Step 1 - Calculate the Time of Birth

To start the process, you need to convert the time of birth into Greenwich Mean Time (GMT), and then from GMT into Sidereal Time, which is the true or real time derived from the orbits of the planets.

Find Out Birth Time in GMT

Note down the time of birth, and then the latitude and longitude of the place of birth - these co-ordinates can be found in an atlas, or on the Internet at heavens-above.com, an online resource offering useful astronomical data. The birth time should be entered as am or pm from the 12-hour clock system (rather than using the 24-hour clock).

If the birth occurred during Summer Time in the UK, the relevant hour is deducted to arrive at the actual time of birth at GMT.

If the birth was outside the UK, then the time should be converted into Greenwich Mean Time. For example, if the subject was born in Calgary, Canada, then you must subtract seven or eight hours (depending on current UK Summer Time), as the sun rises seven hours later in Calgary than it does in Greenwich, London.

Convert to Sidereal Time

The next stage is to find the Sidereal Time at noon from the relevant ephemeris which is written in after the GMT time of birth. The amount of time before noon or past noon then has to be worked out. For example, someone born at 2 am GMT has an interval of ten hours before noon, whereas someone born at 6.40 pm has an interval of 6 hours and 45 minutes after noon. The interval is added (pm births) or subtracted (am births) to the Sidereal Time at noon to form a new Sidereal Time.

One more slight adjustment is required - the 'acceleration on the interval'. This is because Sidereal Time is actually four minutes per day faster than GMT. Ten seconds should be added (pm birth) or subtracted (am birth) for every hour of difference. Someone born at 2 am (ten hours to noon) will require a deduction of 100 seconds, (1 minute 40 seconds), whereas a birth occurring at 6.45 pm (6 hours and 45 minutes from noon) requires an addition of 67.5 seconds. This minor adjustment gives the correct Sidereal Time at Greenwich at birth.

Convert to Local Sidereal Time

The final process is to find the Local Sidereal Time for all births outside the 0-degree Greenwich longitude. This is called the longitude equivalent and requires the multiplication of the longitude figure by 4.

For example:

Longitude for Edinburgh, UK, is 3.161 degrees west x 4 = 12.6 minutes to be DEDUCTED from Sidereal Time.

Longitude for Auckland, New Zealand is 174.767 degrees east x 4 = 699 minutes (11 hours 39 minutes) to be ADDED to Sidereal Time.