The fundamental astronomical and geographic principles involved in Lagna calculation
In Vedic astrology the zodiac is centered on the Sun. So Sunrise time AT THE PLACE OF BIRTH is important. This is what determines your Lagna or the ascendant, which is the central keystone of your horoscope.
Ascendant or Lagna is the zodiac sign rising on the eastern horizon at the time of birth, at the place of birth.
Let us say a child is born at Chennai on 15th January 2007, at exactly Sunrise time, which is 6.39 AM. On 15th January Sun is in 1 degree Capricorn. So the sign Capricorn is just rising on the eastern horizon.So the Lagna is also 1 degree Capricorn. See the picture bellow to get an idea.
As Earth takes 24 hours to spin around its axis, all the 12 zodiac signs will rise on the eastern horizon. So in a day there will be 12 Lagnas. Hence 24 hrs divided by12 = 2 hours is the time span of each Lagna.
Each sign has 30 degrees. Hence 2 hours = 120 minits divided by 30 degrees = 4. So every 4 minits the Lagna increases by 1 degree.
The degrees and time I am mentioning are rough rounded off figures. There may be minor fractional variations depending on season and place etc.
Hence if the child is born at Chennai after 1 hour, i.e. at 7.39, the Lagna will increase by – 60minits divided by 4 = 15 degrees. So at 7.39 the Lagna will be 15 degrees Capricorn. Similarly if he is born 2 hours after sunrise, the Earth would have spun more and the zodiac sign Capricorn would have gone further up and the next zodiac sign Aquarius would be just rising on the eastern horizon. So at 8.40 the child born at Chennai will have Lagna 1 degree Aquarius.
Instead of 15th January suppose the child is born on 30th January at the same place Chennai and at the same time 6.39 AM. On 15th January Sun was in 1 degree Capricorn and as it travels @ 1 degree per day appx., on 30 January Sun will be in 15 degrees Capricorn. So the child’s Lagna in this case will be 15 degrees Capricorn.
So on a rough note, the fundamental rule is – if born at Sunrise time Lagna is same as the sign in which Sun is placed and the Lagna degrees (Longitude) are same as Sun’s degrees. If the birth is later or before Sunrise, increase or decrease the degrees @ 1 degree per every 4 minits.
When the place of birth changes, the latitude and longitude also change, sometimes the time zone also changes and because of this the Sunrise time also changes accordingly. This is the reason why Lagna changes from place to place.
In the days before computerization when calculations were made manually, the Ephemeris used to give the “Sidereal time” at NOON 12.00 for every day. Taking this sidereal time, with the help of a “Table of ascendants”, which used to give a chart of the variations for the Lagna for various places on Earth and time differences, Lagna calculations were made.
The daylight saving time is in operation in many countries. As this is an artificial advancing of time by one hour, it has no actual effect on Sun’s motion. So if born during the period when daylight saving time is in operation, we have to take the local time + 1 hour which is the actual real time.