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The astronomical basis of the Hindu lunar day

In Vedic timekeeping, a tithi is a "duration of two faces of moon that is observed from earth", known as milа̄lyа̄ (Newar: 𑐩𑐶𑐮𑐵𑐮𑑂𑐫𑐵𑑅, मिलाल्याः) in Nepal Bhasa,^[1] or the time it takes for the longitudinal angle between the Moon and the Sun to increase by 12°. In other words, a tithi is a time duration between the consecutive epochs that correspond to when the longitudinal angle between the Sun and the Moon is an integer multiple of 12°. Tithis begin at varying times of day and vary in duration approximately from 19 to 26 hours.^[2] Every day of a lunar month is called tithi.

Panchanga

A Hindu muhurta (forty-eight minute duration) can be represented in five attributes of Hindu astronomy namely, vara the weekday, tithi, nakshatra the Moon's asterism, yoga the angular relationship between Sun and Moon and karana half of tithi.^[3]

Tithi plays an important role along with nakshatra in Hindus' daily as well as special activities in selecting the muhurta. There are auspicious tithis as well as inauspicious tithis, each considered more propitious for some purposes than for other.^[4]

There are 30 tithis in each lunar month, named as:

Sl.No	Krishna paksha (dark fortnight)	Shukla paksha (bright fortnight)	Deity and properties^{[citation needed]}
1	Prathama / Padyami	Prathama / Padyami	The presiding deity of the first lunar day is Agni and it is good for all types of auspicious and religious ceremonies.
2	Dwitiya / Vidiya	Dwitiya / Vidiya	Vidhatr or Brahma rules this lunar day and is good for the laying of foundations for buildings and other things of a permanent nature.
3	Tritiya / Thadiya	Tritiya / Thadiya	Gauri is the lord of this day and is good for the cutting of one's hair and nails and shaving.
4	Chaviti	Chaviti	Yama/Ganapati is lord of the 4th lunar day, which is good for the destruction of one's enemies, the removal of obstacles, and acts of combat.
5	Panchami	Panchami	The Naaga or Serpents rule this day, which is favourable for administering medicine, the purging of poisons, and surgery.
6	Shashthi	Shashthi	Karttikeya presides over this day and is favourable for coronations, meeting new friends, festivities, and enjoyment.
7	Saptami	Saptami	The 7th lunar day is ruled by Surya; one may begin a journey, buy conveyances, and deal with other such things of a movable nature.
8	Ashtami	Ashtami	The Rudra rule this day, which is good for taking up arms, building of one's defenses, and fortification.
9	Navami	Navami	The Ambikaa rules this day, which is suitable for killing enemies, acts of destruction, and violence. Inauspicious for ceremonies and journeys.
10	Dasami	Dashami	The day is ruled by Dharmaraja and is auspicious for acts of virtue, religious functions, spiritual practices, and other pious activities.
11	Ekadasi	Ekadashi	Rudra rule this day; fasting, devotional activities, and remembrance of the Supreme Lord Vishnu are very favourable. This day has special religious significance in Hinduism and Jainism—usually observed by fasting.
12	Dvadasi	Dwadashi	The Vishnu or Aditya rules this day, which is auspicious for religious ceremonies, the lighting of the sacred fire, and the performance of one's duties.
13	Trayodasi	Thrayodashi	The day is ruled by Kamadeva and is good for forming friendships, sensual pleasures, and festivities.
14	Chaturdashi	Chaturdashi	Kali rules this day, suitable for administering poison and calling of elementals and spirits.
15	Amavasya (new moon)	Purnima or Paurnami (full moon)	The Pitru-devas rule the New Moon, suitable for the propitiation of the Manes and performance of austerities. Purnima is ruled by Moon and is suitable for merry making and fire sacrifice.