94 VISIBILITY AND CONJUNCTION OF PLANETS [CH. VII Thus are obtained the minutes of arc of the latitudes (of the two planets which are in conjunction in longitude). Two things deserve mention here. One is that the revolution-numbers of the nodes of Mercury and Venus, stated in Hindu works on astronomy, as says Bhaskara II 1 , are those increased by the revolution-nnmbers of their res- pective sighra-kendras. The result is that when we subtract the longitude of the ascending node of Mercury or Venus from the longitude of its sighrocca, we obtain the longitude of the planet (Mercury or Venus) as diminished by the longitude of its ascending node. The second is that in finding the celestial latitude of a planet we should use the heliocentric longitude of the planet and not the geocentric longitude. Brahmagupta (628 A. D.) and other Hindu as- tronomers have, therefore,prescribed the use of the true-mean longitude in the case of Mars, Jupiter and Saturn, and that of the longitude of the planet's sighrocca as corrected for the planet's mandaphala in the case of Mercury and Venus. 2 A rule relating to the determination of the distance between the two planets which are in conjunction in longitude : 9-10. From those latitudes obtain the distance between those two given planets by taking their difference if they are of like directions or by taking their sum if the are of unlike directions. 3 The true distance between the two planets, in minutes of arc, being divided by 4 is converted into ahgulas^ Other things should be inferred from the colour and bright- ness of the rays of the (two) planets or else by the exercise of one's own intellect. 5 1 See SiSi, II, viii. 23. 8 See BrSpSi, ix. 9. Also see SuSi, ii. 56-57 ; SiSe, xi. 15 ; and SiSi, II, vi. 20-25(i). a Cf. MBh, vi. 54. 4 Gf. MBh, vi. 55. 5 Set SuSi, vii. 18(ii)-23(i).
