22 MEAN LONGITUDE OF A PLANET Rules for finding the laghvahargana occur in the Brahma-sphuţa-siddhanta¹, the Śisya-dhi-vṛddhida², the Siddhanta-sśekhara, and the Siddhanta-siromani*, etc., but the process described in the above rule is slightly different from them. A rule for finding of the mean longitudes of the Sun, Mercury, and Venus: 31. Divide the grahadeha (for the Sun) by 70: the result is in days, etc. Then multiply one-fifth of the grahadeha by 2: the result is in vighatikas. These (days and vighatikas) subtracted from the grahadeha are stated to be (the degrees, minutes, etc. of) the mean longitudes of the Sun, Mercury, and Venus.³ The grahadeha is the same as grahatanu defined above. The following is the rationale of the above rule. Rationale 1. The grahadeha is in terms of mean civil days. This has to be converted into mean solar days. The difference between civil and solar days in a yuga 1555200000 1577917500 = 22717500 days. Therefore, this difference per mean civil day 22717500/1577917500 of a day 1/70 of a day-2/5 of a vighaṭikā approx. = ¹ i. 42-43. 2 I, i. 37. ³ ii. 40-41 (i).
- I, i(e). 12-13.
The same rule occurs also in ŚiDV, I, i. 39 and in GL, i. 10(i). Similar rules occur in BrSpSi, i. 44; MSi, i. 26 ; Sise, ii. 42, 43; Sisi, I, i(d). 15; SiSā, i. 105; KPr, i. 4;. KKau, i. 16; KKu, i. 7; and SK, i. 6 (i).
