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Ken Ward's Astrology Pages

Astrology: Times for calculating houses and planets - idiot's guide Part 2

This page deals with calculating the Greenwich Sidereal Time of birth, which is used for calculating the position of the house cusps. The previous page deals with calculating GMT, which is used for calculating the positions of the planets.

The special correction of 6 seconds per day is ignored on this page.

How to find the time for calculating the house cusps at birth

The time you need is Greenwich sidereal time (GST).

The procedure is simply:

1. Calculate the Greenwich Mean Time of birth
2. Calculate the Greenwich Sidereal Time of birth

Calculate The Greenwich Mean Time Of Birth

This is used to calculate the positions of the planets. We have already done this on a previous page.

1. Determine the birth time, longitude and Time Zone
2. Deduct any daylight saving time from the birth time
3. If the Time Zone is East, then subtract this from the birth time. If West, then add it.
4. This is the Greenwich Mean Time for the birth time.
5. Bring the figure to within 24 hours, if necessary. If you need to subtract 24 hours, then change the date to the next day. If you need to add 24 hours, then change the date to the previous day. Otherwise, you use the birth date.

Calculating The Greenwich Sidereal Time Of Birth

This is used to calculate the positions of the house cusps.

1. Get the GST from the ephemeris for the appropriate date
2. Add the Greenwichl Mean Time of birth
3. Bring the figure to within 0-24 hours
4. Add the correction for GMT (10 seconds per hour of the GMT)
5. If the longitude is West, then subtract the time equivalent of the longitude (1 hour per 15º, or part thereof). If the longitude is East, then add this figure.
6. And you've got the Greenwich Sidereal Time for finding the house cusps in tables of houses

Note 1: The largest correction for Step 3, the GMT correction, is 24 hours x 10 seconds, which is 4 minutes. Many birth times are not known even to this accuracy. Doing the calculation to the nearest hour will introduce an error no greater than 10 seconds, which is insignificant.

Note 2: In step 4, the largest correction for longitude is 13x10 seconds or two minutes 10 seconds. If the calculation is done to the nearest time zone, the error will usually be less than 10 seconds. Doing the calculation to the nearest whole value of the time zone is accurate enough.

If greater accuracy is required, we need to rectify the birth chart: we cannot rely on the accuracy of the birth time.

Western Birth In Northern Hemisphere

Ann was born in Idaho City (43N50, 115W50), USA on 18 July 1963 at 2:18 p.m. The Time Zone is UT-8 (120º W) and Summer Time was in operation (PDT=UT-7).

Calculate Greenwich Mean Time:

1. Standard Time Zone (PST) is UT-8
2. Birth time = 2:18pm-1 (Daylight Saving Time)=1:18pm (or 13:18)
4. This is the Greenwich Mean Time of birth

Now use this value to calculate the Greenwich Sidereal Time of birth:

1. Greenwich Sidereal Time at midnight is: 19:40:20
19:40:20+
21:18:00=
40:58:21
Reduce to within 24 hours: 16:58:21
3. Add correction for GMT acceleration @10 secs/hour:
210+3 secs, or 3min 33sec:
16:58:21+
00:03:33=
17:01:54
4. Longitude correction for 115W50 at 15º per hour=
7 hours, 43 mins 20 secs
Subtract for West:
17:1:54-
07:43:20=
09:18:34
5. This is the Greenwich Sidereal Time at birth, which is used to calcite the House Cusps.

Western Birth in Northern Hemisphere Date Change

Brian was born in Los Angeles (34N03, 118W15), USA on 3 February 1985, at 8:42pm. There was no daylight saving in operation and the Time Zone is PST (UT-8 hours, 120º).

Calculate Greenwich Mean Time:

1. Standard Time Zone (PST) is UT-8
2. Birth time = 8:42pm-1 (no Daylight Saving Time)
Time of birth=20:42 (24 hour-style)
20:42+
8:00=
28:42
4. Reduce by 24 hours: 4:42, 4 February 1985
5. This is the Greenwich Mean Time of birth

Now use this value to calculate the Greenwich Sidereal Time of birth:

1. Greenwich Sidereal Time at midnight, 4 Feb 1985, is: 8:56:24
2. Add GMT of birth: 8:56:24+
4:42:00=
13:38:24
3. Reduce to within 24 hours (not necessary):
4. Add correction for GMT acceleration @10 secs/hour:
40+7 secs, or 47sec:
13:38:24+
0:47=
13:39:11
5. Longitude correction for 118W15 at 15º per hour=
7 hours, 53 mins 0 secs
Subtract for West: 13:39:11-
7:53:00=
5:46:11
6. 5:46:11 on 4 Feb 1985 is the Greenwich Sidereal Time at birth, which is used to calcite the House Cusps.

Eastern Birth In Northern Hemisphere

Ivan was born in Leningrad (59N55, 30E15) on 1 July 1959, at 11:15am. The time zone is UT+3.

In this example, I am not sure of the details for this place. The TZ could be 3 or 4, but it probably is 3. They use Moscow time, even though logically their normal TZ would be UT+2. Currently, the time zone is 4, and 3 (summer time). I do not have a copy of the International Atlas to hand, so the definitive answer here must wait. If it were a real client, then I could:

• Rectify the chart

It isn't necessary, really, to know whether there is daylight saving, or commercial convenience in time, if we know the TZ for sure. Here, I assume the TZ is UT+3, and this takes into account any daylight saving.

Calculate Greenwich Mean Time:

1. Standard Time Zone is UT+3
2. Birth time = 11:15am (no Daylight Saving Time)
Time of birth=11:15 (24 hour-style)
3. Subtract 3 (East): 11:15-3=8:15
4. Reduce by 24 hours: 8:15
5. This is the Greenwich Mean Time of birth

Now use this value to calculate the Greenwich Sidereal Time of birth:

1. Greenwich Sidereal Time at midnight, 1 Jul 1959, is: 18:45:02
8:15:00=
27:00:02
3. Reduce to within 24 hours (not necessary): 3:00:02
4. Add correction for GMT acceleration @10 secs/hour on 8:15:00:
80+3 secs, or 1min 23 sec:
3:00:02+1min 23sec=3:01:25
5. Longitude correction for 30E15 at 15º per hour=
2 hours, 1 mins 0 secs
2:01:00=
5:02:25
6. 5:02:25 is the Greenwich Sidereal Time at birth, which is used to calcite the House Cusps.

Eastern Birth In Northern Hemisphere With Date Change

Ruth was born in Jerusalem, Israel (31N46, 35E14) on 16 May 1959, at 1:05am. The Time Zone is EET, which is UT+2 hours. The UT time of birth is therefore:

Calculate Greenwich Mean Time:

1. Standard Time Zone is UT+2
2. Birth time = 1:05am (no Daylight Saving Time)
Time of birth=1:05 (24 hour-style)
3. Subtract 2 (East): (add 24) 1:05-2=23:05
4. Reduce by 24 hours: 23:05 15 May 1959
5. This is the Greenwich Mean Time of birth

Now use this value to calculate the Greenwich Sidereal Time of birth:

1. Greenwich Sidereal Time at midnight, 15 May 1959, is: 15:27:54
23:05:00=
38:32:54
3. Reduce to within 24 hours 14:32:54
4. Add correction for GMT acceleration @10 secs/hour on 23:05:
230+1 secs, or 3min 51 sec:
14:32:54+
3:51=
14:36:45
5. Longitude correction for 35E14 at 15º per hour=
2 hours + 20 mins 56 sec
2:20:56=
16:57:41
6. 16:57:41 is the Greenwich Sidereal Time at birth for 15 May 1959, which is used to calcite the House Cusps.

Southern Hemisphere Births

We can find the planets' positions in the ephemeris by calculating the position from the UT of birth, using the appropriate date, either the birthday, or the day before (eastern) or the day after (western), if necessary. There is no difference between a Northern birth and a Southern birth in this respect. Therefore, what has been said above for Northern births is directly applicable to Southern births. The only difference is that we add or subtract 12 hours (depending on whether the time is greater than 12, or less than 12) to get the sidereal time. However, when we use the sidereal time to find the house cusps, we reverse the sign found in the tables.

Eastern Birth In Southern Hemisphere

Kiri was born in Gisborne (38S40, 178E01) New Zealand, on 6 March 1944, at 2pm. The Time Zone is UT+12. Therefore the UT of birth is:

Calculate Greenwich Mean Time:

1. Standard Time Zone is UT+12
2. Birth time = 2:00pm
Time of birth=14:00 (24 hour-style)
3. Subtract 12 (East):  14:00-12=2:00
4. Bring into 24 hour range, if necessary: 2:00 6 March 1944
5. This is the Greenwich Mean Time of birth

Now use this value to calculate the Greenwich Sidereal Time of birth:

1. Greenwich Sidereal Time at midnight, 6 March 1944, is: 10:54:24
2:00:00=
12:54:24
3. Bring to within 24 hours, if necessary 12:54:24
4. Add correction for GMT acceleration @10 secs/hour on 2:00:00:
20 secs
12:54:24+
00:00:20 =
12:54:44
5. Longitude correction for 178E01 at 15º per hour=
11 hours + 52 mins 4 sec
11:52:04=
24:46:28
6. Bring into 24 hour range: 0:46:28
7. For a Southern birth, add 12 hours: 12:46:28
8. 12:46:28 is the Greenwich Sidereal Time at birth for 6 March 1944, which is used to calcite the House Cusps. However, for a Southern birth, the signs in the tables are reversed. See below:
 Ascendant from tables Latitude (North) Time 38 39 12:44 22sa8 21sa29 12:48 23sa0 22sa21

Because we reverse the signs when using tables for Northern latitudes to get the Southern values we read Sagittarius as Gemini. So an approximately extracted Ascendant gives us: 22ge15 (Accurately 22ge18).

Western Birth In Southern Hemisphere

Charles was born in Stanley, The Falkland Islands (51S42, 57W51), on 13 January 1972, at 6:30 am. The Time Zone is UT-3.

Observation: The standard time zone is -4, which is logical (-4 is 60º West, and the longitude is 57W51) It was summer time in the Falklands, so the time zone is -4+1=-3. We can use this value and ignore specific daylight saving conversations, because this has already been done with the time zone.

Calculate Greenwich Mean Time:

1. Standard Time Zone is UT-3
2. Birth time = 6:30 am
Time of birth=06:300 (24 hour-style)
4. Bring into 24 hour range, if necessary: 9:30 13 January 1972
5. This is the Greenwich Mean Time of birth

Now use this value to calculate the Greenwich Sidereal Time of birth:

1. Greenwich Sidereal Time at midnight, 13 January 1972, is: 7:26:20
2. Add GMT at midnight: 7:26:20+
9:30:00=
16:56:20
3. Bring to within 24 hours, if necessary 16:56:20
4. Add correction for GMT acceleration @10 secs/hour on 9:30:00:
90+5=95, or 1 min 35 sec
16:56:20+
00:01:35 =
16:57:55
5. Longitude correction for 57W51 at 15º per hour=
3 hours + 49 mins 44 sec
Subtract for West: 16:56:20+
3:49:44=
13:06:36
6. Bring into 24 hour range: 13:06:36
7. For a Southern birth, add 12 hours:
13:06:36+
12:00:00=
25:06:36
8. Bring into 24 hour range: 1:06:36 (otherwise just 13:06:36-12)
9. 1:06:36 is the Greenwich Sidereal Time at birth for 13 January 1972, which is used to calcite the House Cusps. However, for a Southern birth, the signs in the tables are reversed. See below:

Because we reverse the signs when using tables for Northern latitudes to get the Southern values we read Leo as Gemini. The Aquarius is 8aq39.