पञ्चसिद्धान्तिका (आचार्य वराहमिहिर - सूर्य, रोमक, पौलिश, वासिष्ठ एवं पितामह सिद्धान्त)

142 PAÑCASIDDHĀNTIKĀ V. 3
Now, for the visibility in the east: we are now looking eastward and successive points on the ecliptic
are lower and lower towards the horizon. Therefore in figs. 1, 2 and 3, other things being the same,
the ecliptic alone is to be represented as being directed downwards, as in Fig. 4.
M¹ C
M
South S L North
Fig. V. 4
Therefore, in each case taken up for consideration above, the direction of the ayana being
changed, we see that for the ayana and latitude having different directions, 'the result' is subtrac-
tive, and having the same direction it is additive. Thus step (v) is explained.
Now for step (vi). We have already said that the Moon will be visible if it does not set within two
nāḍīs after sunset, or if it rises before 2 nāḍīs before sunrise. (As visibility depends actually on other
factors like the keenness of the eyesight of the observer, we have only to take the authority of the
Śāstras in this matter). So in the evening we have to find the time by which the Moon will set after
sunset, i.e. the segment constituting the corrected difference in longitude will set. As the distance
between the rising and setting points in always in 6 rāśis, this time is equal to that of the rising of an
equal segment in the east, which can be calculated by using the vināḍīs of the ascensional difference
of the rising sign, which being six rāśis away, is the seventh from the Sun (or Moon). If this time is
greater than 2 nāḍīs, the Moon would not have set, and therefore be visible. In the matter of visibility
in the east, the same explanation holds, except that now the time of rising of the segment in the east
is wanted, using the ascensional difference of the rising sign in which the Sun (or Moon) itself is
situated, and hence the instruction to use that sign.
This instruction to use the ascensional difference of the same sign as the Sun in the case of visibility
in the east is implied by the use of the word vā, though not explicitly stated, and can also be inferred
from the nature of the explanation. TS-NP do not seem to have noted the difference in the methods
to be pursued in the operation. Another mistake they have made is that they have discarded the
correct reading, ayanānukūlavikṣipte (v. 2) and chosen the incorrect reading apamānukūlavikṣipte and
accordingly, have given the condition for additiveness or subtractiveness, "If the moon's latitude is