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Ken Ward's Astrology Pages

Astrology: Times for calculating houses and planets - idiot's guide Part 1

clear.gif (807 bytes) This page deals with calculating GMT, which is used for calculating the position of the planets. The next page deals with calculating Greenwich Sidereal Time of birth, which is used for calculating the house cusps, ascendent and MC.

On this page:

How to find the time for calculating the planets position at birth

The time you need is Greenwich Mean Time (GMT). For practical purposes this is the same as:

  • Universal Time (UT)
  • Universal Co-ordinated Time (UCT).
  • British Standard Time (BST), and
  • Western European Time (WET)

The formula is simply:

Clock Time - Daylight Saving - Standard Time Zone (East Longitude), or

Clock Time - Daylight Saving + Standard Time Zone (West Longitude)

The rule is: Add for West and subtract for East.

If this time is within the range 0 to 24 hours, then this is the time you use to look up (calculate) the position of the planets in an ephemeris. If the time is less than 0 or greater than 24, you need to use a date before or after the birth date.

Time Zone

If you use the Time Zone as stated, then it isn't necessary to bother with daylight saving time, etc. For example, Atlantic Standard Time is +4 hours. Atlantic Daylight Time is +3 (or +4-1 daylight saving). India Standard Time is -5:30. India Daylight Time is -6:30 (or -5:30-1 daylight saving).

If you use the current time zone, which takes into account daylight saving, etc, then the formula is simply:

Clock Time -  Current Time Zone (East Longitude), or

Clock Time  + Current Time Zone (West Longitude)

The current time zone is used below.

Finding The Correct Date Of Birth

If the above sum is less than 24 hours (negative), you use the previous day from birth. That is, if you need to add 24 hours to bring the time into the range zero to 24, then use the previous day in the ephemeris.

If the above sum is greater than 24 hours, and you need to subtract 24 hours, then you use the next day in the ephemeris.

Otherwise, of course, you use the birth date.

We can find the time changes for the date, using an atlas, or astrodienst.

Western Birth In Northern Hemisphere

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

2:18pm +7 (Current Time Zone)=


The date to look up in the ephemeris is the birth date, 18 July 1963.

Alternatively, we can use the daylight saving information and the standard time zone:

2:18pm -1 (daylight saving)+8 (Standard Time Zone)=


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 summer time and the Standard Time Zone is PST (-8 hours, 120). The GMT time of birth is:

8:42pm+8 (Current Time Zone)=

28:42-24 (Bring within 24 hours)

4:42am (4 February 1985, the next day)

We therefore look up the planets in the ephemeris for the next day, 4 February 1985, using the time 4:42am.

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, we deduct the 3 hours from the birth time to get UT:

11:15am - 3 hours (Current Time Zone)=


Eastern Birth In Northern Hemisphere With Date Change

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

1:05-2 hours (Current Time Zone)

(need to add 24 hours)


23:05 p.m.  (on 15 May 1959)

We therefore look up the planets in the ephemeris for 15 May 1959 at 23:05 p.m.

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 Sourthern births.

Eastern Birth In Southern Hemisphere

Kiri was born in Gisbourne New Zealand, on 6 March 1944, at 2pm. The timezone is UT-12. Therefore the UT of birth is:

2pm-12 (Current Time Zone)=


We look up the planets for 2am on 6 March 1944.

Western Birth In Southern Hemisphere

Francis was born in Montevideo, Uraguay (34S53, 55W11), on 13 January 1972, at 6:30 am. The TZ is UT-3, so the UT of birth is:

6:30am+3 (Current Time Zone)=


We therefore look up the planets in the ephemeris on 13 January 1972 at 9:30 am.