To caste a horoscope, following accurate details are required:

Date of birth
Time of birth
Place of birth.

Time of Birth: It is recorded for India as IST - Indian Standard Time (and for other countries as GMT - Greenwich Mean Time). GMT is 5½ hours ahead of IST. But for casting a horoscope, Local Mean Time is used, which depends on the place of birth.

Place of Birth: Logitude and latitude of the place of birth are used to find the time correction to be applied to the IST, to find out the Local Mean Time.

Longitude: It is the arc drawn from North Pole to South Pole to mark the location in degrees of a particular place from a central place (Greenwich). All places are located either to the West or to the East of Greenwich. For example, the longitude of Bombay is 72° 50' E.

Latitude: It is the arc drawn to mark the location in degrees of a particular place North or South of Equator. For example, the latitude of Bombay is 18° 58' N.

Sidereal Time: It is the time taken by the earth to make one full rotation. It is not exactly 24 hours, but goes on changing according to increase or decrease in the duration of days and nights. It is called Vernal Equinox time and it is a measure of earth's rotation with respect to the equinox, which has a very small retrograde motion. Sidereal time is used for calculation of the Ascendant.

CALCULATION OF ASCENDANT

For making the horoscope, first of all we have to calculate the Ascendant. Ascendant is the rising sign at the time of birth. Let us see how the Ascendant is calculated. This will be done with the help of two books, viz. Table of Ascendants, and Annual Ephemeries, 1993 (for the year of birth) by Lahiri. These two books can be procured from the market.

Details of sample birth are once again given below. We would calculate Ascendant for this.

Date of birth : 10.12.1993,
Time of birth : 3.50 AM IST
Place of birth : Bombay.

We have indicated the Longitude and Latitude of Bombay as above. These were taken from page 101 of the book "Table of Ascendants". This book contains the necessary information about many major cities of India and abroad. If the city required is not given therein, kindly see the Atlas for these details. And, if Atlas is not handy, nearest city can be taken. But for accurate casting of horoscope, accurate details are required.

As made clear earlier, in a northern horoscope, Ascending sign is written in the top centre and then zodiac signs are continued anti-clockwise. Later on planets are placed in the horoscope. So, to calculate the Ascendant sign, the first Step is to find out the Local Mean Time (LMT).

Step 1

Convert the IST to LMT. There are two ways to find out the correction time to IST to obtain LMT::

i. Take the Longitude of Bombay, i.e. 72° 50' N.

Formula for getting the correction time to IST to find out LMT is 15° = 1 hour, or 1° = 4 minutes
(Please note, Longitude is given in degrees (°), minutes (') and seconds (''), [ 1° = 60', 1' = 60''), whereas the time is given in hrs, minutes and seconds [1 hr = 60 minutes, 1 minute = 60 seconds]. Do not confuse the minutes of Longitude with the minutes of time.
As per above formula, divide Longitude 72° 50' by 15 to know the LMT. It comes to: 4 hr 51 mt 20 secs. How? This is shown below to facilitate your understanding of the calculations.
{72°/15 = 4 12/15 (i.e. 4 hrs + 12/15 hrs)
12/15 x 60 (to make minutes) = 48 minutes
Hence, 4 hr 48 minutes
Now 50' /15 = 3 5/15 (i.e. 3 minutes + 5/15 or 1/3 minutes).
1/3 x 60 (to make seconds) = 20 seconds.
Hence, 4 hrs, 48+3 minutes, 20 seconds = 4 h 51 m 20 s.
Now deduct 5 h 30 m 0 s (as GMT is +5h 30 m 0 s of IST) from 4 h 51m 20s:
4 h 51 m 20 s
- 5 h 30 m 0 s
- 0 h 38 m 40s

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