Q: Why do we need a calendar?
To watch and follow day to day life, climatic conditions, happenings throughout the world, calender is needed to specify and to remember to correlate for futures.
Yes. Good reasoning. The first calendar was developed to mark the seasonal changes so that right activities can be done in agriculture without missing the season. Again this happened due to the realization that the time is cyclical and astronomical phenomenon such as seasons recur in a cyclical manner.
Again they needed to know how much time has passed from a specific event, which is not possible without a calendar. When the civilizations developed, they need to keep track of such events. Anyway, since they were more tuned to the nature, they could see the patterns in seasonal variation such as heavy rains or light rain, hotter summer/ warmer summer etc and they needed a measure of time to mark these patterns with respect to it. When civilizations developed even further, they started keep track of phenomenon such as Eclipses, sighting meteors etc...
Q: What is the basis for creation of a calendar?
Calender creation is based on the movements mostly on Sun and Moon.
This is a narrow understanding. We can make calendars for anything which is cyclical. Hence there are various calendars in addition to those which keep track of Sun, Moon, Sun+Moon; such as one which keeps track of eclipses called metonic cycle which recur after 18 years. So basically anything which is cyclical can have a calendar.
Since calendar is used to keep track of astronomical events, there should be sound astronomical basis for creation of a calendar. Every calendar has a define start and end after which a new cycle begins. The start and end of the calendar should also mark with an astronomical event, else it becomes a arbitrary start or end point and thus becomes subjective, whereby different people can have their own start and end.
Q: How many kinds of calendar (around the world) do you know?
To my knowledge, there are two types of calenders among a lot around the
1. based on sun transit into the Aries.(Sowra)
2. based on Moon transit in the Chitrai Nakshtra (Santhana)
There are two widely used calendar used for measure of solar motion, one is for keeping track of season and the other one is for keeping track of Sun for astrological purpose. The calendar which is used for keeping track of Season is called Tropical calendar and year is called tropical year, while the one which is used to keep track of Sun's placement in the nakshatras, which is used for astrological purposes is called the sidereal calendar and the year is called the sidereal year.
Now take up the tropical calendar. Since its purpose is to keep track of Seasons, it should be synchronized with the equinoxes and solstices; which means the equinoxes and solstices should always fall in the same days every year. Why this should be so? This is because, the equinoxes and solstices decide the start of all the seasons on earth. Now you should be able to tell, what should be the correct start of the calendar, so that each year; beginning of the year is marked with such an event and we know that the calendar is not out of track when compared to the seasons. So when should the calendar start. Logically it can start when Sun is at the beginning of Sign Aries or Libra which marks the equinoxes or it can start when Sun is at the beginning of Capricorn or Cancer, which marks the solstices. But for whatever reasons, the tropical calendar starts when Sun is at 9Cp55; which is just a arbitrary position.
Now lets take up the tropical calendar. Since its purpose it to track position of Sun in the Nakshatras, it should be synchronized with the movement of the Sun in the zodiac (nakshatra belt). If we do this, then at the same part of the year, the same nakshatra will be seen on the sky in a certain position in every year. For example if we follow strict sidereal calendar, then Aswini 0 degree will rise with Sun in the eastern horizon, beginning of every year. Since the length of the sidereal year is "almost" the same, the tropical year can be used to mark the rising time of the nakshtras over the sky at any part of the year. However, since the position of equinox is preceeding when seen with respect to the nakshatra belt, minor correction needs to be applied to the calendar, which is known as Ayanamsa Correction. Over time, the ayanamsa correction has accumulated to around 23 degrees. There was a time sometimes near 285 a.d., when the vernal equinox was at the same position as the beginning of the Ashvini Nakshatra, it was the time when the tropical and sidereal zodiac was perfectly synchronized. However now, due to the precision, the equinox actually falls at around 6 Pi 45. Have you wondered what should be the length of the day based for the tropical year? Since we are interested in the tracking the movement of the Sun w.r.t the Nakshatras, each day in this calendar is 1 degree motion of Sun in the sidereal zodiac. We can call this as the sidereal day and end of 360 such days we can reach the end of the year which is marked by the last degree of Revati so that the next degree will marks the 1st day of the next sidereal year. Now you should know, why in the nakshatra based dasa, the length of the year should be 360 solar days and not 360 civil days or 365 civil days (of 24 hrs each). The day length of a sidereal day is a bit more than the tropical day. Hence if we use tropical days in a dasa system, a correction needs to be applied at various places in the dasa duration.
Over the ages various civilizations used various calendars based on their need to track various astronomical phenomenon. It is interesting to study them and it can give insights into their civilizations.
In India we also use the soli-lunar calendar, which is based on Tithis. We need the tithis for observances of various festivals. Since this is based on the both Moon and Sun, the feminine and masculine principles of nature, this calendar is extremely important. Also since this year is also dependent on the Sun, the year length is same as Sidereal Solar year (the name of the month is based on the position of the Sun in that month) and the day length is the one tithi each (12 degree separation of Moon from the Sun). If we consider the soli-lunar year has 12 months like the solar year, then 1year in this calendar is around 11 days shorter than the tropical year. Thus every three years, a correction of 1 extra month is applied to synchronise this calendar with the tropical calendar, which is known as Mal Masa. Since this month is used only for adjusting purposes, this cannot be used for observances of any festivals and the original calendar will still take precedence. When we talk of start of month in this calendar, it can be mapped with either Amavasya or Purnimasya. There are two groups in India who follow either of them, the first one is called Amanta and the 2nd one is called Shuklanta. It seems like Amanta month is corruption which crept in with passage of time. It is suggested by great astronomer and astrologer Varahamihira to follow Shuklanta Calendar. This is also evident from the names of the month, which is derived from the approximate position of the Moon during the start of the month. Thus the first month Chaitra begins when Moon is in Chirtra Nakshatra. This can happen when Moon is placed in opposition to Sun at the beginning of the year; which shows that the name is derived from the Shuklanta principle (end of sukla paksha or bright half). Think on this and you will understand more.
There are so many more calendars which I will discuss when time permits.