Bharatiya Jyotihshastra (History of Indian Astronomy), translated by R. V. Vaidya v2.c12.s2.t6.7

v2.c12.s2.t6.7

A Karaṇa work is required to give the positions of planets as at the beginning or epoch of that work, for finding the planetary positions. These positions are termed ‘kṣepaka’. The ‘kṣepakas’ as mentioned by the Pañcasiddhāntikā prove to be true for the mean Meṣādi (Sun’s entry into Aries), falling on the 14th Lunar day of the dark half of ‘amānta’ Caitra of Śaka 427 ; i.e. on Sunday, the 20th of March, 505 A.D. There is no doubt that some of them are true for the moon and others for the midnight of that date. This point will be more clearly explained in the study of the Sūrya-Siddhānta later on. The next Śukla ‘pratipadā’ (i.e. the first lunar day) after the 14th day of the dark half of Caitra, i.e. the first day of the light half of Vaiśākha, is seen to fall on Tuesday. Evidently this very ‘pratipadā’ was called the ‘Caitra-śukla-pratipadā’, by Varāhamihira (otherwise, by no other method the Caitra-śukla-pratipadā of Śaka 427, can be shown to fall on Tuesday), and it was no doubt correct that the ‘ahargaṇa’ is required to be calculated from that day. Varāhamihira appears to have adopted this epoch, because it is convenient to calculate positions from those on the 1st day of the light half. It is a well known fact of mathematical astronomy that the ‘ahargaṇa’ calculated with the help of any ‘Karaṇa work’ proves to be correct within a day, and it has to be verified with the help of the day of the week. It may now be questioned as to why the Vaiśākha śukla-pratipadā could have been called the Caitra-śukla-pratipadā by Varāha. I have absolutely no doubt that the śukla-pratipadā following the day for which the Kṣepakas are given is the first day of the Vaiśākha-śukla-halfmonth of Śaka 427 according to the ‘amānta’ system. The Caitra is defined as that ending lunar month which ends while the sun is in Meṣa (Aries). As the sun is found to be in Meṣa at the end of that amāvāsyā which fell on the day, next to the day for which the elements have been given irrespective of whether a mean or a true Meṣa was reckoned; and hence, according to the above definition the amānta lunar month which ended with the amāvāsyā must be termed as Caitra, and Vaiśākha commenced on Tuesday i.e. the next day. But a lunar month belonging to the pūrṇimānta system ends on the full moon day, and hence, the full moon day belonging to that light half which commenced just after the ‘kṣepaka’ day will coincide with the end of the lunar month. The calculations made on the basis of ‘kṣepakas’ given by Pañcasiddhāntikā show that the sun still occupied the Meṣa sign, and hence that lunar month naturally received Caitra as its name. The author does not find any other convincing theory to explain how that month could be termed Caitra. The pūrṇimānta system has remained in vogue in Northern India from a very ancient time. Even though the ‘pūrṇimānta’ system is still in vogue there, the lunar months are not named according to the system described above; but it appears that the system must have been in use in its pure form at the time of Varāhamihira.