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

497
Comm. The directions are clear in the wake of the
previous commentary. In marking off the Valana, Bhās-
kara says ‘from the west in the last quarter and from the
east in the first quarter’. In the first quarter, the Moon
will be visible immediately after Sunset in the western
sky and the illuminated part of the Moon will be towards
the western side of the disc in which direction the Sun is
situated. The eastern point and the western point of the
disc are easily discernible on the background of the
horizon. Drawing first the diameter ‘WE’ of the disc
along the vertical circle in case the Moon is due west or
else along a small circle parallel to the prime-vertical called
Upa-Vṛtta, we have the east point of the disc namely e
vertically above the disc in the first quarter. (The figure
119 is shown for the first quarter). Then we are directed
to mark off the Valana E′ G′ in the form of a Hsine'.
Since the Valana is already computed as directed in verse
(5), to mark off this Valana we have to lay the scale
perpendicular to WE. Such a point G′ on the circum-
ference of the disc is to be marked from which the Hsine
equals the Valana computed. It is to be noted here that
it is not marking G′ E′ from G′, for, G′ is not known,
nor E′ for the matter of that, but locating G′ knowing
the magnitude of the Valana. Though we happened to
mention before that GE is the Valana, in the light of
what Bhāskara mentions in verse (8) we have to under-
stand that in the first quarter, the practice is to mark
off Valana from the point e marked on the disc before hand.
Though it does not matter when we take EG to be the
Valana E′ G′, we have to notice the practice followed.
The direction given in the course of the commentary
“केन्द्राद्वलनोपरिवृत्ताद् बहिरपि खटिकया सूत्रमुच्छाद्यम्” meaning
thereby that from the point M, we have to mark off
along MG′ where G′ is the point pertaining to Valana, a
line with the help of a chalk and a fine thread, namely
MC. This practice is still in vogue adopted by masons to
draw straight lines.
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