Quick answer: A birth chart is built in four steps. Your date and time become a Julian Day; the ephemeris gives each planet's ecliptic longitude for that instant; your longitude and the date give the local sidereal time, which fixes the Ascendant and Midheaven; and a house system divides the rest of the wheel from those angles.
Most articles about birth charts start after the chart exists. How to read your birth chart begins with a wheel already drawn. This one is about where the wheel comes from, because the calculation is the part that is actually checkable, and knowing it tells you exactly which of your details matter and why.
Step 1: The Moment Becomes a Number
Astronomy does not work in calendars. Calendars have leap years, a ten-day gap in October 1582 when most of Europe switched from Julian to Gregorian, and time zones that were invented in the nineteenth century and have been edited ever since.
So the first thing any chart program does is convert your birth moment to a Julian Day number: a continuous count of days since a fixed origin in 4713 BC, with the fraction of a day after the decimal point. Noon on 1 January 2000 is Julian Day 2451545.0. That number has no months, no leap years and no politics in it, and every position that follows is computed from it.
Two conversions happen before that number is reached, and both are where charts go wrong.
Your clock time becomes Universal Time. That means applying the time zone that was legally in force at your birthplace on your birth date, which is not always the one in force today. Countries have changed zones, adopted and abandoned daylight saving, and used wartime double summer time. A chart calculated with today's rule for a 1943 birth in Britain can be an hour or two out.
Your birth time is assumed to be local clock time, not solar time. If you were told a time by a hospital clock, that is what goes in.
Step 2: The Ephemeris Gives the Planets
An ephemeris is a table of where each body is, and modern chart software does not read a table, it evaluates the same underlying model the table was printed from. Ours uses the Swiss Ephemeris, which is derived from NASA's JPL planetary and lunar ephemerides.
What comes back for each body is an ecliptic longitude: an angle from 0 to 360 degrees measured along the ecliptic, which is the plane of Earth's orbit, starting from the vernal point. Divide that angle by 30 and you have the sign; the remainder is the degree within it. A planet at 217.4 degrees is at 7.4 degrees of Scorpio, because Scorpio starts at 210.
This is worth stating plainly: the signs in a chart are arithmetic on one number per planet. Nothing about them is looked up in a table of meanings at this stage.
The tropical zodiac used here is measured from the vernal point rather than from the constellations, which is why the sign a planet occupies is not the constellation it appears in front of. That is a real difference and not an error, and it is the difference between the tropical and sidereal systems rather than a mistake in either.
Step 3: Your Longitude Gives the Angles
Planets are the same for everyone born at that instant anywhere on Earth. The angles are not, and this is where your birthplace enters.
The chart program computes the local sidereal time: how far the Earth has turned, measured against the stars rather than against the Sun, at your longitude at your moment. From that and your latitude it derives:
- The Ascendant, the ecliptic degree rising on the eastern horizon.
- The Midheaven, the degree crossing the meridian overhead.
These move fast. The Ascendant advances roughly one degree every four minutes, so a whole sign passes in about two hours. That single fact explains why an exact birth time matters far more than people expect, and why a chart cast without one has to say so rather than assume noon.
Latitude matters here too, and it matters more than most people realise. The Ascendant and Midheaven are not 90 degrees apart except at the equator, and the further from it you were born, the more unevenly the ecliptic meets the horizon.
Step 4: A House System Divides the Rest
With two angles fixed, the remaining ten house cusps are produced by a house system, and the systems disagree because they divide different things: space, time, or the ecliptic itself.
Placidus, which this site uses throughout, divides the TIME a degree takes to rise from the horizon to the meridian. Whole sign gives each house an entire sign. Others divide the celestial equator or the prime vertical. They agree closely near the equator and diverge toward the poles, which is why the same chart can put a planet in the ninth house under one system and the tenth under another while every planetary position stays identical.
At high latitudes the time-based systems break down entirely, because some degrees never rise at all. That is not a bug in the software; it is a fact about the sky above the Arctic Circle, and it is one of the standing arguments for whole sign houses.
What This Means for Your Chart
Three practical consequences follow from the chain rather than from any interpretation.
Date alone gives you the planets. If you know only your birth date, the planets and most aspects are calculable and the angles and houses are not. That is why a chart without a time is a real chart with real limits rather than a guess.
Minutes change the angles, not the planets. A ten-minute correction moves the Ascendant about two and a half degrees and moves the Moon about five arcminutes. If a correction changes your Sun sign, the date was wrong, not the time.
Place changes everything angular. The same instant in two cities gives the same planets and a different Ascendant, different houses and a different Part of Fortune, since that is measured from the Ascendant.
Where the Errors Actually Come From
In order of how often they bite, and none of them is astrological.
The recorded time is wrong or rounded. Hospital times get written down late and rounded to a quarter hour.
The historical time zone is wrong. This is the silent one, because the chart looks perfectly normal and is simply an hour out.
The place is a large city entered as a point. Within one metropolis this is negligible; across a country it is not.
The interpretation is being blamed for an arithmetic problem. If two sites give you a different Ascendant, one of them has a different birth time or a different time zone in it, not a different opinion about astrology.
Frequently Asked Questions
What information does a birth chart calculation need?
Three things: the date, the time as exactly as you can give it, and the birthplace. The date fixes the planets, the time and place together fix the Ascendant, the Midheaven and all twelve house cusps.
Why does birth time matter so much?
Because the Ascendant moves about one degree every four minutes, so it crosses a whole sign in roughly two hours. The planets barely move in that window, which is why a wrong time damages the houses and the angles rather than the signs.
Is astrological calculation the same as astronomy?
The positions are. A chart uses the same ephemeris data astronomers use, and the tropical longitudes it reports are a straightforward transformation of it. What is not astronomy is the interpretation laid on top, which this site treats as a symbolic tradition rather than as a physical claim.
