The Lunar Day  
   
  Thithis are based on the distance of Moon from Sun. Sun’s orbital path is the ecliptic. Other planets have different orbits. When Moon is in conjunction with Sun (we are aware that the paths of all planets are almost identical with that of Sun), which means they are at the same point of the zodiac, New Moon occurs. When the luminaries are 180 degrees away, it is Full Moon. In short, if Moon is approaching the Sun, it is waning. If it is moving away from the Sun, it is waxing.

To calculate the lunar day:

  There are 15 lunar days in each Sulapaksha and KrishnaPaksha.

Find the distance of Moon from Sun. If Moon’s longitude is more than 180 degrees, subtract 180. Divide the distance by 12 and the quotient + 1 is the number of the Lunar Day.

For instance, if Sun is in Virgo 22 and Moon is in Aquarius 14, Moon is 142 degrees away from Sun (314 – 172). When we divide 142 by 12, the quotient is 11. So, it is the 12th lunar day, called Dhwadhasi (11 + 1).

If Sun is in Scorpio 12 and Moon is in Leo 16, the distance of Moon from Sun is 276. 274 – 180 = 94. Dividing 94 by 12, we get the quotient 7. Therefore, the thithi is 7 + 1 = 8, Ashtami.

Importance of the lunar day:

  Lunar day is considered for Muhurtha or fixing time for auspicious events. Certain lunar days are auspicious and others are bad depending on the event. Generally, Prathama, Ashtami and Navami are avoided for all occasions. In North India, Navami is not considered as bad.
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