How is a birth chart calculated?

A birth chart is a map of the sky at one exact moment — the instant and place you were born — and calculating it is a precise astronomical problem, not a matter of opinion. It starts with three inputs: your birth date, your birth time, and your birthplace. From those, the calculation works out where every body in the solar system stood, as seen from that spot on Earth, and lays them onto the 360-degree wheel of the zodiac.

The Sun's position is easy — it moves slowly and stays in one sign for about a month. The demanding parts are the fast-moving points: the Moon shifts about a degree every two hours, and the Rising sign (Ascendant) and house cusps rotate through the entire zodiac every twenty-four hours, roughly a degree every four minutes. Getting those right means knowing the exact time and the exact geographic coordinates, then converting your local birth time to universal time correctly — which is where accuracy is won or lost.

Sigastra computes planetary positions with Swiss Ephemeris, the high-precision ephemeris derived from NASA's JPL data and trusted by professional astrology software worldwide. It gives the position of the Sun, Moon and planets to the arc-second for any moment across millennia. When Sigastra shows you a placement, it is the real position of that body in the real sky — computed, never guessed.

This matters because astrology's interpretations are only as trustworthy as the astronomy beneath them. A chart built on a rough approximation can put a planet in the wrong sign or a cusp in the wrong house, and every reading downstream inherits that error. We start from the exact numbers so the interpretation has something true to stand on.

The single biggest source of error in birth charts is not the planets — it is time itself. To place your Ascendant and houses, the calculation must convert your local birth time to universal time using the offset that applied at your birthplace on your birth date. And time zones have changed constantly through history: countries shifted zones, adopted and dropped daylight saving, and redrew borders.

Take someone born in Yugoslavia in 1965. The rules for standard time and any summer-time adjustments then were not the same as today's rules for that same location — and a naive calculator that applies the modern offset can land the birth time an hour off, which is enough to move the Rising sign into the wrong sign entirely. Sigastra uses a full historical timezone database, so a 1965 birth is calculated with 1965's rules, not 2020's. That accuracy on the time axis is our quiet advantage: it is the difference between a chart that merely looks right and one you can actually trust.

It is worth being clear about what is calculation and what is not. The positions, the houses, the aspects — all of it is deterministic mathematics. Run the same birth data twice and you get the identical chart, to the arc-second, every time. Nothing about the sky is invented by a language model.

This is a real distinction in an age when you can ask a chatbot for "your natal chart" and get a fluent-sounding answer with planets in plausible-but-wrong places, because the model is predicting text rather than computing astronomy. Sigastra computes the chart first, from real ephemeris data, and only then writes the interpretation in plain language on top of the exact numbers. The words are there to make the astronomy meaningful — they never stand in for it.

So how does astrology work at Sigastra? In two clearly separated layers. The first layer is astronomy: exact, deterministic, and identical for everyone who runs the numbers — your planetary positions, your houses, the geometric aspects between your planets. The second layer is interpretation: the meaning astrology has traditionally assigned to those placements, written back to you in human language, in your own tongue.

We keep the layers honest and distinct. The astronomy is exact and verifiable. The interpretation is thoughtful and candid, offered for reflection and self-knowledge rather than prediction — a mirror to think with, not a fortune to obey. Knowing which is which is what makes a reading you can actually trust.