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

412 ∴ Sin EMA = a/d sin ẐAM = ÊMA expressed in radian measure since ÊMA is very small ∴ ÊMA (expressed in radian measure) = a/d sin z I where z is the apparent zenith-distance of the Moon ie. zenith- distance as seen by the observer (in contradistinction to the geocentric zenith-distance of the Moon namely ẐEM). In particular, when z = 90°, ÊMA = a/d which is the maximum parallax known as the horizontal parallax ie. the parallax when the Moon is situated on the horizon of the observer. Also the parallax is zero when M is situated at Z as is seen from formula I and as is rightly remarked by Bhāskara in the words 'खमध्ये नास्ति लम्बनम्'. In fig. 91, ÊMA is the angle by which the line of sight of the observer namely AM is depressed from the geocentric line of sight EM. Since the plane of the paper represents a vertical through the Sun and the Moon, the depression of either the Sun or the Moon or the excess of the depression of the Moon over the Sun are all in the vertical plane. This depression is called Drik-lambana because it is a lambana or depression in the Drik-maṇḍala or vertical. This Drik-lambana varies as sin z as is seen from formula I where a, and d may be taken to be constants. (Both a and d vary slightly a varying slightly from place to place on the Earth, the Earth being an oblate spheroid, and d varying from position to position of the Moon). The maximum horizontal parallax is given by a/d in radian measure which is equal to, according to Bhāskara's