Tools of the Trade

Yesterday I was talking to Maria Kay Simms, whose publishing house, ACS Publications,  produces many astrological reference works such as the American Ephemeris series and the American Atlas. I asked her how many people still hand-calculate charts in the computer age, and mentioned that I haven’t done so in decades. She replied that those who sit for a professional astrologer’s certification exam still have to demonstrate the ability to manually create a natal horoscope (probably to show that they understand its scientific basis), something I had forgotten. I decided to pull together all of the documents I once used in my own practice in order to refresh my memory on exactly how complicated it was.


First and foremost was an ephemeris showing the planetary positions for each day, at either midnight or noon, over a range of years. The American Ephemeris series (calculated for midnight) has become the “gold standard” for this, but many computer programs use the Swiss Ephemeris.

Next came a table of houses (I used Dalton’s Spherical Basis of Astrology, a Placidus house system, but there are numerous others). This showed the house cusps for hourly intervals of Greenwich Mean Time.

Third was a table of proportional logarithms, which allowed easy interpolation between listed positions for the planets and house cusps in order to zero in on their exact positions for the time of birth. All that was needed in the way of math skills was the ability to solve simple algebraic equations.

Fourth was a book of longitudes and latitudes for major cities in the area of the nativity being examined, coordinates that could then be interpolated between two listed locations to arrive at the exact degrees and minutes of longitude and latitude for the birthplace. I used Eugene Derney’s 1945 Longitudes and Latitudes in the US.

Because of the uneven application of Daylight Savings Time in the USA after World War II, it was necessary to have Doris Chase Doan’s Time Changes in the USA.

Although it wasn’t needed during my early days, an ephemeris showing the positions of the major asteroids became necessary when their use in astrology became fashionable. (To be honest, I never used it since I don’t bother with asteroids.)

A textbook explaining the step-by-step process of chart calculation was required. I used Jeff Mayo’s How To Cast A Natal Chart, augmented by minor technical tweaks (like “acceleration on the interval”) that I found here and there.

Finally, a horoscope form to manually record all of the calculations was needed. I always used a very basic one but there were many more elaborate versions. (I also had a fill-in-the-blanks sheet showing all of the steps to make sure I didn’t miss any, but I seem to have lost all of them in our recent move.)

I have been gratified to see that the hand-calculated charts I created back in the early ’70s perfectly match those produced by my current computer program, right down to the exact minutes of arc. Although I still use my ephemeris for quick look-ups of retrogradation, ingresses, eclipses and other phenomenon, I doubt very much that I will ever manually generate another chart.

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