A natal chart maps the positions of the Sun, Moon, planets, and other calculated points for a particular birth date and time. It also includes the Ascendant and houses when the birth time and location are available.

Synastry keeps the two natal charts separate and compares them. It does not create a new chart. The comparison may include aspects between the planets, contacts with the chart angles, and house overlays.

For example, an astrologer might examine an aspect between one person’s Mercury and the other person’s Moon. In common astrological interpretation, Mercury is associated with communication and the Moon with emotional responses. The aspect is then considered alongside the rest of both charts rather than treated as a complete description of the relationship.

An aspect is an angular relationship between two points in the charts. If one person’s planet is approximately 90 degrees from a planet in the other person’s chart, the two planets form a square. If they are approximately 120 degrees apart, they form a trine.

Different aspects are interpreted in different ways within astrology. Squares and oppositions are often associated with pressure, disagreement, or the need for adjustment. Trines and sextiles are often associated with an easier exchange between the two planets. A conjunction can be supportive, difficult, or both, depending on the planets involved and the rest of the charts.

An aspect does not determine whether a relationship is good or bad. A synastry reading considers how several aspects work together and whether similar themes appear elsewhere in the charts.

An orb describes how far an aspect is from being mathematically exact. A perfect square is 90 degrees. If two planets are 88 degrees apart, the square has an orb of two degrees.

Astrologers use different orb limits depending on the planets, the type of aspect, and their interpretive method. A tighter orb is often given more weight, but orb size is not the only factor considered in a relationship reading.

A natal chart is divided into twelve houses associated with different areas of life. A house overlay describes where one person’s planets fall within the other person’s houses.

For example, one person’s Venus may fall in an area of the other person’s chart associated in astrology with home, partnership, or shared resources. The overlay gives the astrologer another way to consider where the relationship may be felt most strongly in each person’s life.

House overlays depend on an accurate birth time because the Ascendant and house cusps change throughout the day. If a birth time is unknown, house overlays for that person cannot be treated as reliable.

An unknown birth time does not remove every part of synastry. Many planetary positions and cross-chart aspects can still be compared. However, the Moon and other time-sensitive factors may require additional caution if their positions change meaningfully during the possible birth-time range.

The reading should state this limit clearly. It should not use a guessed Ascendant, houses, or planet-to-house overlays as if the birth time were confirmed.

Astrologers use synastry to explore how two people’s chart patterns meet. This can include communication, attraction, emotional responses, sources of support, and areas where the two people may approach the same situation differently.

Synastry may help give language to a recurring relationship pattern, but it cannot establish why an event happened with certainty. The history of the relationship and the choices made by both people still matter.

Synastry cannot guarantee that a relationship will last, prove that two people are compatible, or determine whether someone is a soulmate. It cannot reveal another person’s private thoughts or diagnose a psychological condition.

Challenging aspects are not automatically negative, and easier aspects do not guarantee a healthy relationship. Their relevance depends on the rest of both charts and on the relationship itself.

Synastry compares two separate charts and focuses on the contacts between the people. A composite chart uses mathematical midpoints between the two natal charts to create a separate set of relationship-chart positions.

Within astrological practice, the composite chart is used to consider shared relationship patterns. It is a mathematical construction rather than a chart of an actual moment in the sky. Neither method replaces the other, and neither provides a final verdict on the relationship.

You can read a fuller comparison in Synastry vs. Composite Chart, or learn how the midpoint chart is built in What Is a Composite Chart?.

Horoscope Room considers synastry together with the relationship information you provide. A chart pattern can be relevant in different ways depending on whether the relationship has just begun, has lasted for years, or has ended.

The context does not change the chart calculation. It helps the reading address the relationship question you are actually asking.

Learn more about this process on How Horoscope Room Works, or start a personalized relationship report.