Surya Siddhanta
विक्षेपाग्राल् लिखेद् वृत्तम् ग्राहकार्धेन तेन यत् । ग्राह्यवृत्तम् समाक्रान्तम् तद्ग्रस्तम् तमसा भवेत् ॥
From the extremity of the latitude describe a circle with a radius equal to half the measure of the eclipsing body : whatever of the disk of the eclipsed body is enclosed within that circle, so much is swallowed up by the darkness (tamas). [The phraseology of the text in this passage is somewhat intricate and obscure; it is fully explained by the commentary, as, indeed, its meaning is also deducible with sufficient clearness from the conditions of the problem sought to be solved. It is required to represent the deflection of the ecliptic from an east and west line at the moment of greatest obscuration, and to fix the position of the centre of the eclipsing body at that moment. The deflection is this time to be determined by a secondary to the ecliptic, drawn from near the north or south point of the. figure. The first question is, from which of these two points shall the deflection be laid off, and the line to the centre drawn. Now since, according to verse 10, the latitude itself is to be measured upon the line of deflection, the latter must be drawn southward or northward according to the direction in which the latituda is to be laid off. And this is the meaning of the last part of verse 8; " in accordance," namely, with the direction in which, according to the previous part of the verse, the latitude is to be drawn. But again, in which direction from the north or south point, as thus determined, shall the deflection be measured? This must, of course, be determined by the direction of the deflection itself: if south, it must obviously be measured east from the north point and west from the south point; if north, the contrary. The rules of the text are in accordance with this, although the determining circumstance is made to be the agreement or non-agreement, in respect to direction, of the deflection with the moon's latitude—the latter being this time reckoned in its own proper direction, and not, in a lunar eclipse, reversed. Thus, in the case for which the figure is drawn, as the moon's latitude is south, and must be laid off northward from M, the deflection, t)"w", is measured from the north point; as deflection and latitude are both south, it is measured east from N. In an eclipse of the sun, on the other hand, the moon's latitude would, if north, be laid off northward, as in the figure, and hence also, the deflection would be measured from the north point : but it would be measured eastward, if its own direction were south, or disagreed with that of the latitude. The line of deflection, which is Mv" in the figure, being drawn, and having the direction of a perpendicular to the ecliptic at the momemt of opposition, the moon's latitude for that moment, Ml", is laid off directly upon it. The point I" is, accordingly, the position of the centre of the shadow at the middle of the eclipse, and if from that centre, with a radius equal to the semi-diameter of the eclipsing body, a circle be drawn, it will include so much of the disk of the eclipsed body as is covered when the obscuration is greatest. In the figure thf> eclipse is shown as total, the Hindu calculations making it so, although, in fact, it is only a partial eclipse.]
english translation
vikSepAgrAl likhed vRttam grAhakArdhena tena yat । grAhyavRttam samAkrAntam tadgrastam tamasA bhavet ॥
hk transliteration by Sanscript