What is a synastry chart?

Synastry is the oldest form of relationship astrology. You take two birth charts, lay one over the other, and look at the angles between one person's planets and the other's. Those angles are called aspects. Where they are easy, the two of you flow. Where they are tense, the relationship asks something of you.

It is not new. In the second century Ptolemy wrote in the Tetrabiblos that marriages tend to last when the Sun of one person and the Moon of the other stand in harmony. Astrologers have been comparing charts in this way ever since.

This calculator reads the contacts between your two charts and sorts them into four lenses: desire, home, work and abundance. A relationship is not one thing, so I do not think it deserves one score.

How to read your synastry aspects

Five aspects carry most of the meaning. The closer the angle is to exact, the more strongly you will feel it, which is why each contact above shows how many degrees it is from exact.

The tense aspects are not bad news. Squares and oppositions are where attraction often lives, and where a couple grows. A chart with only easy contacts can be pleasant and a little sleepy.

The four lenses

Desire and the honeymoon

Venus and Mars are the planets of attraction. When one person's Venus meets the other's Mars, desire has a place to land. The Moon and Venus together tell you how tenderness moves between you. If you want to keep the honeymoon blooming, this is the lens to tend.

Home and family

The Moon is how each of you feels safe. Moon to Moon shows whether your daily rhythms agree, and Sun to Moon is the classic sign of two people who can make a home.

Working together

Mercury is how you think and speak, Mars is how you act, and Saturn is how you commit. These contacts show how you will plan a business, run a household, or get through a renovation without a silence at dinner.

Abundance and staying power

Jupiter is where you grow each other. Saturn is the glue. A relationship with Jupiter contacts feels lucky, and one with Saturn contacts lasts. The fortunate ones have both.

Do I need a birth time?

No. With two birth dates the calculator can place the Sun, Mercury, Venus, Mars, Jupiter and Saturn accurately, which covers most of what synastry reads. The Moon moves about thirteen degrees a day, so without a birth time its contacts are marked as approximate.

If you add a birth time and city for both of you, the Moon becomes exact and the calculator also reads your rising signs. To find your own first, use the sun, moon and rising calculator.

Synastry chart or composite chart?

They answer different questions. A synastry chart compares two charts and shows how you affect each other. A composite chart blends the two into a single chart of midpoints, a chart of the relationship itself. Most astrologers begin with synastry, because it keeps both of you in view.

Is astrological compatibility real?

I want to be honest with you. When researchers have tested astrology against data, it has not held up. One study looked at more than ten million marriages in England and Wales and found that Sun signs made no difference to who married whom. Even Jung, who ran his own marriage experiment, decided his striking early results were not proof.

I still love it, and I use it. Not to predict whether two people will last, but as a way of asking better questions. Where do we flow? Where do we need care? What have we never said out loud? A chart is a good excuse for a real conversation.

More to explore

Find your own placements with the sun, moon and rising calculator, see which day of the week you were born on with the birth day calculator, or read your Human Design chart.

Common questions

Is this synastry chart calculator free?

Yes. The reading on this page is free, and so is the couple report by email if you would like it. There is nothing to buy.

What are the best synastry aspects for marriage?

The tradition names a few. The Sun of one person in harmony with the Moon of the other is the oldest, from Ptolemy. Venus with Mars brings lasting desire, Moon with Moon brings ease at home, and Saturn contacts bring staying power. No single aspect makes or breaks a marriage.

Can I use it for a business partner, a friend or a family member?

Yes. Synastry reads any two people. For a business partner, look first at the working together lens: Mercury, Mars and Saturn.

What if we have many hard aspects?

Then you have a relationship with energy in it. Squares and oppositions are where couples grow. Read each one as a practice, not a problem.

Is my birth data saved?

No. The chart is calculated in your own browser and nothing you enter is stored or sent. If you ask for the couple report, we keep your email and the signs of the two charts, never the birth dates.

How accurate is it without a birth time?

Accurate for every planet except the Moon, which is marked approximate. The rising sign needs a birth time and a city.

With love from Bali,
Myrah