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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.

This change in effect is more prominent longitudinally and is comparitively much less latitudinally e.g. it remains day if we move South to North - Sri Lanka to Russia. But there is still a difference in sunrise mariginally 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.

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.

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

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

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	40⁰	50⁰	60⁰	70⁰	80⁰	90⁰
Distance covered by 1' of Longitude	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

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 100^s of time. And from a centre point it is only ±50^s. i.e. less than a minute! Thus when time of birth is accurate only to a minute level, taking the centre 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 areial 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.

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 :

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

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')

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 substracted 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. Alongwith 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 compa red to latitude.