The Hindu calendar defines nine measures of time (Sanskrit: मान IAST:māna):[2] Of these, only the last four are in active use[3] and are explained here.
The candra māna (Sanskrit: चन्द्र मान) of the Hindu calendar is defined based on the movement of the Moon around the Earth.
A candra māna varṣa or lunar year is made up of 12 consecutive candramāsa.
[6] In other words a tithi is the time taken for the Moon's elongation (on the ecliptic plane) to increase by 12°.
[7] The saura māna (Sanskrit: सौर मान) of the Hindu calendar is defined by the movement of the Earth around the Sun.
A saura māna varṣa or sidereal year is the time taken by the Sun to orbit the Earth once and return to the starting point with respect to the fixed stars.
The starting point is taken to be the position of the Sun when it is in opposition to Spica (Sanskrit: चित्रा, romanized: citrā).
A ṛtu (Sanskrit: ऋतु)[note 5] is the time taken by the Sun to move sixty degrees on its orbit around the Earth.
The six ṛtu of the year are known as Nākṣatra māna (Sanskrit: नाक्षत्र मान) is defined with respect to the fixed stars, so all elements are sidereal in nature.
A dina (Sanskrit: दिन) is the time taken by the celestial sphere to complete one sidereal rotation around the Earth.
[17][note 7] In reality, this movement is caused by the diurnal rotation of the Earth on its axis.
[18] Sāvana māna (Sanskrit: सावन मान) of the Hindu calendar defines civil time.
Apart from the four māna explained above, the concept of nakṣatra is an important characteristic of the Hindu calendar.
This term has multiple meanings:[20] The four māna explained above are used in combination in the Hindu calendar.
In order to synchronise these two parts of the calendar, an additional cāndramāsa is introduced into some cāndra māna varṣa.
[4][note 10] Human life is regulated by the rising of the Sun and not by the movement of the Moon through a 12° arc.
Even though the Moon moves into the trayodaśī arc soon after sunrise (at 6:54AM), that entire sāvana dina is considered to be dvādaśī tithi.