How is a birth chart calculated?

A birth chart is calculated from three inputs — your date of birth, your exact time of birth, and your place of birth — converted into the precise positions of the Sun, Moon and planets as they appeared from that spot on Earth at that moment. It is far more astronomy than mystery. Here is each step.

Step 1 — The three inputs

Your date fixes where the planets sit along their orbits. Your time fixes which signs are rising and setting, and therefore the houses. Your place fixes the local horizon — the same minute looks like a different sky from London than from Sydney. Remove any one of the three and part of the chart cannot be built.

Step 2 — Planetary positions from an ephemeris

An ephemeris is a table of where every celestial body sits at a given moment. Modern astrology software uses the Swiss Ephemeris, derived from NASA JPL's DE431 data and accurate to a fraction of an arcsecond across thousands of years. This is the astronomy step, and it has exactly one correct answer — there is no interpretation in it.

Step 3 — The Ascendant and the houses

Using your time and place, the software finds the Ascendant, or Rising sign — the exact zodiac degree climbing over the eastern horizon at your birth — and divides the sky into twelve houses, the life areas of the chart. Because the sky turns a full circle each day, the Ascendant moves about one degree every four minutes.

Step 4 — The aspects

Finally the software measures the aspects — the geometric angles between planets, such as the 90-degree square or the 120-degree trine. These are pure geometry, calculated directly from the positions found in step two.

Where two honest charts can still differ

If you have seen slightly different charts of yourself, two legitimate choices usually explain it. The first is the house system — Placidus, Whole Sign and others divide the houses by different rules. The second is the zodiac — tropical, used by Western astrology, versus sidereal, used by Vedic astrology. Same astronomy underneath, different conventions laid over it.

Why an exact birth time matters most

The planetary positions barely move over a few hours, but the Ascendant and houses move fast. A birth time wrong by even an hour or two can push your Rising sign into the next sign entirely and rotate every house. If you want the structural layer of your chart to be correct, an accurate time is the input that matters most.

The planetary positions have one correct answer. The house system and the zodiac are conventions — and an honest chart tells you which ones it used.

How Stars Lab calculates yours

Stars Lab runs every chart on Swiss Ephemeris data derived from NASA JPL, with exact degrees and house placements — and builds it in both the tropical and sidereal zodiacs from a single set of birth details, so you can see the conventions side by side.