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.

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 0° on 14th April and it advances by 1° everyday or 1 sign every month.

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 28° scorpion 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 28° Scorpio. After that it increases by the sign every 2 hours or 1° 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 9°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.