Ayana literally means movement in Hindu calendar and the ayana days are June 21 and December 21. When the sun apparently moves towards north of the equator, its movement is called uttarayana; when it moves towards the southern hemisphere the movement is called dakshinayana. The apparent path of the sun (ecliptic) is 23.5 degrees north or south of equator in one year. The regino between the equator and the northern part of the sun’s path is called Karka Vritta (Tropic of Cancer) and that of the south of the equator is called Makara Vritta (Tropic of Capricorn).
The ecliptic crosses the equator twice a year around March
21 and September 21. The points are calle equinoxes. They are also known as the
first point of Aries and the first point of Libra. The sun is thus the farthest
from the equator twice in a year, i.e., around June 21 (summer solstice) and
December 21 (winter solstice). The apparent journey of the sun from December to
June is uttarayana, (also called devayana); and the movement of the sun from
June to December is dakshinayana (also called pitrayana).
The point when the sun crosses the equator to move from the
south to the north is called the Makara Sankranti day, which falls around
December 21. But due to the precession of equinoxes (moving backward) and
computation of time and days, warranted by the necessity to reckon time by
sidereal year, this date is changed to January 14 every year. Karka vritta and
makara vritta are called ayana vritta. Around June 21, the sun’s rays fall
vertically on karka vritta; and around December 21, the sun is exactly above
makara vritta. These two days are called ayana days.
Ayanamsha (precessional effect) is the angular distance
between the equinox and a fixed point on the ecliptic. The angular precession of
equinoxes is 47 seconds per every hundred years.
The obliquity of the ecliptic with the equator is denoted by
the Greek letter omega. Ayana, an apparent motion of the sun, accounts for the
fact that the picture of a constellation visible in the night skies at a given
time and place, though moving from month to month, repeats itself exactly at the
same time and place.
The spring equinox is taken as first and the year is called Sayana
year (tropical year), whereas if a point nearer to the ecliptic Revati point is
taken as the first day of the year, it is called nirayana year (sidereal year).
Nirayana year is longer than Sayana year by 51 palas (29 seconds)
approximately.