Calendars
A calendar divides time into long intervals – days, months, years. A lunar calendar rests on the 29.53-day synodic month (12 months ≈ 354 days), a solar calendar on the tropical year (≈ 365.2422 days), and a lunisolar calendar reconciles the two by adding a leap month. The Julian calendar (from 45 BCE) took the year as 365.25 days, i.e. every fourth year is a leap year; it drifted ahead by about a day in 128 years. In the Gregorian calendar, introduced in 1582, a year divisible by 4 is a leap year, but a century year is not unless divisible by 400: 1900 and 2100 are not leap years, 2000 is. Since 400 years contain 97 leap years, the mean year is 365.2425 days. Uzbekistan uses the Gregorian calendar in civil life; Navruz is celebrated around the vernal equinox. The Jalali solar calendar prepared by Omar Khayyam and colleagues around 1079 was also very accurate, and Biruni compared the calendars of different peoples in his book on the chronology of ancient nations.
In your notebook list the century years from 1900 to 2100 and mark by the rule which are leap years. Compare the result with a classmate.