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Ken Ward's Astrology Pages

Astrology: Calculating Local Sidereal Time using the Noon Ephemeris.

clear.gif (807 bytes) We have found out the sidereal time for the day of Mary's birth at Greenwich and we need to find her local mean time of birth.

On the next page, we will use the midnight ephemeris to do these calcualtions, but on this page we will use the noon ephemeris. The midnight ephemeris is recommended, but traditionally the noon ephemeris was used. This page is for completeness only. The use of the noon ephemeris is rather awkward compared with the midnight version. You can safely ignore this page.


Example: Mary was born  7 May 1982 at 11 hours 17 minutes AM, New Berlin, New York, USA. The longitude is 75 degrees, 20 minutes west of Greenwich. And the latitude is 42 degrees 37 minutes North of the equator.

Her local time of birth has been determined as:

10 hours 15 minutes and 40 seconds

From the ephemeris, the sidereal time at noon on the day of her birth was:

3 hours 0 minutes 0 seconds

Local Sidereal Time

Mary's Local Sidereal Time of birth is the sidereal time at Greenwich at the instant that Mary was born.

She was born before noon, and the time before noon, from her local time is:

12 hours (noon) -

10 hours 15 minutes 40 seconds

This comes to:

1 hour 44 minutes and 20 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. In our example of Mary, the correction works out as 17 seconds. Therefore the time before noon is:

1 hour 44 minutes and 20 seconds +

17 seconds

This is:

1 hour 44 minutes 37 seconds

The local sidereal time (LST) is the difference between this and the sidereal time at Greenwich:

3 hours -

1 hour 44 minutes 37 seconds

Which comes to:

1 hour 15 minutes 23 seconds

In other words, Mary was born 1 hour 44 minutes and 37 seconds before noon, so we subtract this from the sidereal time at noon at Greenwich  to get her local sidereal time.

Time Zone Correction

We need a further correction for each 15 degree time zone. This is 10 seconds per zone. Five time zones make 50 seconds. Adding this correction we get the sidereal time for her place of birth as:

1 hour 16 minutes 13 seconds.

This is called the Local Sidereal Time (LST).