Casting a chart by hand, once

Interpolate a planet's position from a daily ephemeris and assemble a chart without software, then check your work against the engine.

Lesson 5 of 6 in Casting a Chart · free to read · no account, no email

Why you should do this exactly once

Nobody casts charts by hand any more, and nobody should. Do it once anyway. The hour it takes buys you a permanent, physical understanding of what the wheel is, and it retires an entire category of vague anxiety about whether the software is doing something mysterious.

The whole job is five things. Convert your birth time to Universal Time using the zone history. Look up the planets in an ephemeris for the day, and for the following day. Interpolate between those two rows for your hour. Work out your local sidereal time, and look up the Ascendant, Midheaven and cusps in a table of houses for your latitude. Draw the wheel and mark the aspects.

The astronomy has already been done for you by the people who made the tables. What you are doing is the assembly, and the assembly is where a chart stops being an image and becomes a set of measurements you could defend.

The interpolation, worked

An ephemeris gives positions at one fixed hour — usually midnight or noon at Greenwich. Your birth is almost certainly not at that hour, so you work out how far through the day you were born and take that fraction of the planet's daily motion.

Suppose the Moon is at 14°22′ Leo at midnight on the 5th and 27°01′ Leo at midnight on the 6th. Subtract: it travelled 12°39′ in that day. A birth at 18:00 UT is three quarters of the way through, so take three quarters of 12°39′, which is 9°29′. Add that to the starting position and the Moon was at 23°51′ Leo.

Do the same for everything else and notice how the work distributes itself. The Moon moves twelve to fifteen degrees a day, so the interpolation is the whole job. The Sun moves about one degree, so a rough fraction is fine. Saturn moves a few arcminutes and Pluto barely stirs — for those you can read the row and stop.

Two things to watch. A planet near a station is not moving at a steady rate, so linear interpolation is at its worst exactly where a planet's condition matters most; a careful hand-caster uses three rows instead of two. And a planet that changes sign between the two rows needs the arithmetic done in continuous degrees, or you will subtract 29° from 2° and get a lunatic answer.

The angles, and the check that matters

For the angles you need local sidereal time: the sidereal time at Greenwich for your date and hour, corrected for your longitude. Then a table of houses — a printed book organised by sidereal time and by latitude — gives you the Ascendant, the Midheaven and the intermediate cusps directly. That is what those tables were for, and they were the astrologer's most-used possession for centuries.

Then you draw. Mark the Ascendant on the left, the Midheaven at the top, lay the cusps in, place each planet at its degree, and rule the aspect lines across the middle.

Now do the thing that makes the exercise worth an hour: cast the identical chart in Umbriel and compare, line by line. Everything should agree to within a few arcminutes. Where it does not, you have found something specific and useful — a missed daylight-saving hour, an interpolation done across a sign boundary, a longitude correction applied the wrong way round.

That comparison is the real lesson. You have just built the thing with your own hands and then checked it against an independent computation, which is the same standard this whole course applies to every number it prints. After this you will never again wonder what a chart is.

What that looks like

A common hand-casting error, and a useful one: the Moon at 28°40′ Cancer one midnight and 11°55′ Leo the next. Subtracting naively gives a negative number and a nonsensical result. Done properly, 28°40′ Cancer is 118°40′ and 11°55′ Leo is 131°55′, so the Moon travelled 13°15′. Every hand-caster makes this mistake once. Making it is what teaches you that the zodiac is a single continuous circle of 360° and the signs are only labels on it — the same lesson the very first module of this course started with.

What to take away

• The five steps by hand: convert to UT, look up two days in an ephemeris, interpolate for your hour, get the angles from a table of houses by sidereal time and latitude, then draw.
• Interpolation is almost all Moon: twelve to fifteen degrees a day against the Sun's one and Pluto's almost nothing.
• Interpolate in continuous degrees, never across a sign boundary — and use three rows near a station, where motion is not linear.
• Finish by casting the same chart in software and comparing line by line. The discrepancies are the lesson.

The common mistake

Interpolating a planet that changes sign between two rows by subtracting the degree numbers. The zodiac is one continuous 360° circle and the signs are labels on it; do the arithmetic in longitude and the problem disappears.

Try it on your own chart

Cast your own chart by hand. Get an ephemeris for your birth year, convert your time to UT, interpolate the Moon carefully and everything else roughly, look up your angles in a table of houses, and draw the wheel on paper. Then cast it in Umbriel and put the two side by side. Note every discrepancy and find out what caused it. Keep the paper one.

Check yourself

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

Keep going

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