Birth chart — how AstroStella computes it

The website route accepts a calendar date, an optional clock time, and coordinates. It converts the local time to UTC with the timezone of those coordinates, then posts date, lat, lng, and houseSystem PLACIDUS to the ephemeris service. The response includes a longitude for each body, the sign that longitude falls in (30° sectors starting at 0° Aries), a house number, and a retrograde flag when speed in degrees per day is negative.

Without a clock time the route still sends noon as a placeholder so the Sun and Moon can be labeled, and the page tells you the Ascendant is not reliable. Houses are angles on the local horizon. They move about one degree every four minutes of time. A chart that omits the time is a list of planetary signs, not a house chart.

This is not a sun-sign horoscope. The Sun’s sign is one longitude among the set. Mars, Venus, and the Moon can sit in other signs on the same timestamp. The sign pages on this site document each 30° sector. The chart is the only place those sectors are combined for one birth.

Entertainment interpretations in the app are generated after this calculation. They do not replace the longitudes. If a sentence in a reading cannot be traced to a position in the returned chart, it is commentary, not an ephemeris fact.

A concrete call looks like a birth in Paris at local noon on 21 March: the route resolves Europe/Paris, converts that noon to UTC, and posts houseSystem PLACIDUS. The Sun comes back near 0° Aries. The Ascendant does not, because noon is not sunrise. Swap the clock to 06:30 and the rising degree moves by hours of right ascension even though the Sun’s sign stays Aries. That pair of responses is the difference between a date lookup and a chart.