The term "11 sep tithi" functions as a noun phrase. In this construction, "11 sep" (September 11th) acts as a specific adjectival modifier, and "tithi" is the core noun. A tithi is a fundamental unit of time in the Hindu lunisolar calendar, representing a lunar day. Therefore, the phrase refers to the specific lunar day that corresponds to the Gregorian calendar date of September 11th for a particular year.
A tithi is defined by the time it takes for the longitudinal angle between the Moon and the Sun to increase by 12 degrees. A lunar month contains 30 tithis, which are divided into two fortnights (pakshas): the waxing phase (Shukla Paksha) and the waning phase (Krishna Paksha). Because the Gregorian calendar is solar-based and the Hindu calendar is lunisolar, the specific tithi corresponding to September 11th is not fixed; it varies each year. The calculation requires a Panchang, a Hindu astrological almanac, which aligns the lunar and solar cycles for a specific location and year. Furthermore, a tithi's duration can vary from approximately 19 to 26 hours, meaning a single Gregorian date can sometimes encompass parts of two different tithis.
The practical application of determining this specific lunar date is primarily for religious and cultural observances. Individuals use this information to identify the correct day for festivals, fasts (vrats), and ancestral remembrance ceremonies like Shraddha, which are scheduled according to the lunar calendar. Ascertaining the tithi for September 11th of any given year is a necessary step for anyone needing to perform rituals or celebrate events tied to the Hindu lunar cycle on that specific solar date.
