पृष्ठम्:महाभास्करीयम्.djvu/२८४

LIMITS OF VISIBILITY OF THE PLANETS The remainder of this chapter deals exclusively with the planets. Minimum distances of the planets from the Sun when they are visible: 199 44. Venus is visible when it is 9 degrees away from the Sun; Jupiter, Mercury, Saturn, and Mars are observed when they are respectively further away by two degrees in succession (i.e., when they are respectively 11°, 13°, 15°, and 17° away from the Sun). 45. Ve. as, which moves in its proper orbit but. appears retrograde, is visible, due to profusion of its rays, when it is (only) 4 or 4 degrees away from the Sun. The degrees above are "the degrees of time" (called kalabhāga). One degree of time is equivalent to 60 asus. Thus Venus is visible in the east if the time taken by the part of the ecliptic between the Sun and Venus to rise above the eastern horizon amounts at least to 9x 60 asus; and in the west, if the time taken by the part of the ecliptic between the Sun and Venus to set below the western horizon, or the time taken by the diametrically opposite part of the ecliptic to rise above the eastern hori- zon, amounts at least to 9x60 asus. It may be pointed out that a sign sets in the same time that the seventh, i.e., the diametrically opposite sign, takes to rise. According to Lalla, Mercury and Venus, when in regression, are visible when they are respectively 12 and 8 degrees distant from the Sun. According to Sripati, Mercury and Venus, when near their apogees, are visible when they are respectively 14 and 10 degrees distant from the Sun; and when in regression, they are visible when respectively 12 and 8 degrees distant from the Sun.5 1 The same degrees of visibility are prescribed also in BrSpSi, vi. 6; x. 32 ; SIDVE, I, vii. 5 (i); and Sise, ix. 8 (i), 12. 2 When Mercury or Venus is in retrograde motion, it is nearer to the Earth and so its size is a little enlarged. 3 Venus in both the cases being that corrected for the visibility corrections. 4 SiDV, I, vii. 5 (ii). 5 Sise, ix. 9. The Greek astronomer Ptolemy (150 A. D.) also considered the heliacal rising and setting of the planets and defined the limits of visibility of each planet when in the sign Cancer (i.e., when the equator and the ecliptic are nearly parallel). His limits are; for Saturn, 14°; for Jupiter, 12°45'; for Mars, 14°30'; and for Venus and Mercury, in the west, 5°40' and 11°30' respectively. Vide Syntaxis, xiii. 7-9.