astrology: calculating local mean time, greenwich mean time (Universal Time)next.gif (1008 bytes)

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Ken Ward's Astrology Pages

Astrology: Calculating the Chart - UT or GMT.

clear.gif (807 bytes) While few people are actually going to work out astrological charts by hand, that is, using paper, pencil, it is, I believe, important to know how to do it. It may also be true, that unless an astrologer spends a great deal of time doing charts by hand, they will never fully understand astrology. By doing charts by hand, you will begin to understand more about the movement of the planets and how they combine to affect the lives of people on Earth.. Another reason is that astrology is a sacred science and by doing the mundane routine things, our minds enter a meditative state wherein we realise things we might not have been able to know or understand otherwise.

The tools you require to make a horoscope are (in addition to writing and drawing tools) are an atlas and an ephemeris. An ephemeris is a book or booklet containing information about the positions of the planets, etc.

The information you require is the date and time and place of birth.

The place of birth is expressed in longitude and latitude.

Universal Time, or Greenwich Mean Time, is required to calculate the positions of the planets.

What is Universal Time?

Suppose someone is born in New Berlin on 7 May, 1982, at 11:17 according to the hospital clock. This is the Civil Time of birth (Clock time). At Greenwich, the time will be different. It will be later than this civil time because the sun has already passed over Greenwich and the civil time is some hours later than the civil time at Greenwich. (See diagram)

Universal Time is the time at Greenwich for the given civil time, taking into account any daylight saving time.

Calculating Universal Time

For example, Mary was born  7 May 1982 at 11 hours 17 minutes AM, New Berlin, New York, USA. Perhaps you think everyone knows that New York is in the USA, but do be careful. Many place names are duplicated, especially between the USA and European countries. So don't be afraid of the obvious.

Now use an atlas to look up the longitude and latitude for this place. I get:

75W20

And for the latitude:

42N37

That is, it is the longitude is 75 degrees, 20 minutes west of Greenwich. And the latitude is 42 degrees 37 minutes North of the equator.

Note: From Greenwich, the time difference is one hour for each 15 degrees of longitude.

Eastern Standard Time (EST) for New York is based on 75 degrees longitude. That is 5 hours (75/15=5). When it is 7 am in New York, it is 12 noon in Greenwich, because New York is west of Greenwich. For western longitudes, add the time difference to the local time. (And for eastern longitudes, subtract the time difference).

We need to find the time at Greenwich (GMT) in order to calculate the chart. Because the sun rises in the east, the times in the east are earlier than those in Greenwich, and those to the west are later than Greenwich.

Sun movement diagram

gmt.jpg (9526 bytes)

If you look at the diagram above, you will notice that the sun is in the East. It hasn't reached Greenwich yet, so the time in the East is earlier than that at Greenwich. The sun will reach Greenwich before it reaches places in the West, so places in the West have a time later than those in Greenwich.

So if you live to the East of Greenwich, then the sun rises earlier, and you get your noon before Greenwich does. If you live to the West, then you get your noon later than Greenwich. Therefore, Greenwich Mean Time (GMT) is later for those in the West and earlier for those in the East. This GMT of birth is also called Universal Time.

Longitude 75W GMT 75E
Time Difference +5 0 -5
Local Time 7 AM Noon 5 PM

So if you were born at 7 AM at longitude 75 West, it would be noon at Greenwich (7am +5 hours). Similarly, if you were born at 5 PM at longitude 75 East, it would be noon in Greenwich (5pm - 5 hours).  

For our example birth date we need to add five hours to get the GMT: So the birth time, so far, is:

11 hours 17 minutes AM + 5 hours = 16 hours 17 minutes

However...

This is not GMT yet, because the clocks may have been changed, so we need to check on summer time (daylight saving) before we can get the birth time in GMT. The stars do not change when we change our clocks, so any Summer time (or other changes) need to be accounted for.

Summer Time (Daylight Saving)

In some countries, time is changed to take advantage of the light and this is called Daylight Saving Time. When this is in force, you often need to subtract one hour (although sometimes 1/2 hour or 2 hours). In summer time the clocks are advanced one hour, so we need to remove this hour, when summer time is in existence. Summer time was in existence for our example, so the GMT time of birth was:

16 hours 17 minutes - 1 hour = 15 hours 17 minutes GMT

This is the time at Greenwich when Mary was born. We can discover whether summer time was in operation by consulting a special atlas or by checking this url:

http://www.astro.com/atlas/horoscope

Rule for determining the GMT (or UT - Universal Time) of birth (East and West)

You need to know the time zone for the birth. You get UT by adding the Time Zone if the birth was West of Greenwich, or by subtracting the time zone if the birth was East of Greenwich. You also need to subtract any daylight saving time.

Examples

Eastern Birth

Kiri Te Kanawa was born in Gisborne, New Zealand on 6 March 1944 at 2pm. The longitude is 178E01 and the time zone is 12. Daylight saving was not in operation. The birth occurred in the East, so we subtract the time zone. The UT was:

14 hours - 12 hours = 2 hours (2 am)

That is, universal time was 2 am (time on the Greenwich meridian).

Near midnight birth

Charles Aznavour was born in Paris on May 22, 1924 at 0:15 am. The longitude is: 002E20, and the time zone is 1. The birth was in the East, so the time zone is subtracted from the birth time (civil time):

0 hours 15 minutes (quarter past midnight) - 1 hours

Add 24 hours so we can subtract 1 hour:

24 hours 15 minutes (quarter past midnight) - 1 hours = 23 hours 15 minutes

That is, 11:15 pm on 21 May 1924 - the Universal Time here is the day before the civil day of birth.

Next-day Birth

A birth occurred in New York, NY, United States of America on Tuesday 26 June 1990 at 20:03.The latitude is  40N43 and the longitude is 74W00. The time zone is 5 and daylight saving was in operation. Because the birth in West, the time zone is subtracted. Taking into account summer saving the UT of birth is:

20 hours 3 minutes +5 (Time Zone) - 1 (Daylight Saving) = 24 hours 3 minutes

Because the time is greater than 24, we subtract 24 and add one to the day. So the UT birth time is:

0:03 am on 27 June, 1990

That is, the UT at Greenwich is 3 minutes past midnight on 27 June 1990, which is the day after!

When the birthday changes, we use the new date to look up the sidereal time in the ephemeris and find the planets

Local Mean Time

LMT is required to calculate the house positions (the ascendant, in particular). It is the accurate local time.

Mary was born  7 May 1982 at 11 hours 17 minutes AM, New Berlin, New York, USA

She was born very close to the time zone of 75 degrees, but she was born 20 arc minutes to the west of this zonem (75W20), so her sun rose a little later than shown for the time zone. (Do not confuse minutes of arc in latitude and longitude with minutes of time ... not 20 minutes of time, but 20 minutes of arc!)

The sun moves 15 degrees for every hour. So it moves one degree in four minutes of time. It therefore moves 43 minutes of time for every 20 seconds of arc. Her local mean time of birth is therefore:

11 hours 17 minutes (Civil Time of birth) -

1 minute 20 seconds (longitude correction, calculated above) - 

1 hour (Summer time) =

10 hours 15 minutes 40 seconds (local mean time)

So she was born at 10 hours 15 minutes and 40 seconds. (We will keep the seconds, but they aren't really significant because the birth time was measured in minutes and not seconds!)

When we convert to GMT, we add the allowance for longitude West and subtract for longitude East.

In contrast, when we convert to local time, we subtract the allowance for births West of the Time Zone, and add for births that are East of the Time Zone. Mary's GMT time of birth is therefore the above time with the time zone added, or:

10 hours 15 minutes 40 seconds +

5 hours

Which is:

15 hours 15 minutes and 40 seconds (7 May 1982 );

This is Greenwich Mean Time when Mary was born. .

Summary and Alternative Method for Local Mean Time

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.

To get GMT, we use the longitude. The longitude is 75 degrees and 20 minutes: 75W20. Now for every degree the sun travels, the time changes by 4 minutes. So the difference between where Mary lives and Greenwich is 75 x 4 minutes. This works out to be 300 minutes or 5 hours. The other "bit" is the 20 minutes of arc, or one third of a degree. This is one third of 4 minutes. So the time difference is:

5 hours 1 minute 20 seconds.

To get GMT we add this figure to her birth time and take account of Summer time (Daylight Saving Time, DST). So her GMT is:

11 hours 17 minutes + 5 hours 1 minute 20 seconds -1 hour (DST)

This works out to be 15:18:20, which is the time in Greenwich when she was born. We aren't finished yet because we have to take into account the fact that Mary wasn't born exactly on the time zone (75 degrees), but at a longitude of 75 degrees 20 minutes.

Local Mean Time

She was born at 10 hours 7 minutes (allowing for Summer Time), but she was born 20 minutes of arc west of the time zone (at 75 degrees). When she was born, the time on the 75 degree of longitude was the time she stated. This was the time at 75 degrees, but 20 minutes of arc to the West of this zone, it was only:

10 hours 17 minutes - 1 minute 20 seconds

Or

10 hours 15 minutes and 40 seconds.

In every hour, the sun travels 15 degrees around the earth. In one minute it travels 60/15 minutes or 4 minutes. And to travel 20 minutes of arc, it takes 4 x 20/60 minutes or 1 minute 20 seconds.

This is her local time of birth. To get the time at Greenwich, we need to add the time zone, which in this case is 5 hours.

So her local birth time in GMT is:

15 hours 15 minutes and 40 seconds.

The local mean time is similar to the UT, except that the local mean time is computed from the exact longitude of birth and not the time zone.

The local mean time of birth is the corrected Universal Time of birth. The UT is used to calculate the position of the planets and the local time is used to calculate the ascendant and the cusps of the houses.