The Tropical Year Calendar
Julian/Gregorian
The tropical year is the time, taken by the Sun, to pass from the vernal equinox to the vernal equinox. It equals 365 days, 5 hours, 48 minutes, and 46 seconds, or 365.2422 days. The tropical year is used, to keep track of seasons, planting, and harvesting. There is a need to create a calendar, with a whole number of days per year to accurately follow the tropical year without losing synchronization with the seasons over a period of time.
We begin with a calendar of 365 days per year. Our calendar year is shorter than the tropical year by 0.2422 days. So to correct (approximately), we add 1 day every four years (leap year). Thus, three calendar years are 365 days long; the fourth calendar year is 366 days long. The average length of the calendar year in days now becomes (3 x 365 + 366)/4 = 365.25 days.
This calendar system was actually instituted for use in the Roman Empire by Julius Caesar around 46 BC. But since the Julian calendar was 0.0078 days (11 minutes and 14 seconds) longer than the tropical year, errors in timekeeping gradually accumulated. Between 46 BC and 1582 AD, this accumulated error amounted to a total of: 0.0078 x (1582 + 46) = 12.7 days. In 1582, Pope Gregory XIII reformed the calendar by specifying that all years divisible by 4 are to be leap years except for century years, which must be divisible by 400 to be leap years. Now, in 1200 years:
- A total of 300 years (including all century years {since any century year = N x 100, where N = an integer}) are divisible by 4 and are therefore candidate leap years.
- A total of 900 years are not divisible by 4 and are therefore regular years.
- Twelve-century years are possible leap years.
- But only 3 century years (out of the 12) are divisible by 400 (i.e., {400, 800, 1200}, {1600, 2000, 2400}, etc.), so only 3 century years are actually leap years.
Since 12 - 3 = 9, Gregory's rule eliminates 9 leap years out of 1,200. Thus: 300 - 9 = 291 years are actual leap years, and 900 + 9 = 909 years are regular years. The average length of the year becomes (291 x 366 + 909 x 365)/1,200 = 365.2425 days, with an error of 365.2425 - 365.2422 = 0.0003 days per year, or one day every 3,333.3 years.
The Gregorian calendar came into use in Roman Catholic countries in October 1582 when the seasons were brought back into step by eliminating 10 days from the calendar then in use. Thursday, October 4, was followed by Friday, October 15 (which caused some apprehension and anxiety amongst the people, especially those with birthdays on the eliminated dates!).
Britain and its colonies did not introduce the Gregorian calendar until September 1752 by which time an additional one-day correction was required (actually, {1752 - 1582} x 0.0078 = 1.33 day). Some British documents from the period before the British reform actually contain two dates, an old and a new.
Hijri
The Hijri calendars are purely Lunar and hence the year is also lunar and not tropical, measuring 354 days and 355 days during the leap year. An extra day is added to the last month 11 times in a block of 30 years. Usually 2nd,5th, 7th, 10th, 13th,15th,18th,21st, 24th, 29th year in the cycle of 30 years. In the course of 33 years, the beginning of the Hijri year runs through the whole course of seasons. The months and seasons are not in sync.
This practice of adjustment varies in different countries and sometimes at different periods in the same country.
This Hijri calendar is not tropical.
Akbar tried to adjust the Hijri calendar with the seasons and termed it "The Fasli year" (To synchronise the month with harvest/फसल time)used only for commercial purposes.
This facilitated in collection of harvest tax. (Harvest time and Hijri month varied every year ). The commencement of this year was in July. It was more or less a mix of the " Vilayati Year (used in some parts of Bengal), Amli Era of Odisha and Bengali San" with Hijri month names.
In most parts of India, it was a Solar year, however different customs of different parts of India caused a divergence of reckoning. It was later discontinued.
The original Hijri calendar was followed for all other aspects.
Hindu
In contrast just see our Hindu calendars, where the months are Lunar and the year is Solar. In fact, the Hindu calendar is lunisolar. The 5-yearयुग and intercalation system of adjustment is followed to keep in step with seasons and the actual position along the ecliptic.
One month, known as अधिक मास is added every 30 months, therefore in a युग of 5 years 59 days get added, thus the synchronisation with the seasons is nearly real-time. (My blogs on the Vedic period give out details)
This was available in the Vedic period, earlier than 1500 BCE, even before the Europeans could think of it!
