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

174 PAÑCASIDDHĀNTIKĀ VII.1 At a solar eclipse, z.d. is practically equal for the Moon and the Sun, and in measuring the angular distance between the Sun and the Moon relative parallax can be applied to the Moon, the Sun being supposed unaffected. We can write: Relative parallax = Moon's parallax −Sun's parallax = sin z.d. × 28.65 × earth's radius × (1/Moon's dis. − 1/Sun's dis.) When the Sun or Moon is at the horizon, sin z.d. is 120, and the relative parallax, (hereafter we shall call it merely parallax), called horizontal parallax, is a maximum, and this Siddhānta takes it as equal to 49'. ∴ Parallax = 49' sin z.d./120. Also, we can see from the fig. 1a that the Moon is depressed, away from the zenith by parallax, along the vertical circle, ZM, increasing the z.d. and that is why it is called lambanam, i.e. 'depression', in Sanskrit. This general parallax has to be resolved into two parts, correction to longitude, (p. long.) and cor- rection to latitude (p. lat.). This is shown in fig. 1b. Z Zenith N Nonagesimal (Lagna − 90°) O Orient ecliptic point (Lagna) A First point of Aries MAQ = ω Fig. VII. 1-b.