Sidereal year
A sidereal year (, ; from Latin "asterism, star"; also sidereal orbital period) is the time taken by the Earth or another planetary body to orbit the Sun once with respect to the fixed stars. Hence, for the Earth, it is also the time taken for the Sun to return to the same position relative to the Earth with respect to the fixed stars after apparently travelling once around the ecliptic. It equals for the J2000.0 epoch. The sidereal year differs from the solar year, "the period of time required for the ecliptic longitude of the Sun to increase 360  degrees", due to the precession of the equinoxes. The sidereal year is 20 min 24.5 s longer than the mean tropical year at J2000.0 . At present, the rate of axial precession corresponds to a period of 25,772 years, so sidereal year is longer than tropical year by 1,224.5 seconds (20 min 24.5 s, ~365.24219*86400/25772). Before the discovery of the precession of the equinoxes by Hipparchus in the Hellenistic period, the difference between sidereal and tropical year was unknown. For naked-eye observation, the shift of the constellations relative to the equinoxes only becomes apparent over centuries or "ages", and  pre-modern calendars such as Hesiod's "Works and Days" would give the times of the year for sowing, harvest, and so on by reference to the first visibility of stars, effectively using the sidereal year. The Indian national calendar, based on the works of Maga Brahmins, as are the calendars of neighbouring countries, is traditionally reckoned by the Sun's entry into the sign of Aries and is also supposed to align with the spring equinox and have relevance to the harvesting and planting season and thus the tropical year. However, as the entry into the constellation occurs 25  days later, according to the astronomical calculation of the sidereal year, this date marks the South and Southeast Asian solar New Year in other countries and cultures.