The Ascendant is defined as the intersecting point between the Ecliptic and the Celestial Horizon and is the Eastern Celestial Horizon. It is represented by the formula
Tan L = Sin E / cos E cos w – Sin w Tan A
where A is Latitude of the place, w is the maximum declination of the Sun, E is the Right Ascension of East Point ( Sayana Kala Lagna ) & L – the Lagna.
Raseenam Udayo Lagnam – Udaya Lagna, the Ascendant, the pivot of the horoscope, is the Eastern Celestial Horizon. In Astrology everything is based on Lagna, Sarvam Lagnepi Chinthayel.
180 degrees opposite to the Ascendant is the Descendant, the Astha Lagna and is the Western Celestial Horizon.