Calendar

I really like astronomy. The Stars and planets above our head in night sky are astonishing. One of the related topics in astronomy is calendar, the system which people in countries use to calculate the days and month and other events in a year.
We just passed the first day of new Gregorian year. Today is the first day of Islamic calendar. We will reach to Chinese New Year's Day in a month. And in March there will be Persian New Year's Day (نوروز).
Mostly the old calendars are lunar type. A lunar calendar is a calendar oriented at the moon phase. The only widely used purely lunar calendar is the Islamic calendar, whose year always consists of 12 lunations.
A solar calendar is a calendar whose dates indicate the position of the earth on its revolution around the sun. Gregorian and Persian calendars are examples of solar calendars.
Chinese calendar is lunisolar, The reason for this is that a year is not evenly divisible by an exact number of lunations, so without any correction the calendar year will drift with respect to the seasons.
The interesting question is always popped: Why this day is New Year's Day?
Gregorian calendar decided the birth of Christ (actually a day between two most saying) and Islamic calendar decided the first day of great journey of Prophet Mohammad.
Chinese calendar uses a bit complex mathematical and astronomical calculations to determine the first day. In one sentence, the Chinese New Year is the second New Moon after the winter solstice.
The Persian year starts at vernal equinox. (The time when the sun crosses the celestial equator and day and night are of equal length, marking the beginning of spring.)
The Persian mathematicians used a simple and universal method for the calendar which impress me too much.