Place of Birth, Time Zone and Time Correction

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In astrology place of birth is equally important as date and time of birth. This lesson tells you how the place of birth affects the calculations and how much precise we should be in specifying the place of birth - whether a street, a colony, a city or a country. Some important formulas to compute distance are described when longitudes
and latitudes are known. The lesson also explains importance of time zone and time correction.
In astrology place of birth plays an important role. If it is day in India, it is night in America. So the effect of Sun is reversed. Similarly effect of other planets also change.
e.g. it remains day if we move South to North - Sri Lanka to Russia. But there is still a difference in sunrise marginally from one Latitude to other even if longitude remains the same.

Let us understand what is longitude or latitude; how we measure it and how much distance has how much effect in calculation of horoscopes.

Latitude: These are imaginary lines, parallel to equator. The equator represents zero degree latitude.

North pole is 900N and South Pole is 900S.

Longitude : These are imaginary vertical lines parallel to the prime meridian, which pass through Greenwich where the British Royal Observatory is located. The prime meridian is at 00 longitude and we count 1800E in East, to 1800W in West. 1800E and 1800W coincide and represent the same vertical line just opposite to prime meridian.

Unlike parallel of latitude, all meridians are of equal length. Any place on earth can be uniquely assigned a longitude and latitude and any such coordinates define a single point on earth. Delhi has a longitude of 77013'E and latitude of 28039'N.

Since earth is spherical, every degree of longitude does not represent equal distances.

Span of 1' of Longitude or Latitude :

Let us determine the distance represented by 1' of the longitude or latitude; that is, how much distance changes the coordinates by 1'.

Earth's mean radius "a" = 6371 km.

Considering the earth as sphere
10 of longitude at latitude f= p. acosØ.km. .........1
180

At Delhi (f = 280 39' N)
10 of longitude = 97.6 Km.

Thus 1' of longitude at Delhi

=1.626Km.
» 1 mile
and 10 of latitude = p x a Km. .........2
180

For all longitudes

10 of latitude = 111.2 Km.
or 1' of latitude= 1.853 Km. » 1.16 mile

For all practical purposes, in India we can consider 1 mile, making a difference of 1' in longitude or latitude or combined difference of both.

From formula 1 and 2 it is obvious that the distance in North or South makes a variation in latitude, which is constant for all places on earth. However the longitude at least changes as much as latitude and the variation becomes more and more prominent as latitude increases. This is obvious because of the fact that equator the circumference of earth is maximum, where as it reduces as latitude increases and rate of change of circumference also increases with the increase in latitude.

A table can be drawn for distance covered by 1' of longitude at different latitudes.

Distance covered by 1' of Longitude

Latitude
00
100
200
300........
800
900

Distance in E-W direction
(Km.)
1.853
1.825
1.741
1.605
1.419
1.191
0.926
0.634
0.321
0.00

Distance covered by 1' of Latitude
For all Longitudes 1.853 Km. in N-S direction

Variation of Longitude and Latitude in a City

For latitudes like in India a city, which has span of 40 Kms. or a distance of 25 miles, can make a difference of 25' in longitude and latitude. This is particularly so in case of Delhi and Mumbai, where the city stretches to over 40 kms. diagonally. In Delhi, where the accepted coordinates 280 39' N & 770 13' E are for New Delhi Railway Station, easily makes a difference of over 25' in longitude and latitude from one end to the other.

For example Nangal in South-West of Delhi has coordinates 280 33'N and 770 06'E, whereas Vikas Kunj in North-East coordinates of 280 45'N and 770 18'E, thus reflecting a difference of 12' in longitude and 12' in latitude, with a total difference of 24'. Similarly in Bombay Dahisar in North has coordinates 190 16'N and 720 51'E, whereas Colaba in South has coordinates 180 54'N and 720 49'E, making a difference of 22' in latitude and 2' in longitude, again making a total difference of 24'.

However small cities are normally only half or even less than half the size of Delhi or Bombay. Towns are only a few kms. in length or breadth, thus making a total difference of few minutes in all. If a centre point is chosen then the difference does not exceed more than 1' or 2' in latitude and longitude combined. For this reason for most of the towns and places, 1' accuracy in longitude or latitude is just sufficient, whereas in metropolitan cities a further breakup into small area is advisable.

To understand the total difference caused by longitude and latitude, let us convert the maximum combined difference in Delhi or Bombay (~25') into time. We find it is equivalent to 100s of time. And from a centre point it is only ±50s. i.e. less than a minute! Thus when time of birth is accurate only to a minute level, taking the center point of even the metros for longitude or latitude is not going to add much to the inaccuracy in results.

Computation of Distance:
We have seen above that there is a direct relationship between distance and the longitude or latitude. We can easily compute the aerial distance between the two points on earth if we know their coordinates accurately.

For rough computations of distance, we may simply add up the difference of longitude and latitude and equate that to miles.

Example : Calculate the distance between Delhi and Mumbai approximately.

Coordinates of Delhi 280 39' N 770 13'E
Coordinates of Mumbai 180 58' N 720 50' E
Difference 90 41' and 40 23'
or 581' and 263'
adding the two, distance between Delhi and Mumbai is 844' » 844 miles
Taking 1 mile = 1.6 Km. difference between Delhi and Mumbai is 1350 Km. approximately.

Distance between two points on the earth's surface having longitude and latitude L1, f1 and L2, f2 respectively can be computed accurately by first computing the angular distance between the following points by the following formula :

Cos d = SinØ1.SinØ2+CosØ1.CosØ2.Cos (L1-L2)

then computing the required linear distance by the following formula
S = 6371p d/180 kms.
where d is expressed in degrees.

(Note: The formula does not work well for very small values of d)

Example : Calculate the distance between Delhi and Mumbai, taking the following coordinates.

Delhi : L1 = 770 13' E Ø1= 280 39' N Mumbai: L2 = 720 50' E Ø2 = 180 58' N

Cosd = Sin280 39'.Sin180 58'+Cos280 39'. Cos180 58'.Cos(770 13'-720 50')

on solving, Cosd = 0.983324945

or d = 10.477956950

so, s = (6371*p*10.47795695)/180

or s = 1165 kms.

Note : - The result is accurate up to a few Kms. The inaccuracy is mainly due to flattening of the earth, which has been ignored in the present formula.

Longitude and Time
Sun is the best time-keeper throughout the world. It Sun regularly rises and sets every day. Local time can be measured by the shadow cast by the sun. All the places on a meridian have midday at the same moment. If the earth rotates from West to East, places East of Greenwich are ahead of Greenwich time and those in the West behind it. Since the earth rotates 3600 in 24 hours, every 150 there is a difference of 1 hour.

Standard Time
Local time of places which are on different meridians differ. In India there will be a time difference of about 1 hour and 45 minutes in the local time of Dwarka in Gujarat and Dibrugarh in Assam. It will be difficult to prepare a time-table of trains which move from one corner to another. It is therefore necessary to adopt the local time of some central meridian of a country as the standard time for the country. In India 820 30'E is treated as the standard meridian. The local time at this meridian is taken as the standard time for the whole country.

Some countries have a great longitudinal extent and so they adopt more than one standard time. For example USA has as many as 5 standard times. The earth has been divided into 24 time zones of one hour each. A few countries like India adopt a time zone in between the two, like 5½ hour zone.

Time Correction
In countries with high latitudes, day duration changes drastically from say 6 hours in winters to 18 hours in summers. At poles this difference becomes so large that there is day for six months and night for six months.

In Northern latitude May, June are longer, whereas in Southern latitude December January are longer and they have summer at this time. In summer the sun rises very early. To take the advantage of sun light, the clocks are advanced by one hour during summer for about six months and it is set back to original position during winter. This advanced time is called "summer time" or "day light saving time". At some places the correction is done for 2 hours and it is called as "double summer time". Similarly sometimes it is only half an hour correction. Since this adjustment is only to save light, it is subtracted before we do any astronomical calculation.

Summary
For astrological purposes we need date, time and place of birth. City of birth is normally sufficient as place of birth. Along with this one should know the standard time zone and the day light saving time correction to know exactly and correct the time of birth.
Longitudinal distance play vital role in time gap compared to latitude.

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One Solar day is the time between two successive passages of the sun across the meridian as observed at a particular place. In astrology, we are interested in motion of stars. We want to know the time period of earth when any star is observed to return to the same position. Such a clock is called a sidereal clock and its time, being regulated by stars is called sidereal time.

There are 365.2422 days in the year. During this period, the earth makes one revolution around the Sun. If we look from remote stars, we see that the earth has made 366.2422 circles about its axis in 365.2422 days. Each sidereal day is thus :

Duration of one sidereal day = Duration of sidereal year/No. of sidereal days

= 365.2422/366.2422
= 23 hr. 56 m 4.09 s

(shorter by 3 min 55.91 seconds than the mean solar day of 24 hours.)

GMT and Greenwich sidereal time coincide at one instant every year at the autumnal equinox (around September 22). Thereafter, the difference increases until half a year later it is 12 hours (around March 23). After one year, the times again coincide.

To compute sidereal time manually, please refer to "Tables of Ascendants" by N.C. Lahiri.

The 10th House or Mid-Heaven :
The point of intersection of the ecliptic of the given time with the meridian of the place is the tenth house for that moment. If A is the sidereal time expressed in degrees and B is the tenth house, then tenth house can be computed as :
tan B = tan A Sec w
where w is the inclination of earth’s equatorial plane to the ecliptic.

Ascendant :
The ascendant is the point of intersection of the ecliptic with the eastern horizon of the place. if A is the sidereal time in degrees + 900
then

tan B = tan A cos w ....(1)

From A & B, we can evaluate D the declination & q =90 - angle between meridian & ecliptic, as:

sin D = sin A sin w ....(2)

sin q = sin w cos b ....(3)

From D & q, we compute the ascendant as :

tan E = sin D tan(j + q) ..(4)

where j is the latitude of the place.

Ascendant = B + E ....(5)

The value of the ascendant as calculated above is the sayan value. Subtracting ayanamsa we get nirayana ascendant. The above formulae gives accurate values of ascendant. In absence of scientifc calculators, the ascendant & the tenth house can be computed using tables of ascendant by N.C. Lahiri. However, calculation of ascendant using tables gives rise to errors because of tables not available for the given place or for the given sidereal time. This error some times goes upto many minutes of arc.

Approximate computation of Ascendant :
The ascendant can also be computed approximately orally from date of time of birth. It does not vary much because of latitude or longitude, if local time is taken. So take the standard time & roughly estimate the local time. Take the date of birth and estimate the Sun degree, using the fact that it is 00 on 14th April and it advances by 10 everyday or 1 sign every month.
Month Sun’s entry Month Sun’s entry
April Aries October Libra
May Taurus November Scorpio
June Gemini December Sagittarius
July Cancer January Capricorn
August Leo February Aquarius
September Virgo March Pisces

If the date of birth is December 13th, then as per table given Sun enters into Sagittarius on 15th Dec. Hence it is approximately on 280 scorpio on 13th December in any year.

Since at Sun rise, Sun is at the eastern horizon, so the Sun degree matches with the ascendant degree. Hence at Sun rise on 13th December any year, the ascendant is approximately 280 Scorpio. After that it increases by the sign every 2 hours or 10 every 4 minutes. More accurately, it crosses six signs from Sun rise to Sun set and next six signs from Sun set to Sun rise. So if time of birth is 1 PM and since sunrise is approximately at 7.15 AM in Delhi at that time and day duration is only about 10 hours the ascendant after 5hrs. 45 minutes of sunrise shall be 3 signs and few degrees away than the Sun, that is, it will be just crossing Aquarius and reaching Pisces.

Actual calculations show that ascendant on 13.12.56 at 1 PM at Delhi is 90 7’ Pisces.

Traditionally, we calculate ascendant from Ishta Ghati using Palabha, Lankodaya, Udaymaan and Sun degrees for different signs. However because of better trignometric methods described above we are not putting it down here.

To make the task easy, Panchangs give ending time of ascending signs on daily basis for a given place. These tables can be directly used to get fairly correct ascendant sign. An interpolation of time between the two signs also gives us ascendant accurate to a degree normally sufficient for most of the astrological predictions.

Ascendant calculation for foreign births :
To utmost surprise, at least 50% astrologers commit mistake in calculating the ascendant for foreign births. This is mainly because they convert the time of birth to Indian Standard time. This way if a birth has taken place in the morning in New York, it becomes evening in India. Then when we calculate the horoscope, we find sun placed in the 7th house, whereas it should be in the ascendant by virtue of the definition of ascendant itself.

The trigonometric method above holds good for any date, time and place of birth. You do not have to change anything anywhere. However, if you want to compute ascendant orally, then also go for the method prescribed above. It will give approximately correct value of ascendant for any longitude or latitude (south latitude also).

To make the task further easy, take any Panchang which gives ending moment of signs for given local time of birth. Compute the local time by making oral correction for longitude to time zone and look into the table to get ascendant accurate to few degrees. This works because of the basic fact that ascendant varies only by a few degrees due to latitude.

Further note that to calculate planet positions using ephemeris or Panchangs, it is necessary to convert time from other time zone to IST since the Panchangs are giving values at IST only.

This is the common mistake done by most of the astrologers that they convert time to IST for calculation of ascendant and take the time as it is for calculation of planet degrees. So, remember : Do not convert time to IST for ascendant calculation of foreign births but convert the time to IST for planet degree calculations.