From a birth certificate to a wheel

Name the five steps between three pieces of birth data and a finished chart, and say which input controls which part of the result.

Lesson 1 of 6 in Casting a Chart · free to read · no account, no email

Three inputs, five steps

You hand a program a date, a time and a place. It hands you a wheel. Between those two things are five steps, and knowing them is the difference between using software and understanding it.

One: the place becomes two numbers. A town name is looked up and resolved to a latitude and a longitude. Nothing astrological has happened yet.

Two: the clock becomes Universal Time. The historical zone offset for that place on that date is applied, daylight saving is removed if it was in force, and you are left with UT. The previous module is entirely about how this step goes wrong.

Three: the date and time become one number. Calendars are a mess of months, leap years and a ten-day gap in 1582, so astronomy does not compute in them. It converts to a Julian Day: a single continuous count of days and fractions, running from a fixed instant in 4713 BCE. Noon on 1 January 2000 is JD 2451545.0, and every subtraction between two moments in history becomes ordinary arithmetic.

Four: the planets are computed from that instant, in the uniform timescale, using the ephemeris.

Five: the angles and houses are computed from the local sidereal time and the latitude. Only now does the chart become yours rather than the world's.

Which input controls what

The three inputs do not do the same kind of work, and separating them explains several techniques at once.

Date and time set the planets. Nothing else touches them. Two people born at the same instant anywhere on Earth have identical planetary positions, to the arcsecond.

Longitude sets the clock twice over. It is used to convert civil time to UT, and again to turn UT into local sidereal time. Moving east or west therefore rotates the whole house framework while leaving the planets where they are.

Latitude touches only the horizon. It does not move a single planet. It changes where the horizon cuts the ecliptic, so it changes the Ascendant and the shape of the houses — and it does nothing else at all.

That separation is the entire mechanism behind relocation charts and astrocartography later in this course. You keep the moment, so the planets are untouched, and you change the place, so the houses turn underneath them. A planet that was buried in the twelfth in one city is on the Midheaven in another, and nothing about the sky has changed — only where somebody is standing to look at it.

How precise does each one need to be?

Different inputs deserve different amounts of worry, and knowing the exchange rates stops you agonising over the wrong one.

Latitude is forgiving. A tenth of a degree — about eleven kilometres — is far more than enough; it moves a house cusp by a small fraction of a degree.

Longitude is less forgiving, because it feeds the clock. One degree of longitude is four minutes of time, which is about one degree of Ascendant. So getting the right city matters and getting the right suburb almost never does.

Time is the sharp one, at a degree of Ascendant every four minutes, and the last module priced it in full.

The date looks like the safe one and has a trap in it. A birth shortly after midnight in a zone well ahead of Greenwich is on the previous day in Universal Time, and a birth shortly before midnight in a zone well behind is on the next one. Software handles this; people casting by hand forget it constantly, and the result is a chart a full day out with every planet in a plausible-looking wrong place.

What that looks like

Take one birth and move it, keeping the moment identical. Move it three thousand kilometres east and the planets do not shift at all, but local sidereal time jumps by about three and a half hours, so the Ascendant advances by roughly fifty degrees and every planet lands in a different house. Now move the original birth two thousand kilometres north instead: the planets still do not move, the Midheaven does not move either, but the Ascendant changes and the houses become markedly more lopsided. Two different inputs, two completely different kinds of change.

What to take away

• Five steps: place to coordinates, clock to Universal Time, date to a Julian Day, Julian Day to planets, sidereal time and latitude to angles and houses.
• A Julian Day is one continuous number, which is how astronomy escapes months, leap years and calendar reform.
• Date and time set the planets; longitude sets the clock and rotates the houses; latitude touches only the horizon and moves no planet at all.
• That separation is exactly why relocation works: keep the moment, change the place, and the houses turn under planets that never moved.

The common mistake

Thinking the birth place moves the planets. It does not, ever. It moves the horizon, and the horizon moves the houses — which is a big enough change to transform a reading without a single planet shifting by an arcsecond.

Try it on your own chart

Cast your own chart, then cast it again with the place changed to a city far to the east or west — same date, same clock time. Note what stayed still (every planet) and what moved (every house cusp). Then change the latitude only, by going a long way due north or south. This is relocation, and you have just found out what it can and cannot do before the lesson that teaches it.

Check yourself

The same questions the lesson asks in the app. Open one to see the answer.

Keep going

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