Month and Year

Say you are in a train standing in platform and that if you look through the window you could see only another train standing in the adjacent track. When any of the train moves, you cannot come to a conclusion whether your train or the adjacent one is moving. Only by having a look at the ground you could come to a conclusion.

Thus you can determine the position of any object only with respect to some other object. To determine the position of the Sun in the ecliptic, you should have some reference object. This reference object should be fixed when you see from the earth. Stars other than the sun are at a very very large distance from the earth. So we will have these stars as our fixed frame of reference and say that the sun moves in the background of these stars.

Now, it's time to define our larger units of time. The time taken by the sun to move around the earth in this ecliptic path once is defined as a solar year, which is equal to 365.25 days. (i.e. in one solar year the Sun goes around the zodiac once). In other words, if the Sun is very close to a star in the Zodiac, it will come again to the same position in about 365.25 days.

We've seen about days and years. Let's go to some thing in-between. 365 days is a longer unit of time. Let us reduce this by a factor of 12. How do we define our new unit? We shall divide zodiac into twelve equal zones each 360°/12 = 30° wide. Each zone is associated with a zodiacal constellation. You can see the names and shapes of these constellations in the following figure.

A keen reader would have readily observed that these zodiacal constellations do not have the same width or an even spacing! In other words, some constellations of Zodiac may entirely lie within a zone of 30° while some of them may extend beyond a zone. Why is it like that? The stars are randomly distributed in the universe. We try to group them so that we can form an observable pattern, which is easily recognizable. So you cannot except them to be of even width and even spacing.

As a result, the twelve equal zones do not exactly correspond with these constellations.
The Sanskrit names of the twelve zodiacal constellations along with the Greek names and the figures associated with them are as follows:

Even though a constellation may extend beyond a single zone, each of the zones will be prominently occupied by a single constellation. Based on this constellation, each of these zones is assigned a ‘rasi’ or a ‘sign of the zodiac’. The sequence of rasis enumerated above is from west to east. Now, digest these facts. If we divide the ecliptic into twelve zones, you can start from any point. But genearlly, people follow some convention. In the Indian system, the beginning point of the first zone is a fixed point on the ecliptic, which is opposite the star Chitra in the Kanya rasi (179° 59’ away from Chitra). In the Western system, the beginning point of the zodiacal division is known as the first point of Aries which corresponds to the vernal equinox, where the ecliptic intersects the celestial equator. Hence there is a difference between starting point of each zone. The beginning point of the Mesha rasi (in Indian convention) is nearly 23°45’ east of the first point of Aries (Western convention).

Based on the Rasis, we define Solar month as the time taken by the sun to cover one rasi. Click here to view the names of Solar Months in different parts of India and their starting day.
You should have heard about Makara sankranti. Solar sankranti is said to occur when the sun is at the beginning point of a rasi. So, when the sun is at the beginning point of Mesha rasi, it is Mesha sankranti (which is also the beginning of the new solar year in India, and occurs around April 14/15); it is Makara sankranti when the sun has completed its stay in the Dhanur rasi and is at the beginning point of the Makara rasi (which occurs around January 14^th ).

So far we were using positions of the sun to define various units. Similarly, we can use the position of the Moon to define various units. Let us see how the moon can be used to define units of time and try to correlate time units based on the solar and the lunar systems. While the Sun takes 365.25 days to go around the zodiac, the moon takes 27.32166 days. This time period is known as the sidereal period of the moon. Another periodic event with respect to the moon is the time when the Sun and the moon are along the same direction. This, as we've already seen, corresponds to Amavasya. The time between two consecutive Amavasyas is 29.530588 days. This value is closer to a Solar month than the Sidereal period of the moon. Hence we take this value as a Lunar Month or Chandra Maasa. This value of 29.530588 days can be arrived at as follows: The sun travels at nearly 0.99^o per day along the ecliptic whereas the moon travels at the rate of 13.17^o per day. Hence the moon overtakes the sun by 12.18^o in one day  24.36^o in two days and so on. Thus after 360/12.8 days (nearly 29 1/2  days) the difference between the Sun and the Moon would have been 360^o, and so the Sun and the Moon would be again in the same direction as seen from the earth to give the next Amavasya. At Amavasya, the sun and the moon are said to be in conjunction. To put it in a different way,  Amavasya repeats in 29 days, 12 hours and 44 minutes.  Click here to view the names of the Chandramaasas.

Using the Lunar months, we can define a  lunar year as the duration of 12 lunar months. This value comes to 354.3670 days.

Like rasi division in the solar system, here we have the nakshatra division. But there we chose 12 as a factor to divide the zodiac because 365 is quite a large number. But here we shall use 27. Thus zodiac is divided into 27 divisions and each one is called as a Nakshatra division. A nakshatra is equal to 1/27^th part of the zodiac, that is, each nakshatra division has a width of 360°/27 = 13°20'. This means that the moon takes slightly more than a day, on the average, to cover a nakshatra division. In fact, this is why the zodiac is divided into 27 nakshatra divisions. You are already familiar with the names of the 27 nakshatras. The beginning point of the first nakshatra namely Aswini is the same as the beginning point of the Mesha rasi. Click here to see the names of other nakshatras

A solar nakshtra at a particular moment is the nakshatra division in which the sun is situated at that moment. Similarly, a lunar nakshatra tells you where the moon is situated at that moment. For example, at 6 a.m IST on January 1^st, 1992, the solar nakshatra was Poorvashadha and the lunar nakshatra was Visakha.

Thus we have lunar and solar systems based on the moon and the sun respectively to keep track of time. Now we shall see how to bring correlation between these two systems. Obviously they are not the same, as 12 solar months has 365~ days and 12 lunar months would make only 354~ days. If we define a lunar year to be made up of 12 lunar months always, the lunar year will keep on lagging behind the solar year at the rate of 10.8893 days per year. Since the lunar year is based on the position of the moon there will be no correlation between the seasons and the lunar year. To avoid this, whenever the Lunar year lags behind the Solar year by more than 29~ days,  an additional month called adhika maasa is added to a lunar year. As the lunar year is being corrected to keep pace with the solar year, this system is known as the luni-solar system. In the following we will discuss the relation between the solar and lunar years and the procedure for introducing the Adhika maasa and also Kshaya maasa ! .

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