So we are done with the requisite astronomical ideas for Calendar system. It is to be reminded again that in this context there are two issues-Solar and Lunar. Solar issues are of two types-Tropical and Sidereal. Most of the official Calendar system (except Islamic Calendar which follows Lunar rules) follows Solar/tropical rules meaning thereby year begins with Sun falling in vernal equinox position when Sun travels over the head in its northward journey, or days length starts to increase, on 22-23 march. We know that this position is ever shifting on Zodiac in clockwise direction, in geocentric view. As per western system the beginning point of Zodiac is accordingly ever shifting (i.e. Aries sign is ever changing its beginning point) with this position, but this hardly matters from the point of view of Calendar system. Indian system follows Luni-Tropical rules meaning thereby though year begins with vernal equinox but months follows Lunar rules of 30 Tithis or 29.5 days as an average. (We have discussed these issues at length in the Part IV of this article.) Gregorian calendar system begins with 1st Jan but officially again vernal equinox is taken to be beginning of year or financial year, though initially there was only tropical rule. Two months namely Jan and Feb are later stage additions in Julian calendar system. In all systems year consists of 12 months, but months are of varying length. Length of year varies as per Tropical or Lunar nature of the Calendar system. Sidereal system which begins when Sun enters into Aries sign of fixed Zodiac, has been abandoned in favor of tropical system in all official systems being followed. So, sidereal system is almost reserved for Astrological and Religious purposes in oriental cultures like India. In analyzing the movements of Sun and Moon there are couples of remaining issues like Analema and Cassini law, but those are of not any significance in Calendar system. So we are omitting those.
So now let us discuss one by one the different Calendar systems.
- Western (English speaking countries) systems Roman/Julian/Christian/Gregorian Systems. Obviously each of the four is successor, extension or upgrade, of the previous one.
Roman: It is the oldest known Calendar system of Europe. It consisted of only 10 months, total 304 days, starting from 1 March as the first month and ending on 31 December. Remaining 61 days were not counted because of the rigor of winter. In the year 673 BC Roman King Numa Pompilius added two more months, namely Jan and Feb each of 31 days, at the end of December. The word January has been derived from God Janus which looks/faces both ways. So January became the first month. (We are omitting the length of different months for ease and clarity). The total number of days in a year used to come only 355 days like in Lunar year. This naturally was not in tune with season, so after every 2 or 3 years, 22 days more were added at the end of February as intercalary month, just as it is also done in Lunar system. Later on Julius Caesare corrected the system further. This is because lots of confusions were arising because of intercalation. The year started about 90 days before the proper season.
Julian: As discussed Julius Caesar, the first King of old Roman Empire, modified Roman calendar and introduced Julian calendar starting from 1 Jan 45 BC. Originally Caesar wanted it to begin it on what is presently known as 25th December 46BC, that was winter solstice. But people opinion went in favor of the lucky moon day falling on 1 Jan 45BC. The first year was as usual 355 days but divided among 12 months. Due to certain technical difficulties of intercalation the ensuing year, 44 BC, was the year of great confusion. It consisted of 445 days. Then the things were set right. Then it was reformed as consisting of average 365.25 days, 365 days in ordinary year and 366 days in leap years, starting from 1 Jan. The 7th month used to be known as Quintilus in Roman calendar. In Roman calendar it was 5th month from the beginning of year that used to be 1 March. In honor of Caesar it was renamed as July, as 7th month of the Julian system. Later on in 8th BC the next month known as Sextilus (as per Roman calendar 6th month), was also renamed as August in honor of King Augusts, the successor of Caesar. Now question is why February consists of 28/29 days. There goes a story, though not authentic, that earlier February, July and August all consisted of 30 days. But when 1 day was added to July and August each in honor of the kings as mentioned above, 2 days has to be deducted from February. Why February? This is because tropical calendars begin on Vernal Equinox falling in the month of March, which also used to be beginning of Roman Calendar. So February practically used to be last month of year.
Christian/Gregorian: This is almost same as Julian Calendar. This was introduced in the year 530 AD. The beginning of the era is regarded to be birth date of Christ, December 25th of 45 Julian Year. This is the beginning of 1st AD. But later on it was discovered from an inscription that Christ lived at the time King Herold who ruled in 4BC. The year is tropical like Julian consisting of 365.25 days. This calendar was further modified by John Paul Gregory IV, the chief Roman Pope, in 1532 AD and came to be known as Gregory Calendar. Actually Gregory discovered that tropical year of Christian/Julian calendar was longer than the actual tropical year(365.2422 days) by .0078 days. Hence the Christian Calendar was losing touch with Christmas and Easter day. He added 10 days to 5th October 1532 AD and made it as 15 October. For this purpose he made the provision for omitting 1 leap year after every 400 years so that century divisible by 400 was not to be counted as leap year, but those divisible by 4 are leap years of 366 days. The length of year was fixed as 365.2425 days as an average. This still leaves an error of 1 day in 3300 years. Since then this system is being followed all over Europe and Catholic states.
In this system days are counted from midnight to midnight, not from sunrise. Number of days in different months in sequence like that in Chritian/Julian Calendar were modified what is being followed till today. Britain accepted the calendar in the year 1752 AD by taking the starting date September 3 as September 14(I don’t know the reason). But Greek Orthodox Church did not accept this Gregory Calendar. In 20th century this Calendar system was accepted almost all over world.
Next part of this post will be devoted to Oriental Calendars , Hindu and Islamic……….