How birth charts are calculated

Why your birth date matters

The date sets the overall sky for that day: the Sun's sign and the general positions of every planet. The fast-moving bodies — the Moon especially — can change sign within a single day, which is why the date alone gives an incomplete picture.

Why your exact birth time matters

Birth time fixes the parts of the chart that change fastest. The rising sign (Ascendant) moves through all twelve signs roughly every 24 hours — about one sign every two hours — so even a small error can change it.

Time also determines the house cusps and the four chart angles. Without an accurate time, these cannot be placed reliably.

Why your birth location matters

Location provides two things: the geographic coordinates that anchor the chart to a point on Earth, and the correct timezone for converting local time to Universal Time. The horizon — and therefore the rising sign and houses — depends on where on Earth you were.

How timezones and daylight saving are handled

To place your birth on the astronomical clock, local time is converted to Universal Time. That conversion needs the timezone rules that were actually in effect at your date and place, including daylight saving. GetBirthChart uses the IANA timezone database to apply the right historical offset instead of assuming a fixed one.

From positions to houses and aspects

Once the exact instant and location are known, the engine computes planetary longitudes from Swiss Ephemeris data. Houses divide the chart into twelve life areas anchored to the horizon, and aspects measure the angles planets make to each other within defined orbs. Developers can inspect the calculation implementation in the published GetBirthChart Python engine (opens in a new tab).

What happens if you don't know your birth time

You can still calculate a meaningful chart. Placements that don't depend on exact time — such as most planetary signs — remain valid. What GetBirthChart leaves out is the rising sign, the houses and the chart angles, because those require an accurate time. It omits them rather than guessing — see birth chart without a birth time — and you can add a time later if you find one.

Astronomy vs astrology

The calculation itself is astronomy: real positions of real bodies at a real moment. What those positions mean is astrology — a symbolic, reflective framework. GetBirthChart keeps the two clearly separate and does not present interpretation as a scientific prediction of events.