Description of absolute dating and numerical age
A year is the orbital period of the Earth moving in its orbit around the Sun.
Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by changes in weather, the hours of daylight, and, consequently, vegetation and soil fertility.
(hṓra) "year, season, period of time" (whence "hour"), Old Church Slavonic jarŭ, and Latin hornus "of this year".
Latin annus (a 2nd declension masculine noun; annum is the accusative singular; annī is genitive singular and nominative plural; annō the dative and ablative singular) is from a PIE noun , which also yielded Gothic aþn "year" (only the dative plural aþnam is attested).
Similarly, "year" can mean the orbital period of any planet: for example, a Martian year or a Venusian year are examples of the time a planet takes to transit one complete orbit.
The Revised Julian calendar, as used in some Eastern Orthodox Churches, currently does a better job than the Gregorian in synchronizing with the mean tropical year.
No astronomical year has an integer number of days or lunar months, so any calendar that follows an astronomical year must have a system of intercalation such as leap years.
Financial and scientific calculations often use a 365-day calendar to simplify daily rates.
Both *yeh₁-ro- and *h₂et-no- are based on verbal roots expressing movement, *h₁ey- and *h₂et- respectively, both meaning "to go" generally (compare Vedic Sanskrit éti "goes", atasi "thou goest, wanderest").
Derived from Latin annus are a number of English words, such as annual, annuity, anniversary, etc.; per annum means "each year", anno Domini means "in the year of the Lord".