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

XVIII.36 XVIII. VĀSIṢṬHA-PAULIŚA – RISING AND SETTING 339 IV: 9° (Fast Retrograde) – 9 in Aries 42-00 Balance No. of days 206-1 V: 16° 12° in Aries 45-00 (12/16 × 60) 4° in Taurus 14-49 (4/17 × 63) —————— 59-49 VI: 85° 47' 26° in Taurus 41-36 (26/30 × 48) 30° in Gemini 51-00 29° 47' in Cancer 53-36 (53-36/54 × 30) —————— 146-12 Therefore, True motion in 543-20 days from 20° to 29° 47' Cancer = 249° 47 Mean motion in 543-20 days: 408 – 43 × 543 1/3 /780 = 284° 42' —————————— Therefore days to be added = 34-55 True motion for 34-55 to be added In Cancer 0-13' for 0-23, and for the balance 34-32 days, In Leo 30 × (34-32 / 54) = 19° 11' Therefore True Mars: 19° 11' in Leo. Motion of Mercury¹ As in the case of other planets, the epoch date is so selected that Mercury rises heliacally in the west on that date. Since, for Mercury, the elongation required for heliacal rising is given as twelve degrees, on the epoch day its longitude is 12 degrees ahead of the Sun. Then, the number of days elapsed from the epoch is obtained and divided by the sidereal period of Mercury, so that the resulting quotient represents the number of risings that had taken place since the epoch, and the remainder the number of days elapsed in the current sidereal cycle. The movement of Mercury in one synodic period being known, (given by the text), the total longitude moved during the elapsed number of risings is also known and adding to this the epoch constant, that is, the position of Mercury on the epoch day, its longitude on the day of the current rising is obtained. However, these figures are on the assumption that the motion of Mercury is uniform, which is not the case. So, what we have got is only the mean position of Mercury and not its true position. Here, as also in the text, 'mean position' does not mean the mean heliocentric position as we now understand, but the mean geocentric position on the assumption of uniform motion. The true position varies from the mean by quite a few degrees. The date of rising also is only a mean date and the true date may be several dates behind or ahead.

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1. Mercury : Translation and Notes by S. Hariharan, Bangalore.