With reference to an article I had read on the internet, here is why the lunar day changes, and thereby tithis overlap.
The Hindu calendar is a lunar calendar, days are based on the movement of the moon. The length of the lunar day VARIES every day, and it makes things complex.
One lunar day is the time taken when there is a 12 degree change in the angle subtended by sun (S) and the moon (M) on the earth (E), i.e. SEM. So, When the SEM is 0° (when sun, moon, and earth are collinear, with the moon in between), the tithi is poornima (full moon). On the next tithi, the moon has moved a little bit further in its orbit around earth, enough to make SEM = 12°. The angle makes it slightly difficult for us to see the full face of the moon, as the moon does not have light of its own. The tithi is "ekam", the paksha is krishna paksha (krishna = dark). The waning of the moon continues. Gradually, we reach a stage when SEM = 180°, and the moon is nowhere between the sun and earth. The tithi is amavasya (new moon), when we can't see the moon at all. Now begins the waxing of the moon (shukla paksha), and it approaches back towards the full moon stage. The whole cycle takes 30 days, a maas (month).
The thing to be noted here is, the 12 degrees is the angle subtended at the earth by the moon and sun! Had it been the change in angular position of the moon in its orbit around the earth (i.e. M¹EM², where M¹ is the position of moon on Day 1 and M² is the position of moon on Day 2), the length of a tithi would have been the same (because the moon would take the same time everyday to traverse 12° in its orbit (constant angular speed). However, it becomes very complex because the tithi changes upon a 12 degree change in the angle SEM (with S fixed while E and M moving in their own independent speeds and orbits), and at every turn of the tithi, the earth is at a new position with respect to the sun, as the earth has also been rotating and revolving all the time!
All these complexities mean that the length of the lunar day (tithi) is not constant, and can vary somewhere between around 19 hours to 26 hours. The solar day always begins at midnight, but the lunar tithi can begin at any time of the day, and moreover, can have any length between 19 to 26 hours.
For convenience, calendars show a solar date's tithi as the tithi at the time of sunrise. So, 1) if a tithi begins after sunrise on a certain solar date and ends before sunrise on the next date, it is not shown on calendars, and you think that a day is missing. 2) And when a tithi begins before sunrise on a certain solar date and ends even after sunrise on the next date, the same tithi is shown for both dates in calendars, and you think a day has been repeated.
This is the reason why the Durga Puja Tithis vary. This year , in October 2015, we have Sasthi on 19th, Saptami on 20th, Ashtami on 21st and Navami and Dashami - both on 22nd.
The Hindu calendar is a lunar calendar, days are based on the movement of the moon. The length of the lunar day VARIES every day, and it makes things complex.
One lunar day is the time taken when there is a 12 degree change in the angle subtended by sun (S) and the moon (M) on the earth (E), i.e. SEM. So, When the SEM is 0° (when sun, moon, and earth are collinear, with the moon in between), the tithi is poornima (full moon). On the next tithi, the moon has moved a little bit further in its orbit around earth, enough to make SEM = 12°. The angle makes it slightly difficult for us to see the full face of the moon, as the moon does not have light of its own. The tithi is "ekam", the paksha is krishna paksha (krishna = dark). The waning of the moon continues. Gradually, we reach a stage when SEM = 180°, and the moon is nowhere between the sun and earth. The tithi is amavasya (new moon), when we can't see the moon at all. Now begins the waxing of the moon (shukla paksha), and it approaches back towards the full moon stage. The whole cycle takes 30 days, a maas (month).
The thing to be noted here is, the 12 degrees is the angle subtended at the earth by the moon and sun! Had it been the change in angular position of the moon in its orbit around the earth (i.e. M¹EM², where M¹ is the position of moon on Day 1 and M² is the position of moon on Day 2), the length of a tithi would have been the same (because the moon would take the same time everyday to traverse 12° in its orbit (constant angular speed). However, it becomes very complex because the tithi changes upon a 12 degree change in the angle SEM (with S fixed while E and M moving in their own independent speeds and orbits), and at every turn of the tithi, the earth is at a new position with respect to the sun, as the earth has also been rotating and revolving all the time!
All these complexities mean that the length of the lunar day (tithi) is not constant, and can vary somewhere between around 19 hours to 26 hours. The solar day always begins at midnight, but the lunar tithi can begin at any time of the day, and moreover, can have any length between 19 to 26 hours.
For convenience, calendars show a solar date's tithi as the tithi at the time of sunrise. So, 1) if a tithi begins after sunrise on a certain solar date and ends before sunrise on the next date, it is not shown on calendars, and you think that a day is missing. 2) And when a tithi begins before sunrise on a certain solar date and ends even after sunrise on the next date, the same tithi is shown for both dates in calendars, and you think a day has been repeated.
This is the reason why the Durga Puja Tithis vary. This year , in October 2015, we have Sasthi on 19th, Saptami on 20th, Ashtami on 21st and Navami and Dashami - both on 22nd.
very nice post! Really it's worth reading maa Duraga always helps her devotees in need, at Home I used to chant durga mantra daily once.
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