Zodiacal Astrology : How To Erect a Zodiacal Chart

Method   Example   Southern Hemisphere Charts
Method

You will need an ephemeris, a set of house tables and a good atlas or gazeteer. Learning to erect a horoscope by hand rather than use a computer is recommended for three reasons. The most important being that the whole process allows the astrologer to get the 'feel' of the chart. Secondly, computer programs are not entirley accurate in the writer's experience and, lastly, they soon become outdated whereas books can last a life time. With practice, all of the calculations listed below can be accomplished in 15 - 20 minutes.

Three different 'kinds' of time are involved the the calculation of a horoscope. Firstly, Local Time - the 'clock' time at the place of birth. Secondly, Greenwich Mean Time (G.M.T.) - the 'clock' time at Greenwich with no summer/daylight savings time correction. Thirdly, sidereal time - the time according to the stars. 24 hours of 'clock' time is very nearly 4 minutes longer than 24 hours of sidereal time - for an explanation of this consult a good book on astronomy.

N.B. Throughout this page references to Greenwich mean Greenwich in London, England, at 0 o longitude. Also, dates are given in universal format i.e. year-month-day.

Steps 1 to 4 deal with finding the planets' positions.

STEP 1   Note the place, date (local) and time (local) of birth.

STEP 2   Note the longitude and latitude of the place of birth.
Use a good atlas or gazetteer.

To find the planets' positions we have to convert the local date and time of birth to the date and time at Greenwich because ephemerides use Greenwich Mean Time (G.M.T.).

Calculations now begin with the time of birth in 24hr. form.

STEP 3   Include the Zone Correction.
Use a good atlas or gazetteer.
If the birth place is east of Greenwich, the zone correction should be subtracted from the local time of birth. If west, it should be added to the local time of birth.

STEP 4   Include Summer Time Correction.
If summer/daylight savings time was in operation on the local date of birth subtract this from the result given in step 3.

These calculations might alter the apparent date of birth at Greenwich.

We now have the date and time at Greenwich (in G.M.T). Use these to find the planets' positions in an ephemeris.

Steps 5 to 9 deal with finding the house cusps. To do this we have to convert the G.M.T. time of birth into the local sidereal time because house-tables use Sidereal Time and we need to find how the zodiac was aligned at the place of birth.

Calculations now begin with the sideral time at noon at Greenwich for the date and time derived in step 4.

STEP 5   Note the sideral time at noon at Greenwich for the date derived in step 4.
Use an ephemeris.

STEP 6   Calculate the Interval to/from noon.
If the time derived in step 4 is after 12 noon (P.M.) then add the number of hours & minutes after 12 noon to the sidereal time at noon at Greenwich.

If the time derived in step 4 is before 12 noon (A.M.) then subtract the number of hours & minutes before 12 noon from the sidereal time at noon at Greenwich.

STEP 7   Calculate the Acceleration on the Interval.
This calculation takes account of the difference between 'clock' time and sidereal time.
For every hour of the Interval include an extra 10 seconds. Add or subtract this as per the Interval.

We now have the sidereal time at Greenwich for the G.M.T. time of birth. If we used house tables to look up the house cusps at this point they would apply to Greenwich only. So, the final step is to convert the sidereal time at Greenwich to the local sidereal time. This correction is called the Longitude Equivalent.

STEP 8   Calculate the Longitude Equivalent.
If the place of birth is east of Greenwich then add 4 minutes of sidereal time per degree of longitude to the sidereal time at Greenwich.
If the place of birth is west, subtract 4 minutes of sidereal time per degree of longitude from the sidereal time at Greenwich.

It is possible to end up with a sidereal time that is greater than 24 hours by this point, so:-

STEP 9   Subtract 24 hours if necessary.

We now have the local sidereal time. Use this to find the house cusps in a book of house-tables by looking at the right page for the latitude of the place of birth.

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Example

STEP 1   Place of Birth. Montreal, Canada
Date of Birth (local) 2001-06-01
Time of Birth (local) 9.30 p.m.
STEP 2   Longitude of the place of birth 73W36
Latitude of the place of birth

 45N30

h . m . s
Time of Birth 21.30.00
STEP 3   Zone Correction +5.00.00
Result

 26.30.00

STEP 4   Summer/Daylight Savings Time -1.00.00
Result

 25.30.00

G.M.T. Date (see Note 1) 2001-6-02
G.M.T. Time (24hr.)

 1.30.00

STEP 5   Sideral time at noon at Greenwich on 2-6-2001

 4.44.05

STEP 6   Interval to noon at Greenwich on 2-6-2001 -10.30.00
Result (see Note 2)

 18.14.05

STEP 7   Acceleration on the Interval (10.5 x 10 = 105 seconds) -00.01.45
Sidereal Time at Greenwich

 18.12.20

STEP 8   Longitude Equivalent (73.6 x 4 = 294.4 min.) -4.54.24
Result (see Note 3)

 13.17.56

STEP 9  

 Subtract 24 hours if necessary

 N/A

Local Sidereal Time 13.17.56

... and that's all there is to it!

Note 1    We have subtracted 24 hours from the time and added 1 day to the date.
Note 2    We have added 24 hours before subtracting the Interval.
Note 3    (1) 73o 36' = 73.6o.
(2) 294.4 minutes = 294 min. 24 sec.

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Southern Hemisphere Charts

The only difference for southern hemisphere charts relates to the house cusps. House-tables are normally given for northern latitudes only, but the conversion to southern latitudes is relatively straight-forward. This conversion relies on the fact that the house cusps for a place in the southern hemisphere at a particular sidereal time will be the reverse of the house cusps at the exact opposite place on the Earth at the same sidereal time. Visualising this will help.

STEP A  Perform all of the steps given under Method, but don't look up the house cusps yet.

STEP B Add, or subtract, 12 hours from the Local Sidereal Time - whichever is easiest. This 'puts' us on the opposite side (longitude) of the Earth.

STEP C Using house tables for northern latitudes, note the house cusps for the same number of degrees of latitude as the place of birth. For example, if the birth place was at 30o South then use the page for 30o North. This 'puts' us at the exact opposite place (longitude and latitude) on the Earth - but our view of the sky has been reversed. So ...

STEP D Reverse the house cusps such that the Asc. becomes the Desc., the 2nd. house cusp becomes the 8th. and so on. Use these reversed cusps in the horoscope.

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