A note on Calendar Systems of the world
Overall there are 9 types of major Calendar systems as followed in different parts of the world, as enumerated here below. The first four of those represent the different stages in the evolution of Calendar Systems of Europe. The remaining ones are oriental systems. I would avoid quoting historical sources unless required in the context of explaining the concepts.
1) Roman Calendar-European
2) Julian Calendar-European
3) Christian Calendar-European
4) Gregorian Calendar-European
5) Shaka Calendar-Indian
6) Vikarmi Samvat-Indian
7) Islamic Calendar
8) NanakShahi-Punjabi Calendar
9) Chinese Calendar
But first of all let us have a brief idea of the following concepts as related to reckoning of time in term of the events of Day, Night, Week, Month, and Year. I would avoid discussing the concepts of Yuga or Aeon because those are based on merely conventions ( except that of precessional arc of around 26000 years), and have no scientific rationale behind them and differs from one part of the world to the other. Then as per convenience of standardization & implementation, time is also classified as Local, Standard and Sidereal. We will come to these issue in subsequent sections.
One thing more, that our discussion will be primarily Geocentric in nature, meaning there by assuming that Earth lies at the center of Solar system, as opposed to heliocentric view. As per Heliocentric view Sun is center of our Solar system. This should be evident from the diagram as enclosed here below. Then there is concept of Celestial Equator, Ecliptic and Zodiacal belt. As discussed earlier Celestial equator (Celestial sphere) represents the equator of earth extended into space i.e. celestial sky. Ecliptic is basically the orbit of earth around Sun but in geocentric view this stands for Sun’s path around Earth. Then Zodiacal belt represents the area around Ecliptic ,spanning by 9 degree to both the side ,up and down, through which a user is likely to look at the belt of 27 asterisms( also called constellations), which go to constitute Zodiacal belt. So, any view of of the 9 planets moving around Earth(in Geocentric view) is in line of sight with asterism and the planet is said to be occupying that sign at that particular time. Three asterisms go to constitute one sign of Astronomy/Astrology. The orbits of all the 9 planets lie within this range of this Zodiacal belt. So, zodiacal belt represents the span of orbits around ecliptic plane in which different planets moves around the earth, as seen from earth. Ecliptic plane is tilted by 23.5 degree to celestial equator. This tilt is gradually diminishing. This tilt is responsible for changing seasons. But for this tilt there would not have been any season. It should be reminded that these concepts are primarily Astronomical matters, which were further used by Astrology. So the different measures of time are divided into following categories.
So as discussed there are three types of solar issues-Year,Month,Day/Night. Division of day and night are different as per local conventions. We will discuss all these concepts one by one. There are two types of Solar Years called Tropical and Sidereal.
Tropical year stand for the time taken by Sun to complete one rotation round the earth starting from the day of vernal equinox which presently falls around 22-23 march. This is, that is the vernal equinox day, is represented by the intersection point of earth’s equator with ecliptic, the path along which Sun moves. This point is not fixed because Tropical system is assumed to be receding back because of earth’s precession. We know that earth’s axis is inclined with normal to ecliptic plane by 23degree 27 minute and this axis precess around normal to ecliptic plane in a precessional arc of around 25920 years. Tropics stands for either of two parallels of latitude about 23.5 degrees to the north and south of the equator( called Tropic of Capricorn and Tropic of Cancer respectively) representing the points farthest north and south at which the sun can shine directly overhead and constituting the boundaries of the Torrid Zone or tropic. Zodiac belt is represented by a zone of 9 degree on the both the side of ecliptic plane. The point of intersection of equator with ecliptic called Vernal Equinox point goes on changing its position due to precession of earth. (Accordingly intersection point of two tropics and ecliptic also goes on changing.) In other words now the beginning point of Zodiacal belt is assumed to be vernal equinox point which is regularly going back (at a rate of 50 second per year presently)in clockwise direction of zodiacal belt as seen from earth, presently falling at 5 degree Pisces (2000 AD). So beginning point of Zodiacal belt at the ever shifting position of vernal equinox is regarded to be first sign of Zodiac that is Aries. Accordingly the beginning points of all signs go on changing. This is followed in western religions and system of Astrology. The point of vernal equinox is called ‘Office of Christ”. The rate of precession is not constant but varying from year to year. So as per tropical system the beginning point of zodiac that is Aries sign goes on shifting back. It is the faith of western world, though logically sounds a little odd. Ever shifting position of the beginning point of Zodiacal belt is regarded to represent microcosmic evolution of Earth which stands for the journey of the evolution of the of mankind as a whole.
The time taken by Sun to start from vernal equinox point and come back to vernal equinox point, or to say one complete rotation round the tropical zodiac, is called 1 tropical year. As because the vernal equinox point in the beginning and end of one complete rotation will be different so the duration of tropical year is a little shorter than that of sidereal year, as followed in oriental systems, which we are going to discuss now.( remember in Geocentric view all planets including Sun appear to be moving in anticlockwise direction. Aries 0 Degree to Aries 0 degree is sidereal year belt in which Sun moves in anticlockwise direction . And around 5 degree Pisces is the beginning point of tropical year belt where Sun starts its journey in reckoning of tropical year. This 5 degree is decreasing every year in clockwise direction( that is to say vernal equinox point is shifting in clockwise direction) so end point of the journey will be a little before 5 degree Pisces. Hence the path will be relatively shorter.) It is to be, however, reminded that in the time of Rig Veda time(6000 BCE) in India too the year observed was tropical, that is to say year used to start from vernal equinox day which was falling in Delta Cancri( Constellation Pushya) during those days. (According to a few scholars it was constellation Pollux 11 or Punrabasu, which is one sign behind Delta Cancri. There was no concept of division of Zodiac in 12 signs at that time. Even latter in the time of one of the most prominent scripture Taitrya Brahmana(2600 BCE) ,while the constellations already stood demarcated as consisting of 800 minute (13 degree 20minute) each, the first constellation in the beginning of year was Alcyone 2 or Krittika, not Arieti or Aswini. (**Though Presently it is Arieti, which is 180 degree opposite to Spica 16 or Chitra) is regarded to be beginning point by both tropical and sidereal system.**) . This is because vernal equinox was lying in Alcyone 2. Meaning thereby any fix point for the beginning of Zodiac was yet to be chosen. The constellation lying at the vernal equinox point was regarded to be beginning of zodiac. Anyway the length of tropical year according to modern astronomy is 365 days 5 hours 48.76 days. This is decreasing by .0001 days in 1600 years as per the varying rate of precession.
There comes the turn of Sidereal year. We will come back to the issue as to why this is called ‘sidereal’ while discussing the concept of Day. Sidereal year is the time taken by Sun to complete one rotation round the fixed Zodiac. This according to modern astronomy consists of 365 days, 6 hours 9.16 minutes. I will avoid quoting the data from Surya Siddhanta because it is more or less same as that given by Modern Astronomy. But Modern astronomy is more reliable for the obvious reason that it is based on the instrumental verification. Fixed zodiac starts at 0 degree Aries or star Arieti. The point to be understood is that in both the systems the beginning point of Zodiac is 0 degree Aries, but in western system 0 degree Aries is not fixed but moving back in anticlockwise direction, whereas in sidereal system it is fixed at a particular location. So presently 5 degree of Pisces of Sidereal system is 0 degree Aries in Tropical system. Sidereal system is used primarily for oriental religions and astrology. For official purpose in all parts of the world Calendar system used are primarily tropical. However as we will discuss ahead Indian official Calendar systems is Luni-tropical and Religious Calendar systems used for Astrological purpose are Luni-sidereal. Both Tropical and Sidereal are solar concepts. We will discuss the Lunar concepts in next post. It should be mentioned that in 285 AD vernal equinox was lying at zero degree Aries which is also the beginning point of fixed sidereal zodiac. So in that particular year there was no difference between Tropical and Sidereal year. Incidentally it was this year when the Astronomical treaty of India called old Surya Siddhanta was written.