Though the word literally means ‘course’ or ‘path,’ it is generally used to indicate the course of the sun to the north or the south of the equator. The northward course is called ‘Uttarāyaṇa’ whereas the southward course is named as ‘Dakṣi-ṇāyana.’ The former is the period of mid-winter to mid-summer and the latter, that of mid-summer to mid-winter. According to the Sūryasiddhānta (14.5.9), a well-known authoritative work of astronomy, the period of six months from the sun’s entrance (called ‘saṅkrānti’) into Capricorn (Makararāśi) is called ‘Uttarāyaṇa.’ Similarly, the period of six months from the sun’s entrance into Cancer (Karkāṭaka-rāśi) is called ‘Dakṣiṇāyana.’
Hindu religious traditions have always considered death during Utta-rāyaṇa as propitious. The Mahābhārata says that the venerable Bhīṣma waited till Uttarāyaṇa to give up the body.
The word ‘ayana’ is also used to indicate the precession of the equinoxes. According to the later astronomers like Āryabhaṭṭa II (A. D. 1028) the equinoctial point oscillates about a mean point. The amplitude of precessional oscillation is 27 degrees and the period of one complete oscillation is 7200 years. The rate of precession is uniform and is 54 seconds per year. A sidereal year that disregards this precession is called ‘nirayana’ and, the tropical year that takes it into account is called ‘sāyana.’
The total precession of the equinoxes since zero-precession year is called ‘ayanāṁśa.’ The ayanāṁśa at present is calculated to be 22 degrees. During the next 350 years it will gradually increase up to 27 degrees. It will then start decreasing to zero over the next 1800 years.