Ephemerides, time zones and houses: what birth chart calculation really involves

A birth chart is a map of the sky at the moment and place you were born. Calculating it is an astronomy problem with three inputs: your birth date, your birth time and your birthplace. From those we work out where every planet stood as seen from that spot on Earth and place them on the 360-degree wheel of the zodiac.

The Sun is the easy part, since it spends about a month in each sign. The Moon is quicker, about half a degree an hour, and the Ascendant and house cusps go all the way around the zodiac in a day, roughly a degree every four minutes. For those you need the exact time, the exact coordinates, and a correct conversion from local time to universal time.

An ephemeris is a table of where the planets are at any given moment. Ours is a high-precision ephemeris built on NASA JPL data and used by most professional astrology software. It gives the Sun, Moon and planets to the arc-second for any date across thousands of years.

Precision matters because a rough approximation can put a planet in the wrong sign or a cusp in the wrong house, and every reading built on that chart inherits the mistake.

The biggest source of error in birth charts is the clock, not the planets. To place your Ascendant and houses, your local birth time has to be converted to universal time using the offset that applied at your birthplace on that date, and those offsets have changed constantly as countries moved zones and adopted or dropped daylight saving time.

Take someone born in Indianapolis in July 1975. Most of Indiana did not observe daylight saving time between 1971 and 2005, so a calculator that applies today's summer rules will put that birth an hour off, often enough to change the Rising sign. Sigastra uses a full historical time zone database, so a 1975 birth is calculated with the rules of 1975.

Positions, houses and aspects are plain math. Run the same birth data twice and you get the same chart, to the arc-second, every time.

Ask a general chatbot for your natal chart and you will often get a confident answer with planets in plausible but wrong places, because a language model predicts words instead of calculating positions. We calculate the chart first and write the interpretation afterward, on top of those numbers.

Here the work falls into two separate layers. The first is astronomy: your planet positions, houses and aspects, which come out the same for anyone who runs the numbers. The second is interpretation, the meaning astrology has traditionally given those placements, written for you in plain language.

The first layer can be checked against any ephemeris. The second is offered for reflection and self-knowledge, as a mirror to think with. We tell you which is which.