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

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In other words the star is within the Iṣṭa-nāḍi distance
from the Sun and as C is behind A by Iṣṭa-nāḍis, the
setting Sun at C will be behind (ie. has a lesser longitude)
the setting star by Iṣṭa-nāḍis. Hence the star sets then
heliacally. Thus we see that the Udayārka and Astārka
given by the points B and C are respectively in advance
(ie. has a greater longitude) and behind (ie. has a lesser
longitude) of the Udayalagna and Astalagna removed by
an Iṣṭa-nāḍī distance. It will be noted that the arcs AC
and AB are not equal though the equatorial arcs corres-
ponding to them are equal.
Instead of finding C' from A' by the Vilomalagna
method and taking its diametrically opposite point, Bhās-
kara gives an alternative namely to find C from A' by the
Kramalagna method the time being (60-Iṣṭa-nāḍikas).
This is evident.
Bhāskara prescribes only Ākṣa-Dṛk-Karma to be per-
formed here in finding the Udayalagna and Astalagna of
the star because the star's polar longitude is already one in
which the Āyana-Dṛk-Karma is contained.
Thus from fig. 121, when the Sun-rises at A, the star
rises, when the Suns rises at B the star rises heliacally
and when the Sun sets at C, the star sets heliacally.
Verse 15. If in the case of a star, the Astārka
happens to have a longitude greater than the Udayārka on
account of a very big northern latitude, that star does not
set heliacally (and so the question of heliacal rising does
not arise).
Comm. In Fig. 121, the Astārka C happens to have
a longitude less than that of B (because CAB is the dire-
ction of increasing longitude ie. positive direction). At
times it so happens that C lies towards the positive
direction of B ie. it has a longitude greater than that of B.