The earth is a spheroid (sphere flattened at the poles) rotating once a day around its axis; the earth also revolves around the Sun once a year.

Thus each of the 12 zodiacal signs seem to rise with the Sun, at the beginning of each of the 12 solar months.

On the first day of Aries the first degree of that sign rises in the Eastern horizon at sunrise, and so on till the last day of Pisces, the sunrise takes place in the last degree of the zodiac, when the solar year ends.

Thus the Lagna (also called the Ascendent) is that particular point of the ecliptic (the apparent orbit of the Sun) considered with reference to the horizon of a particular place.

The degree and the sign in which the Sun rises will be the Lagna at the Sun rise time.

The zodiacal sign which rises at the time of birth is the birth Lagna.