The calendar date and time are converted into a single astronomical moment. Birthplace helps determine the correct time zone and provides the geographical coordinates used to calculate the Ascendant, Midheaven, and houses.

The planetary positions are calculated for that moment. The chart angles and houses also depend on where the birth took place because they describe the sky in relation to the local horizon and meridian.

This is why a complete birth chart asks for all three pieces of information. They do different jobs in the calculation.

The Ascendant is the zodiac degree rising on the eastern horizon at a given time and place. Its degree changes continuously, and the rising sign changes roughly every two hours on average. The actual rate varies by location and zodiac sign.

Even a modest difference in birth time can change the Ascendant degree and the house cusps. In some cases, especially near a sign or house boundary, it can also change the rising sign or the house assigned to a planet.

The Moon moves more quickly through the zodiac than the other commonly used planets. If the birth time is unknown, its exact degree and aspects may be uncertain. On some dates, the Moon may also change signs during the possible birth-time range.

An unknown birth time does not make the entire chart unusable. Many slower-moving planetary positions change little during a single day, and many aspects can still be calculated within a reliable range.

The reading needs to distinguish these more stable factors from details that depend on an exact time. The Moon may require qualification, and the Ascendant, Midheaven, houses, and house-based techniques should not be presented as confirmed without sufficient birth-time information.

This produces a narrower chart, but it still allows a relationship reading to consider many cross-chart planetary contacts.

Synastry compares two natal charts. When both birth times are reliable, it can include house overlays, chart-angle contacts, and other time-sensitive factors.

A house overlay shows where one person’s planets fall in the other person’s houses. Without a reliable time for that person, those houses cannot be calculated with the same confidence. The reading can still consider supported planetary aspects, but it should not claim that a planet falls in a particular house.

If the Moon or another fast-moving factor changes significantly within the possible time range, its cross-chart aspects also need to be treated as uncertain.

A midpoint composite chart calculates new positions from the two natal charts. Many composite planetary midpoints may still be available when a birth time is unknown, although the Moon and other time-sensitive positions may need qualification.

Composite angles and houses depend on the underlying birth information and should not be treated as reliable when that information is uncertain. This is different from saying that no composite information can be used at all.

Horoscope Room allows you to state that a birth time is unknown or approximate. We do not treat an estimate as an exact time.

The reading uses time-sensitive chart features only when the available information supports them. If it does not, houses, angles, overlays, and other dependent claims are omitted or clearly qualified.

If you can provide a rough time window or supporting information, it may help narrow what can be assessed. It does not automatically make every time-sensitive part of the chart reliable.

Birth-time records vary by country and region. Useful places to check include:

• The civil registry or vital-records office where the birth was registered
• An original or long-form birth record
• Hospital or maternity records
• A baby book, birth announcement, or family journal
• A parent or relative who was present

A family member’s memory may provide a useful range, such as early morning or late evening, but it should not be treated as an exact minute unless there is a reliable record.

You can still request a relationship reading if one or both birth times are unknown. Enter the information as honestly as you can and mark the time as unknown or approximate rather than selecting an exact time for convenience.

The resulting reading will focus on the chart factors that remain supported. It will also explain where the missing time limits what can be said.

Learn more about the complete process on How Horoscope Room Works, or read What Is a Natal Chart? before continuing to synastry.