English: The astornomical basis of the Hindu lunar day. Also illustrates Kshaya Tithi (Vaishaka-Krishna-Chaturdashi (i.e. 14th)) and Adhika Tithi (Jyeshta- Shukla-Ekadashi (i.e. 11th))
Each lunar day the Moon increases its angle from the Earth-Sun axis (which itself is moving) by 12 degrees. On Vaishaka-Krishna-Chaturdashi (14th), the Moon covers this distance within a single solar day; hence this day is a Kshaya tithi and is not marked on the lunisolar calendar. On Jyeshta-Shukla-Dashami (10th), the Moon covers this distance over the start of two solar days; hence this days is an Adhika tithi and is marked twice on the lunisolar calendar.
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