"ayanam" means change or signifies movement. Sun moving into each rasi is denoted by the words Meshayanam, Rishabhayanam, etc. Very orthodox brahmins - even though young in age - still do the 96 (ShaNNavati) tarpanams in a year and such people perform tarpanam for each of these ayanams of the sun.
DakshiNAyanam denotes the time when the sun commences its southwardly course during the period Katakam to Dhanus (ADi to mARkazhi), while uttarAyanam denotes its northerly movement commencement (makaram to miyhunam) or thai to Ani.
Almanac mentions these 12 ayanams to help the religious observance of Tarpanam. Also, in the olden days agricultural operations in our country were integrally linked to these ayanams which are called "ஞாற்றுவேல (njARRuvEla)" in Malayalam; this is an example of the ancient link between Kerala people and Tamil - ஞாற்றுவேல = ஞாயிறு + வேளை.
