Some basic concepts of history and geography
... Chinese calendar counts years from the reign of the mythical Yellow Emperor in 2698 BC. The
Islamic calendar numbers years from 622 AD when Muhammad fled from Mecca. Both calendars are
based on lunar cycles. The year 2000 in our calendar is 4697 in the Chinese calendar and 1421 in the
Islamic calend ...
A year is the orbital period of the Earth moving in its orbit around the Sun. For an observer on the Earth, this corresponds to the period it takes the Sun to complete one course throughout the zodiac along the ecliptic.In astronomy, the Julian year is a unit of time, defined as 365.25 days of 7004864000000000000♠86400 SI seconds each (no leap seconds), or 7007315576000000000♠31557600 seconds total.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 fertility.In temperate and subpolar regions, generally four seasons are recognized: spring, summer, autumn and winter, astronomically marked by the Sun reaching the points of equinox and solstice, although the climatic seasons lag behind their astronomical markers.In some tropical and subtropical regions it is more common to speak of the rainy (or wet, or monsoon) season versus the dry season.A calendar year is an approximation of the Earth's orbital period in a given calendar.A calendar year in the Gregorian calendar (as well as in the Julian calendar) has either 365 (common year) or 366 (leap year) days. The average length of the year in the Gregorian (modern) calendar is 365.2425 days (taking account of the century rule for leap years). In an informative annex, the International Standard ISO 80000-3 proposes the symbol a (for Latin annus) to represent a year of either 365 or 366 days. In English the abbreviations y and yr are used.The word ""year"" is also used of periods loosely associated but not strictly identical with either the astronomical or the calendar year, such as the seasonal year, the fiscal year or the academic year, etc.By extension, the term year can mean the orbital period of any planet: for example, a ""Martian year"" or ""Venusian year"" is the time in which Mars or Venus completes its own orbit. The term is also applied more broadly to any long period or cycle, such as the ""Great Year"".