The lagna
The lagna is the point of the sidereal (Lahiri) zodiac rising on the eastern horizon at the place and moment; it moves through all twelve rashis in about a day, each taking from under an hour to over two and a half, by the place's latitude.
The rashi rising: the first house of a chart.
The ascendant from the place's latitude and the sidereal time, made sidereal by the Lahiri constant; checked against Swiss Ephemeris within 0.05° in 32 cases, polar ones included, where it is defined. Each rashi's rising time at a latitude is compared with the Surya Siddhanta's (iii.42-45).