Calculating a natal Chart

The four stages of calculation:

  1. Determine the time of birth in Greenwich Mean Time
  2. Find the local sidereal time at birth
  3. Calculate the planets positions
  4. Calculate the planetary aspects.

In this post I will demonstrate stages 1 and 2.  To follow you will need an Ephemeris for 1957 and a Book of Tables or Tables of  Houses.

Stage 1.     Determine the time of birth in Greenwich Mean Time

We adjust the time of birth to Greenwich Mean Time because the Ephemeris is calculated for GMT.

Example used:  Male:  12th October 1975, 2.20 pm, Ahmedabah, India Latitude

  1. Get the recorded time of birth                                            2.20 pm
  2. Convert this time of birth to the 24 hour clock                   14.20
  3. Adjust this time to Greenwich Mean Time
    • To do this you need to find the Time zone used for the location of birth.
    • The time zones are derived from the longitude of the place of birth and adjusted to a round figure, usually to the hour, but sometimes to the half hour.  Births prior to 1880 were recorded in Local Mean Time i.e. True Longitude Time.
    • If the longitude of the place of birth is West of GMT, then the time zone is added to get GMT as West is behind GMT.
    • If the longitude of the place of birth is East of GMT, then the time zone is taken away from GMT as East is ahead of GMT.
    • Many places use Daylight Saving Time so make sure you check whether this is being used.  If it is, take away the amount of daylight saving time.  E.g. usually one hour, but during the war it was sometimes 2.00 hours.

On the date of birth Ahemedabah, India was on the Indian Standard Time Zone of 5.30 East, so you take away this amount of time to get to GMT.
14 hours 20 minutes -05 hours 30 minutes = 08 hours 50 minutes GMT
This is the time we use to calculate the position of the planets in the natal chart.

Stage 2.    Find the local sidereal time at birth

Why do we need to do this?  We need to exactly calculate the angles and house cusps for a natal chart.

In one day, taken from the first point of Aries (the intersection of the ecliptic and the equator),  the Earth actually rotates 24 hours 3 minutes and 57 seconds so we need to adjust the GMT time of birth for that extra time.  This is called ‘Acceleration on the Interval’ – the interval being the GMT of birth.

Rule of thumb is that this adds 10 seconds of sidereal time for every hour of mean time and 1 second for every 6 minutes and for quick calculations this is fine.  However, it is actually 9.52.5 seconds of sidereal time for every hour which transferred to decimal points  = 9.868 seconds for every hour.

The easiest way to add this acceleration is to convert the GMT into minutes.  Using the GMT time of birth of 8 hours 50 minutes:

There are 60 minutes in an hour, so 8 x 60  =   480 minutes.   Add the extra 50 minutes.  So the  total minutes of GMT  = 530

Multiply this sum by   9.86 seconds   =   5230

Divide by 60 to turn into seconds      =    87 seconds

This is 1 minute 27 seconds and is the acceleration

So GMT time of birth is   8 hrs 50 minutes.  Add acceleration of   01’ 27 gives the sidereal time of birth  of 8 hrs 51’ 27

Now we use the Ephemeris

The midnight sidereal time for  12th October 1975 is   01  hour 21 minutes  13 seconds. Add GMT time of birth with acceleration 08 hours  51 minutes  27 seconds = Sidereal time at Greenwich at birth of   10 hours  12 minutes 40 seconds.

To calculate the exact angles and house cusps of a chart we need to bring this time back to the location of birth.

To do this we calculate true longitude time by turning the exact longitude of birth into time.

Longitude of Amendahbad is 72E37 .We multiply this by 4 because the Sun transits  one degree of longitude every 4 minutes.   72°37’  x   4   =   290.4666667’

Divide by 60 to turn into hours and minutes        =       4hrs 50’ 28”

This is the true longitude time for the birth location.

Because birth was East of Greenwich we add this to Sidereal time at Greenwich at birth

10 12 40 + 4 50 28 = 15 hours 03 minutes 08 seconds Local Sidereal Time.

You then look up the nearest sidereal time in the Book of Tables or Tables of Houses and it will give you close to the degree of the Midheaven of the chart.

You then look down the column to the latitude of the place of birth and this will give close to the degree of the Ascendant and house cusps.

Youwill need to make some small calculations to adjust the angles of the chart to the true sidereal time.  The Tables to help you do this are usually in the back of the Book of Tables.

When this is calculated it will give a Midheaven of 18Scorpio07 and Ascendant of 4Aquarius52.

Born in the Southern Latitudes

If someone is born in the Southern Latitudes you add 12 hours to the final calculations and if this bring the sum to over 24  hours then minus 24 hours if necessary:

E.g.   Local Sidereal Time    =     15.03.08   Plus 12 hours   = 27.03.08.  As this is over 24 hours we minus 24 and get the sum of  03 hours o3 minutes 08 seconds for the Local Sidereal time.

If you are using a Northern Hemisphere Book of Tables then you use this time but reverse the sign of the given Midheaven, Ascendant and house cusps to get the correct sign for these placements.  For example in the Northern Hemisphere Book of Tables 3 hours 3 minutes 08 seconds gives a Midheaven close to 18 Taurus 28, we need to reverse the sign so it is actually 18 Scorpio 28.  For latitude 23 South the Ascendant is given as 20 Leo, so we need to reverse the sign to 20 Aquarius.

This will give a Midheaven of 18 Scorpio and an Ascendant of 20 Aquarius  in the Southern Hemisphere.

Please click here to get the calculation sheet to help facilitate these calculations Calculation sheet for Local Sidereal time

Copyright (c) Alice Portman June 2010

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