194 A rule for getting the duration of the Moon's visibility at night in the light half of the month (I quarter): 27. In the light fortnight, find out the asus due to oblique ascension (of the part of the ecliptic) intervening between the Sun and the (visible) Moon (at moonset) both increased by six signs, by the method of successive approximations. These give the duration of visibility of the Moon (at night) (or, in other words, the time of moonset).¹ RISING, SETTING AND CONJUNCTION OF PLANETS The process of successive approximations may be explained as follows: Compute the (sayana) longitudes of the visible Moon and the Sun for sunset and increase both of them by six signs. Then find out the asus (4₁) due to the oblique ascension of the part of the ecliptic lying between the two positions thus obtained. Then A₁ asus denote the first approximation to the duration of the Moon's visibility at night. Then calculate the displacements of the Moon and the Sun for A₁ asus and add them res- pectively to the longitudes of the visible Moon and the Sun for sunset and increase the resulting longitudes by six signs; and then find out the asus (A₂) due to the oblique ascension of the part of the ecliptic lying between the two positions thus obtained. Then A₂ asus denote the second approximation to the duration of the Moon's visibility at night. Repeat the above process successively until the successive approximations to the duration of the Moon's visibility agree to vighatis. The time thus obtained is in terms of civil reckoning. If, however, the use of the Moon's displacement alone be made at every stage, the time obtained will be in terms of sidereal reckoning. A rule for finding the time of moonrise in the dark half of the month (III quarter): 28. Thereafter (i.e., in the dark half of the month), the Moon is seen (to rise) at night (at the time) determined by the asus (due to oblique ascension) derived by the method of succes- sive approximations from the part of the ecliptic intervening bet- ween the Sun as increased by six signs and the (visible) Moon as obtained by computation, (the Sun and the Moon both being those calculated for sunset).* ¹ Cf. Susi, x. 2-4. Cf. Susi, x. 5.
