In Hindu tradition, Tithi is an important part of the Panchangam (Hindu calendar). It is used to determine many important spiritual days, fasting days, festivals, pujas, and religious observances.
But what exactly is a Tithi? How is it calculated? And why does the Tithi sometimes change during the same calendar day?
Let us understand it in a simple and practical way.
What Is Tithi?
Tithi is a lunar day calculated based on the angular distance between the Sun and the Moon.
In simple terms, the Moon continuously changes its position in relation to the Sun. The angular difference between the Sun and Moon increases as the Moon moves along its orbit.
Whenever this angular difference increases by 12 degrees, one Tithi is completed.
Therefore:
360° ÷ 12° = 30 Tithis
A lunar month consists of 30 Tithis, divided into two halves:
- Shukla Paksha – Waxing Moon
- Krishna Paksha – Waning Moon
Each Paksha contains 15 Tithis.
The 30 Tithis in a Lunar Month
Shukla Paksha – Waxing Moon
The 15 Tithis are:
- Pratipada
- Dwitiya
- Tritiya
- Chaturthi
- Panchami
- Shashti
- Saptami
- Ashtami
- Navami
- Dashami
- Ekadashi
- Dwadashi
- Trayodashi
- Chaturdashi
- Purnima
Purnima marks the full moon day.
Krishna Paksha – Waning Moon
The 15 Tithis are:
- Pratipada
- Dwitiya
- Tritiya
- Chaturthi
- Panchami
- Shashti
- Saptami
- Ashtami
- Navami
- Dashami
- Ekadashi
- Dwadashi
- Trayodashi
- Chaturdashi
- Amavasya
Amavasya marks the new moon day.
How Is Tithi Calculated?
The calculation of Tithi is based on the difference between the geocentric ecliptic longitudes of the Moon and Sun.
The basic formula is:
Tithi = (Moon’s longitude − Sun’s longitude) ÷ 12°
The resulting angular difference determines the current Tithi.
For example, suppose the angular difference between the Sun and Moon is 36°.
36° ÷ 12° = 3
This corresponds to the third Tithi, Tritiya.
Similarly:
| Angular Difference | Tithi |
|---|---|
| 0°–12° | Pratipada |
| 12°–24° | Dwitiya |
| 24°–36° | Tritiya |
| 36°–48° | Chaturthi |
The same pattern continues until the full 360° cycle is completed.
Note: In practical Panchangam calculations, the actual Tithi timing is determined using precise astronomical positions and local time. So the simple formula is useful for understanding the concept, while Panchangam calculations provide the exact start and end times.
How Are Amavasya and Purnima Formed?
Amavasya and Purnima are two important points in the lunar cycle.
Amavasya
Amavasya occurs when the angular difference between the Sun and Moon reaches 0° at the beginning of a new lunar cycle.
At this stage, the illuminated portion of the Moon is not normally visible from Earth.
This is why Amavasya is commonly associated with:
- Ancestor-related rituals
- Tarpanam
- Spiritual practices
- Meditation and prayer
Purnima
Purnima occurs when the angular difference between the Sun and Moon reaches approximately 180°.
At this stage, the illuminated side of the Moon faces Earth, making the full Moon visible.
Purnima is considered significant for various Hindu religious observances, fasting practices, and temple rituals.
Why Is Tithi Important in Hindu Tradition?
Tithi plays an important role in determining many Hindu religious observances.
For example:
- Ekadashi – Traditionally associated with Lord Vishnu worship and fasting
- Pradosham – Associated with Lord Shiva worship
- Sankashti Chaturthi – Observed in connection with Lord Ganesha
- Ashtami – Important for several religious observances
- Amavasya – Significant for ancestor-related rituals
- Purnima – Important for various festivals, prayers, and fasting observances
This is why people often check the Tithi along with the calendar date before planning a religious ceremony or spiritual observance.
Why Does Tithi Change at Different Times?
This is one of the most common questions about Tithi.
A Tithi is not exactly the same as a 24-hour calendar day.
It is based on the changing angular relationship between the Sun and Moon. Since their apparent motions are not uniform relative to one another, the duration of a Tithi can vary.
As a result, a Tithi may:
- Begin early in the morning
- End during the afternoon
- Begin at night
- Continue into the following calendar day
This is why a Panchangam normally gives the start and end time of each Tithi.
Is Tithi the Same as a Calendar Date?
No. Tithi and calendar date are different.
A calendar date is based on a civil calendar system, while Tithi is based on the relative position of the Sun and Moon.
Therefore, one calendar date can contain parts of two different Tithis.
For example, one Tithi may continue during the morning and end in the afternoon, after which the next Tithi begins.
This is also why simply looking at a normal calendar may not be enough to determine the exact Tithi at a particular time.
Why Is Panchangam Used to Find Tithi?
A Panchangam provides detailed information about the traditional Hindu calendar, including:
- Tithi
- Nakshatra
- Yoga
- Karana
- Sunrise and sunset
- Important fasting and festival timings
For religious activities, it is useful to check the exact beginning and ending time of the Tithi rather than relying only on the calendar date.
This becomes especially important when a particular ritual must be performed during a specific Tithi.
Tithi Calculation in Simple Terms
You can remember the basic concept like this:
Sun–Moon angular difference → Every 12° = One Tithi
Therefore:
360° ÷ 12° = 30 Tithis
So:
1 Lunar Month = 30 Tithis
15 Tithis = Shukla Paksha
15 Tithis = Krishna Paksha
This simple calculation helps explain the basic astronomical principle behind the Hindu lunar calendar.
Frequently Asked Questions About Tithi
1. How many Tithis are there in a lunar month?
There are 30 Tithis in a lunar month—15 in Shukla Paksha and 15 in Krishna Paksha.
2. What is Tithi based on?
Tithi is calculated based on the angular distance between the Sun and Moon.
3. How long does one Tithi last?
A Tithi does not have a fixed 24-hour duration. Its actual duration varies depending on the relative motion of the Sun and Moon.
4. Which Tithi is Amavasya?
Amavasya is the 15th Tithi of Krishna Paksha.
5. Which Tithi is Purnima?
Purnima is the 15th Tithi of Shukla Paksha.
6. Can two Tithis occur on the same calendar date?
Yes. One Tithi may end during a calendar day and the next Tithi may begin on the same day.
Conclusion
Tithi is more than just a date mentioned in a Hindu calendar. It is a lunar time measurement based on the changing angular relationship between the Sun and Moon.
Understanding Tithi makes it easier to understand important observances such as Amavasya, Purnima, Ekadashi, Pradosham, and other Hindu religious days.
When planning a puja, fasting, ritual, or spiritual observance, it is helpful to check both the calendar date and the exact Tithi timings in a reliable Panchangam.
Once you understand the simple principle of 12 degrees = one Tithi, the basic concept behind Tithi calculation becomes much easier to understand.