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

415 presumed that all the other planets including the Sun would be traversing the same linear distance during the course of a day. This led to a wrong estimate of the Sun’s distance as well as his spherical radius. Also, they supposed wrongly that the parallax of the Sun also would be equal to 1/15th of his daily arcual motion. Their estimate of the spherical diameter of the Moon was, however, very near the truth, for, they argued, that if 790′-35″ angular motion per day corresponded to 11858¾ Yojanas in linear measure, to what linear measure did the angular diameter of the Moon namely 32′-0″-9′′′ corres- pond? The answer was 11858¾ × 32′ 1/400 ------------------ = 480 Yojanas. 790′-35″ [चित्र: Fig. 94] Fig. 94 It may be here pointed out that there is a relation between the angular radii of two celestial bodies as seen from each other and their mutual horizontal parallaxes (Fig. 94). Let E and M bet he centres of the Earth and the Moon respectively. E M̂ A = horizontal parallax of the Moon = angular radius of the Earth as seen from the Moon and B Ê M = Angular radius of the Moon = Hori- zontal parallax of the Earth as seen from the Moon. Thus, we see that the Earth will be seen from the Moon, as a Moon with an angular radius equal to 57′. In other words our Earth will be a Moon to our Moon, having nearly 16 times the area of our Moon’s disc.