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Ken Ward's Astrology Pages

Astrology: Calculating Local Sidereal Time for a Birth in the Southern Hemisphere.

clear.gif (807 bytes) The following is an example of a birth that occurred in the Southern Hemisphere. It also occurred in the East.

Kiri Te Kanawa born in Gisborne, New Zealand on Monday 6 March 1944 at 2pm. TZ:12.00, Lat 38S40, Long 178E01

Everything in the table below is the same as when we calculate an Eastern birth. The correction for a Southern birth is shown after the table.

Local Birth Time

Kiri was not born on the Time Zone meridian, so some corrections are needed to find her local birth time. Time Zone 12 is 180 degrees, so she was born to the West of the Time Zone. She was therefore born earlier than the civil time of her birth.

The difference between her birth time and her local time of birth is:

180 deg - 178 deg -

1 arcminutes =

1 deg 59 arcminutes

15 degrees of longitude represent one hour in time, so this difference is:

(1 deg 59 arcminutes)/15=


0.1322222 hours =

7 minutes 56 seconds

Her local time of birth is therefore:

14 hours -

7 minutes 56 seconds


13 hours 52 minutes 4 seconds

Local Sidereal Time

Using the midnight ephemeris, we simply add the local time of Kiri's birth to the sidereal time in the ephemeris for her day of birth. This will be for 6 March 1944:                        

10 hours 54 minutes 24 seconds +13 hours 52 minutes 4 seconds

This comes to 24 hour 46 minutes and 28 seconds.

We subtract 24 hours to bring this time within the normal range:

0 hour 46 minutes 28 seconds


Because sidereal time is faster than regular time, we need to correct this figure by allowing 10 seconds for every hour. A sidereal day is 4 minutes short of a regular day, so we need to accelerate regular time to accord with sidereal time.

Because we added

13 hours 52 minutes 4 seconds

The acceleration will be (@10 seconds/hour) about 140 seconds (or 2 minutes 20 seconds).  We add this to our sidereal time so far:

0 hour 46 minutes 28 seconds +

2 minutes 20 seconds =

0 hours 48 minutes 48 seconds

Time Zone Correction

We need a further correction for each 15 degree time zone. This is 10 seconds per zone. One time zones make 10 seconds. Subtracting this correction (because the TZ is East) we get the sidereal time for his place of birth as:

0 hours 48 minutes 48 seconds - 120 seconds =

0 hours 46 minutes 48 seconds

Correction for Southern Births

To find Kiri's sidereal time of birth for the Southern Hemisphere, we add 12 hours (reducing the figure to 0 to 24, as necessary).

Kiri's sidereal time of birth is therefore:

0 hours 46 minutes 48 seconds +12 =

12 hours 46 minutes 48 seconds.

Even though they are correctly written, some computation programs often do not show this figure (with 12 hours added) because they make the correction elsewhere in their program.