Understanding Place of Birth, Time Zone and Time Correction
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.
This change in effect is more prominent longitudinally and is comparatively much less latitudinal 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.
These are imaginary lines, parallel to equator. The equator represents zero degree latitude.
North pole is 900N and South Pole is 900S.
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 mileand 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
|Distancein E-W direction (Km.)
Distance covered by 1' of LatitudeFor 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'ECoordinates of Mumbai 180 58' N 720 50' EDifference 90 41' and 40 23'or 581' and 263'adding the two, distance between Delhi and Mumbai is 844' » 844 milesTaking 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.
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.
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.
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.
With the knowledge of coordinates we can even determine the distance between two points. Longitudinal distance play vital role in time gap compared to latitude.