Siddhanta Shiromani (Crown of Treatises)

From the phrase "understanding" original: boddho and so on, it is explained that the lack of light from the Sun and Moon occurs at their conjunction original: syote. As will be stated: "at the time of the New Moon, they are equal in zodiacal signs," and so forth. The duration of time starting from the beginning of this conjunction until its end is what constitutes a Lunar Month cāndramāsa. This means that the interval from one New Moon amāvāsyā to the next New Moon is a Lunar Month.

From this month, thirty days are reckoned; this specific duration of time constitutes the "Lunar [days]." Twelve of these months make a Lunar Year cāndra-varṣa. This is how the terminology is defined.

That which has been called a Lunar Month is also considered a single Day and Night of the Ancestors pitṛ-ahorātra. By these same days, thirty such days make a Month of the Ancestors; twelve of these months are calculated as a Year of the Ancestors.

As it will be stated: "Day by day, [the Sun] is seen by them... this is the cause of the lack of sight referring to the moon's phases." For those dwelling on the upper part of the moon, their day and night are determined by the waning and waxing phases of the moon. In the Surya Siddhanta a foundational Sanskrit text on astronomy as well, it is said:

In this context, at the end of the month (New Moon amāvāsyā), it is midday for the ancestors. At the end of the phase (Full Moon pūrṇimā), it is midnight. The day for the ancestors begins in the second half of the eighth day of the Dark Fortnight kṛṣṇa-pakṣa the waning moon, and their night begins in the second half of the eighth day of the Bright Fortnight śukla-pakṣa the waxing moon. Thus it is calculated. [21]

The interval between two successive sunrises is a Solar Civil Day arka-sāvana-dina. This is also known as a Terrestrial Day medinī-dina or Earth Day bhū-vāsara, resulting from the Earth's rotation. Its measure is the duration of time that elapses between two sunrises...