The concept of standard time was generated to have a uniform time for a country or a zone. The meridian passing through Greenwich (near London) has been chosen as the zero longitude. Therefore the local time at this meridian at any epoch or moment is known as Greenwich Mean Time (GMT). Standard Time of the world is reckoned from Greenwich. The world is divided into standard zones. The difference between standard time of any country known as zonal standard time and the GMT is the Zonal time for that country or zone. Indian standard time is based on the coordinates 820 30‟E and 230 11‟N and is 5 hours 30 minutes ahead of G.M.T.
To calculate ascendant or the lagna we require date of birth; standard (or zonal) time of birth;and Longitude and latitude of the birth place.In some cases, time of birth is given in Ghatis and Palas. In that case, convert ghatis and palas in hours and minutes by applying the relation that 24 minutes make one ghati and 24 seconds make one pala (or vighati) and add the result to the sunrise time (in IST) to abtain birth time in hours and minutes.
The birth time is converted to LMT. This is obtained by multiplying the difference of longitude of the place form the standard meridian (820.30 „ in case of IST) by 4.
The LMT correction is either (+) or (-) depending upon whether the towards East or West from the standard meridian for that country. A chart is given in the Tables of Ascendants giving LMT correction to get LMT for various places.
After obtaining LMT, we convert it into Sidereal Time of the epoch/moment. For this we use Lahiri‟s Tables of Ascendants. First we note down the sidereal time at 12 noon local mean time for 820 30 „ E longitude the year 2000 for the day and month of the given moment from Table No. I. To this time we apply corrections for the given year (Table II) and then for the different localities (Table III). This will represent Sidereal Time for the given date, year and place but for local noon i.e. 12 hrs. Thereafter find time interval (TI) between local mean noon i.e. 12 hours noon and the LMT of the given moment. This TI is to be increased by apply the correction given in table IV. We get increased TI. This increased TI is to be added to or subtracted from the sidereal time obtained for local noon as mentioned above, if the birth is afternoon or before noon respectively. This becomes the sidereal time of the required moment/epoch. The only caution necessary is that for epochs from 1.9.1942 to 14.10.1945 in India. The recorded IST must be reduced by one hour to get corrected IST before LMT correction is applied. Likewise for Britain, USA and other European countries. Summer time correction is applied to their zonal times (from the book Tables of Ascendants by N, C, Lahiri) available, the table nearest to the page where ascendants for the required latitude is given. If exact latitude is not available, the table nearest to the required latitude shall serve the purpose. To be more precise.
Find out ascendants at two consecutive latitudes falling either side of the required latitude and calculate exact degree by interpolation method.
After finding degree of ascendant as above, apply ayanamsha correction as per table given in table IV and you find lagna, its sign position and longitude in degrees and minutes.
Tenth house or M.C is same for all latitudes. It varies with sidereal time only and can be found the sidereal time straightway from the relevant table in the said book. Apply ayanamsha correction to it also.
By adding 6 signs to the lagna and Xth house so arrived, you get VIIth and IVth house degrees respectively. Then the difference between the ascendant and Xth house is divided into three equal parts and by adding this 3rd part to Xth house we get XUth and again adding 3rd part to XIth we get the XII house. By adding 6 signs to XIth and XIIth house we get Vth and VI houses. Likewise, by adding to the ascendant successively the difference between the IVth house and the ascendant, the longitudes of IInd and IIIrd houses are obtained. Adding 6 signs in them we get house positions of VIIth and IXth houses. These longitudes are the mid-points of the above-mentioned house.
To find out the extent of each house, we are to determine the longitude of the ending points of a particular house and the commencement of the next house. This point of demarcation is called bhava sandhi. To find them, divide the difference between cusps I and X by 6 and by adding successively the 6th part to the longitudes from X onwards, we will get sandhis between, X and XI and XII and I. likewise, after finding out the difference behind IV and I houses, and then dividing it by 6 and by adding successively the 6th part to the longitude of lagna, we get bhawa sandhis between I and II, II and III and III and IV. Then adding 6 signs we get sandhis between other opposite houses.