How we calculate and read a birth chart

From birth details to a chart

A chart uses a birth date, local birth time, and birthplace. The location helps determine coordinates and time zone; the calculation uses Astronomy Engine to find planetary positions in the tropical zodiac. Check the date, location, and time carefully, especially around historical time-zone changes.

Houses and angles

When a birth time is known, Astral Meridian uses equal houses: twelve 30-degree sections beginning at the Ascendant. The Midheaven is calculated separately and is not necessarily the tenth-house cusp. Our angle calculation limits latitude to 66 degrees north or south, so angles and houses at higher latitudes are approximate.

When birth time is unknown

You can still cast a chart. We withhold the Rising sign, houses, and angles when the time is unknown. Planetary positions vary through the day; a sign near a boundary, especially the Moon, may depend on the birth time. A placeholder time is not a known birth time.

From chart to reading

The chart calculations provide the placements used in a reading. AI helps turn chart information into plain language. Astrological interpretation is symbolic and intended for reflection, not a guaranteed prediction or medical, legal, financial, or mental-health advice.

Local time and calculation limits

Enter the local clock time at the selected birthplace. The app uses a named time zone to convert it to a UTC instant. Historical time rules and times near daylight-saving transitions deserve extra care. Do not convert the clock time to UTC before entering it.

The diagram above is generated by the same planetary calculation functions used in the product. It demonstrates the method; it is not independent accuracy validation. This page does not claim that every date, time-zone transition, or high-latitude chart has been independently checked against an ephemeris.

Reference and corrections

Astronomy Engine’s documentation describes the calculation library. It does not validate symbolic astrology. See our editorial approach and correction contact if a page’s claim conflicts with this method.