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

515
[चित्रम् : Fig. 121] such that the time in bet-
ween the rising of B and
the rising of A is equal to
the Iṣṭa-nādis. By the time
B rises, A will have gone
up a little above the eastern
horizon, as well as the star
that has risen along with A
and now the position of the
star is such that the time
in between its rising and
the rising of B is equal
to the Iṣṭa-nādis. Hence
B, which is a point of the ecliptic is termed Udayārka
signifying thereby that when the Sun coincides with B,
the star rises heliacally. Thus the Udayārka B is ahead
of the Udayalagna A and the time in between their risings
is equal to Iṣṭa-nādis.
Let now A' be the Astalagna ie. the point which rises
when the star sets. This point will not be, generally,
diametrically opposite to A because, the time between the
rising and setting of a star which is given by double the
rising hour-angle will be far more than half a sidereal day
when the star has a large northern latitude and in the case
of one having a large southern latitude, the time will be
far less than half a sidereal day.
From A' find C' which is behind A' such that the time
in between the rising of A' and C' is equal to Iṣṭa-nādis.
Then the point C diametrically opposite to C' namely C is
called Astārka ie. when the Sun is at C the star sets helia-
cally in the west. It is so because, when the Sun is on
the western horizon setting at C, C' will be the lagna and
when the star is setting A' is the lagna and as these two
lagnas .differ by Iṣṭa-nādis, the time between the Sun’s
setting and the star’s setting is also equal to Iṣṭa-nādis.