If we want to construct Panchangam only on the basis of movement of Sun because Moon is Chhaya Grah,how will the panchangam differ from Lunar one
First of all, the moon is not a chhaayaa graha or a shadow planet but a real planet i.e upa-graha or sub planet. Since our earth has only one natural satellite hence the moon is of core importance in astrology. Shadow planets are rAhu and ketu respectively.
Moon is very important like the sun in astrology because its related to many key factors like various dashaas like vimshottari and ashtottari. Also related with rashi of the person which is used for naming the person. Also the moon is used as reference to study the transits or gochara which is key factor for astrological predictions.
Saayana is related to tropical zodiac which is folllwed in western astrology and nirayana or sidereal is follwed in Indian jyotisha. The difference between the two is due to ayanamsha which is about 23 degrees. So if we subtract the above ayanamsha from saayana then we get the nirayana or indian astrological planetary positions.
Western Astrology relies on what is known as the Tropical Zodiac (Sayana cakra), which uses the Vernal Equinox as the point of reference. The Indian system - Jyotish - uses the Sidereal Zodiac (Nirayana cakra), which adopts a fixed star as the point of reference.
The reason for the discrepancy between these two systems is that there is a "wobble" in the rotation of the earth on its axis. As a result, the Sun does not return exactly to the same point of 0 degrees Aries each spring at the vernal equinox as observed from a fixed star. It misses by a tiny amount (around 50 seconds of arc according to the most widely used reference point). This may not seem to matter much, but when it accumulates over the years, the vernal equinox is seen to slip back and back into the sign of Pisces then eventually Aquarius and so on. This phenomenon is known as precession of the Vernal Equinox.
At the present time, the discrepancy between these two systems is about 23 degrees, 57 minutes. This ever-increasing difference is known as the ayanamsha (moving part). If you subtract the daily specific ayanamsha from the planet's position in a Western tropical chart, you will get its location in the Vedic Sidereal Chart.
A Western Astrologer is going to fix the Vernal Equinox as 0 degrees Aries regardless of whether the Sun is actually observed against the backdrop of the stars in the constellation of Aries on that particular day and year. In fact, eventually, there can be the bizarre situation where the Sun is actually observed by astronomers in the sign of Libra and a Western astrologer says it is in Aries - the opposite sign of the zodiac! This may be a contributing factor to the typical modern astronomer's negative opinions about astrology. However, as all astrology is, at its heart, a divination system, the important thing is how well the astrologer integrates any system with the cognitive faculty of intuition.
Jyotish, in turn, always establishes the planetary position against the backdrop of the fixed stars so that real-time observation matches the chart positions. The Sayana cakra or Tropical Zodiac is used in Jyotish to a limited extend for some aspects of calendar calculations. However, the all-important mapping of the heavens for the purposes of divination strictly uses the positions of the planets against the Fixed or Sidereal Zodiac – the Nirayana cakra.
The Indian calender are of two types sauramana panchangam which you are talking about where the months are counted based on the transit of Sun in various signs known as SANKRANTI. It is followed in some of the states.
The second and more popular is chandramaana panchangam which is based on the movement of moon. This is further divided into poornimaanta and amaanta based on new and full moon cycles.
Masa or month: There are two types of Indian calendars. One is the solar calendar (Surya ganana) based on the entry of the sun in to various zodiac signs and the other is the lunar calendar (Chandra ganana) based on the transit of the moon. Our Panchanga displays the lunar calendar which is the is the most widely followed Panchanga calendar in India. There are two types of lunar calendars. One starts with the Kartika month and counted after Poornima or the full moon till next Poornima. Hence it is called the Poornimanta or Kartikadi system. This is popular in North India. The other starts with the month Chaitra and counted after Amavasya or the new moon till the next Amavasya. Hence it is called Amavasyanta or Amananta calendar. This is popular in South India. The names of the months are the same.
The lunar months are counted by Tithis or the phases of the moon. Every 12 degree difference between the sun and the moon forms a Tithi. The lunar Tithi or day is the Tithi prevailing at the time of sunrise is the day’s Tithi. New moon to full moon to new moon completes a lunar month. As the motion of the moon is not uniform, a Tithi may be longer than a day or shorter than a day. A lunar fortnight, or Paksha, can vary from 13 to 15 days because of this. As the moon takes about 29.53 days to complete one round of the earth, the 12 lunar months amount to 354 days approximately. As it is less by 11 days as compared to the solar year, once in 2-3 year an Adhika Masa or extra month is added to the lunar calendar to make it coincide with the solar calendar. The twelve lunar months take 354 days, 8 hours and 34.28 seconds. This creates a difference of 10 days, 21 hours and 35.16 seconds from the actual solar year of 365 days, 6 hours, 9.54 seconds.
When the accumulated difference between the two systems exceeds 29 days, 12 hours, 44 minutes and 2.865 seconds, an adjustment is made with a extra month (Adhika Masa), which carries the name of the previous or the next month, depending on the proximity of the month. This addition of extra month is done when there is no solar ingress in a lunar month, ie, the sun does not at all transit into any new rashi but simply keeps moving within one zodiac sign in a lunar month, then that lunar month will be named according to the first upcoming transit. It will also take the epithet of adhika or “extra”. For example, if a lunar month elapsed without a solar transit and the next transit is into Mesha, then this month without transit is labeled adhika Chaitra. The next month will be labeled according to its transit as usual and will get the epithet nija (“original”) or shuddha (“clean”).