Birth Chart Synastry

Synastry is the astrological practice of comparing two birth charts to explore a relationship. Enter both people’s birth dates and, if you know them, birth times and UTC offsets. The calculator works out where the Sun, Moon and planets were for each person and lists the aspects between the two charts — the angles astrologers read as harmony or tension — with their traditional meanings.

What synastry is

In astrology, a birth chart is a map of where the Sun, Moon and planets appeared along the zodiac at the moment someone was born. Synastry lays two charts side by side and looks at the angles, called aspects, between one person’s planets and the other’s. Astrologers read close aspects as points of connection: some suggest ease and shared understanding, others friction or strong attraction. It is a tradition of symbolism and self-reflection, not a scientific method — no controlled study has shown that astrology predicts compatibility — so treat the results as a conversation starter rather than a verdict.

How the positions are calculated

The calculator computes real astronomical positions. Planet positions come from the approximate orbital elements published by NASA’s Jet Propulsion Laboratory for the years 1800–2050, and the Moon’s position uses the main terms of Jean Meeus’s lunar theory. Checked against known events — such as the Jupiter–Saturn conjunction of December 2020 and the 2024 total solar eclipse — the positions agree to within about a tenth of a degree, which is ample for working out signs and aspects. Signs use the tropical zodiac, as in most Western astrology.

The major aspects

The orb is how far from the exact angle an aspect can be and still count. Tighter orbs mean stronger aspects in traditional practice. The calculator uses slightly smaller orbs for aspects that don’t involve the Sun or Moon, and you can switch between tight, standard and wide orbs.

Why birth time matters

The Moon moves about 13 degrees a day, so without a birth time its position can be uncertain by up to about 6–7 degrees either side of the noon estimate — sometimes enough to change its sign. The other planets move much more slowly, so the date alone is usually enough for them. The rising sign (ascendant) and houses need an exact time and place, so they are not included here. Enter the UTC offset that applied at birth: for example −5 for US Eastern Standard Time or −4 during daylight saving time.

Aspects commonly highlighted in synastry

• Sun–Moon contacts, especially conjunctions and trines, are traditionally seen as a sign of mutual understanding.
• Venus–Mars aspects are associated with romantic and physical attraction.
• Moon–Moon harmony suggests similar emotional needs.
• Mercury aspects describe how easily two people talk and think together.
• Saturn contacts are linked with commitment and responsibility — and with feeling restricted.
• Squares and oppositions are not “bad”: many long relationships have them; they point to areas that need understanding.

Worked example

With the sample dates, Alex (born 18 April 1992) has the Sun in Aries, and Jamie (born 2 November 1990) has the Sun in Scorpio. The calculator lists each person’s planet signs, then every aspect between the two charts within the chosen orbs, tightest first, and counts how many are harmonious (trines and sextiles) and how many challenging (squares and oppositions). Because Jamie’s birth time is unknown, the Moon is marked as approximate.

Using the printable worksheet

The template pack includes a blank zodiac chart wheel for each person, an aspect grid to fill in, a reference sheet of signs, elements and modalities, and a reflection worksheet with questions to discuss together. They are designed for workshops, journaling and astrology study groups.

A balanced view

Use synastry for curiosity and conversation. Real relationships depend on communication, shared values, respect and effort — things no chart can measure. If you are making important decisions, rely on your own judgement and, where appropriate, professional advice.

Privacy

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