We define day as the time taken by Earth to complete one rotation around its axis.
But with respect to what is the measurement made? 2/n
Earth rotates in an oval orbit around sun. And moves faster at Perihelion and slower at Apihelion. This changes the duration of day as measured w.r.t Sun (Solar day) through the year....
A Tithi is exact measurement of moon in sky by 12 degrees. And due to equation of time, it varies in time but each lunar month is precisely 30 tithis of 12 degrees = 360 degrees
But is precisely always 12 degrees. Synchronized at Amavasya.
This is the auspicious Hindu month called - Aadhik Maas dedicated to dharma, teerthas. 9/n
Ancient Hindu calendars had it inbuilt in it. The End.