Lagna (ascendant)
The lagna is the sidereal sign rising on the eastern horizon at the moment of birth. It fixes the first house of the kundali and changes sign roughly every two hours.
What the lagna is
As the Earth turns, the whole zodiac rises in the east over a day, one sign every two hours on average. The sign and degree on the horizon at the instant of birth is the lagna, and every house of the chart is counted from it. The exact degree depends on latitude as well as time, which is why the birthplace matters.
Signs rise at different speeds: near the equator evenly, at higher latitudes some signs rise quickly and others slowly. Hastro computes the ascendant from the local sidereal time and the obliquity of the ecliptic, then converts to sidereal with the chosen ayanamsa.
Why it matters
The lagna and its lord set the frame for reading the chart: the first house is the self, and the placement of the lagna lord is read first. In muhurat, the lagna rising at the chosen moment is also examined, with fixed signs preferred for a house entry and movable ones for travel.
FAQ
My birth time is uncertain. How much does the lagna move?
About one degree every four minutes; a full sign in about two hours. If your time is within thirty minutes, the sign is usually secure unless the degree shown is near 0° or 30°.
Related: Kundali (birth chart) Navamsa Nakshatra