सिद्धान्तशिरोमणि: गणिताध्याय (भास्कराचार्य - ग्रहगणित, मध्यमाधिकार व स्पष्टाधिकार सटीक)

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Having computed the Asta-lagna of the star, taking
it to be the Sun, compute the lagna in the reverse direction
ie. the lagna which preceeds it by the Iṣṭa-kāla given. If
this lagna be decreased by 180°, it will give the longitude
of the setting Sun at the time of the heliacal setting of the
star. Or again find the longitude of the point of the
ecliptic which is ahead of the Astalagna which takes (60—
Iṣṭanādis) to rise after the Astalagna; this longitude
decreased by 180° gives the longitude of the setting Sun
at the time of the heliacal setting of the star. The heliacal
rising or setting takes place when the longitude of the Sun
equals the longitude of the point of the ecliptic which is
technically called the Udayārka or the Astārka respectively.
The difference between the longitude of the Sun and that
of the Udayārka or the Astārka divided by the daily motion
of the Sun gives approximately the number of days that
have elapsed or are to elapse for the heliacal rising or
setting as the case may be,
Comm. Here we are to carefully differentiate between
the technical words (1) Udayalagna of the star, (2) Asta-
lagna of the star, (3) Udayārka and (4) Astārka of the
star. The word Udayalagna means that point of the
ecliptic which rises simultaneously with the star. The
word Astalagna means that point of the ecliptic which is
rising when the star is setting. The word Udayārka means
that point of the ecliptic which rises when the star rises
heliacally. The word Astārka means that point of the
ecliptic which is setting while the star sets heliacally. Thus
the four points are only points of the ecliptic.
Let A be the Udayalagna of a star on the circle CAB
which is the ecliptic. We know that the position of the
star should be above the eastern horizon, to rise heliacally,
by such a distance that the time in between the rising of
the Udayalagna and that point of the ecliptic which will
be rising when the star rises heliacally must be the Iṣṭa-
kāla nādis. Let B be a point ahead of A on the ecliptic