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

516 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.