Panchang is a Hindu calendar and almanac, which follows traditional units of Indian timekeeping, and presents important dates and their calculations in a tabulated form. The word Panchang is basically made up of 2 words
- Panch (meaning Five)
- Ang (meaning Parts)
So, the word 'Panchang' basically refers to a system of calculating and keeping a date into 5 parts. These 5 parts that make up a particular date are:
The Thithi is calculated with help of position of Sun and Moon in transit.
Tithi = (Moon Longitude - Sun Longitude) / 12 .
If the calculated result is greater than 15, we have to subtract 15 from the result. The net result is Tithi.
TITHI VRIDHI is the tithi present at Sunrise on first day anas well as on the second day.
Seven days of the week are Sunday, Monday, Tuesday, Wednesday, Thursday, Friday & Saturday. In Hindi, they are referred as Ravivaar, Somvaar, Mangalvaar, Buddhvaar, Brihaspativaar/Guruvaar, Shukravaar and Shanivaar resp.
The 360 degrees zodiac is divided into 27 parts to acheive a single nakshatra i.e. of 13 degrees 20 min. There are 27 Nakshatras, Every nakshatra is further divided into four periods (CHARAN) i.e. 3 degree 20 min. A Rashi consists of Two and a quarter of the Nakshatras.
Calculation of NAKSHATRA
Nakshatra = Moon Longitude / 13 deg. 20 min
There are 27 Yogas named as Vishkumbh, Preeti, Aayushman, Saubhagya, Shobhan, Atigand, Sukarma, Dhriti, Shool, Gand, Vridhi, Dhruv, Vyaghat, Harshan, Vajra, Sidhi, Vyatipat, Variyaan, Parigh, Shiv, Sidh, Sadhya, Shubh, Shukla, Brahm, Ainder and Vaidhriti.
Calculation of YOG :
Yog = (Moon Longitude + Sun Longitude) / 13 deg. 20 min
One tithi consists of two karans. there are total 11 Karan, among them seven Char karans are named as Vav, Valav, Kaulav, Taitil, Gar, Vanij, Vishti. Four are fixed Karans named as Shakuni, Chatushpad, Naag kinstughan.
Calculation of KARAN
Karan = (Tithi) / 2
Btw, if you want to read the in depth explanation of tithi calculation along with calculations regarding solar/lunar eclipses and why these factors are used you can read through this PDF document.