite that carries their own computed chart positions, never their birth date, time or place. The other person opens it in Undersky, adds their own details, and the reading for the two of them is worked out on that phone. Neither of you makes an account and nothing is stored on a server in the process.

Published 10 August 2026 by Pacefyx, Inc. We make Undersky, one of the apps below, so check the claims in this section against the store page.

Co-Star is best if you want to compare charts with friends inside a social app. Undersky is best if you want the reading to change depending on what the relationship actually is.

What is synastry?

Synastry is comparing two whole birth charts to each other, rather than comparing two sun signs. It looks at where one person's planets land on the other person's chart.

That is why it can say something about one specific relationship, instead of something general about Leos and Scorpios. It also works for any two people, not just couples. That last part is what most apps ignore.

Which app is best for compatibility?

App	How it handles two people	Best if
Co-Star	Add friends and see how your charts meet, inside the app	You want astrology to be social
The Pattern	Detailed relationship readings and timing between two people	You want psychological depth about the bond
TimePassages	Proper synastry and composite charts to read yourself	You are studying astrology
Sanctuary	Ask a human astrologer about the relationship	You want a person's judgement
Undersky	Pick what the relationship is, and the reading changes with it	You want family and friends read as family and friends

Why does the type of relationship matter?

Because the same two charts mean different things depending on who the two people are. A tense aspect between two charts is one thing in a marriage and quite another between a mother and a nine-year-old.

Most compatibility features were built for dating, then pointed at everyone. That is how you end up with romantic language describing you and your dad. At best it is silly. At worst it is upsetting.

The same two charts mean different things depending on who the two people are.

Undersky asks what the relationship is first: partner, parent, child, sibling, friend or coworker. The reading changes with the answer, and readings about children stay about growing up. The app enforces that itself rather than trusting the writing to remember.

Can you check compatibility without typing in someone else's details?

In Undersky, yes. One person sends an invite carrying their own computed chart positions, never their birth date, time or place. The other person opens it in Undersky, adds their own details, and the reading is worked out on that phone. Neither of you makes an account, and nothing is stored on a server along the way.

This matters more than it sounds. The usual way to compare charts is to type someone else's birth details into an app, or to let an app read your contacts. Both hand over information about a person who never agreed to anything.

What does Undersky's compatibility actually include?

A full reading of the two charts against each other, in the frame of the relationship you chose. It is a one-time purchase, not a subscription, and it stays yours.

You pick the relationship type, and the reading is written for it.
The reading grows as the relationship does, for as long as you keep it.
Every reading has a "Show what this reading came from" button listing what it was built on.
Both people's birth details stay on the phone. There is no account and no server of ours.

Your own chart, daily horoscope and a first look at your year are free before any of this, so you can see how the writing reads before deciding.

Common questions

What is the difference between synastry and a composite chart?

Synastry compares two charts to each other, planet by planet. A composite chart merges the two into one chart that stands for the relationship itself. Synastry describes how you meet. A composite describes what you make together.

Is astrology compatibility accurate for friends and family?

The chart comparison works for any two people. What has to change is the reading itself, because advice written for lovers reads wrongly for a sibling or a child. Look for an app that asks what the relationship is.

Do both people need the app for compatibility?

Yes, you both open it in Undersky. What you never do is type someone else's birth details into your own phone, which is the usual way this works elsewhere. The invite carries computed positions rather than birth data, and each of you enters only your own.

Is there a free compatibility reading?

Undersky's invite flow gives a free reading to the pair who use it. Beyond that, relationship readings are a one-time purchase, and your own chart and daily horoscope are free.