Astronomical basis of the Hindu calendar

[[File:Uttarayana.webm|thumb|Animation that illustrates uttarāyaṇa and dakṣiṇāyana.]]

[[File:Uttarayana.webm|thumb|Animation that illustrates uttarāyaṇa and dakṣiṇāyana.]]

[[File:Devayana.webm|thumb|upright|Animation that illustrates devayāna and pitṛyāṇa.]]

[[File:Devayana.webm|thumb|Animation that illustrates devayāna and pitṛyāṇa.]]

{{Anchor|uttarayana}}”'<big>uttarāyaṇa, dakṣiṇāyana</big>”’

{{Anchor|uttarayana}}”'<big>uttarāyaṇa, dakṣiṇāyana</big>”’

devayāna and pitṛyāṇa are not in active calendric use any longer but form the basis for [[Astronomical basis of the Hindu calendar#pitrpaksha|pitṛpakṣa]].

devayāna and pitṛyāṇa are not in active calendric use any longer but form the basis for [[Astronomical basis of the Hindu calendar#pitrpaksha|pitṛpakṣa]].

[[File:Rthu_(Seasons).webm|thumb|upright|Animation that illustrates the six ṛtu.]]

[[File:Rthu_(Seasons).webm|thumb|Animation that illustrates the six ṛtu.]]

{{Anchor|rthu}}”'<big>ṛtu</big>”'{{refn|The Surya Siddhanta defines ṛtu in terms of various rāśi<ref>Burgess 1935, pp.313 (XIV.10)</ref> assuming that makara ”saṅkramaṇa” coincides with the winter solstice.<ref>Burgess 1935, p.207</ref> Due to the [[Axial precession|precession of the equinoxes]], that assumption is no longer true and hence those definitions of ṛtu are no longer accurate.|group=”note”}}

{{Anchor|rthu}}”'<big>ṛtu</big>”'{{refn|The Surya Siddhanta defines ṛtu in terms of various rāśi<ref>Burgess 1935, pp.313 (XIV.10)</ref> assuming that makara ”saṅkramaṇa” coincides with the winter solstice.<ref>Burgess 1935, p.207</ref> Due to the [[Axial precession|precession of the equinoxes]], that assumption is no longer true and hence those definitions of ṛtu are no longer accurate.|group=”note”}}

To illustrate: consider the Gregorian date [https://www.drikpanchang.com/panchang/month-panchang.html?date=18/09/2021 18th Sep 2021]. Instead of referring to it as “2nd dina of kanyā masa” Hindus will refer to it as ” bhādrapada māsa, śukla pakṣa, dvādaśī tithi”, which is the tithi prevailing at sunrise on that sāvana dina. 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.

To illustrate: consider the Gregorian date [https://www.drikpanchang.com/panchang/month-panchang.html?date=18/09/2021 18th Sep 2021]. Instead of referring to it as “2nd dina of kanyā masa” Hindus will refer to it as ” bhādrapada māsa, śukla pakṣa, dvādaśī tithi”, which is the tithi prevailing at sunrise on that sāvana dina. 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.

[[File:Adhika Tithi.webm|thumb|upright|Animation that illustrates adhika tithi]]

[[File:Adhika Tithi.webm|thumb|Animation that illustrates adhika tithi]]

[[File:Kshaya Tithi.webm|thumb|upright|Animation that illustrates kṣaya tithi]]

[[File:Kshaya Tithi.webm|thumb|Animation that illustrates kṣaya tithi]]

{{Anchor|adhikatithi}}”'<big>adhika tithi and kṣaya tithi</big>”’

{{Anchor|adhikatithi}}”'<big>adhika tithi and kṣaya tithi</big>”’