Two panchangams can disagree for six reasons. (1) Location: sunrise, sunset and every window built on them, such as Rahu Kalam, change from city to city. (2) Calculation system: the classical Vakya tables and the modern Drik (Thirukanitha) ephemeris place the Sun and Moon slightly differently, so tithi and nakshatra end times can differ. (3) Ayanamsa: it shifts nakshatra, rasi and sankranti times, but not tithi. (4) The definition of sunrise: the Sun's upper edge with refraction, or its centre on the true horizon, differ by a few minutes. (5) Timezone and daylight saving. (6) Rounding and units, such as nazhigai counted from sunrise. None of these makes either panchangam wrong -- check which city and which system each one used.
☕ Same sky, different measuring tapes
1. Which city. Rahu Kalam is one-eighth of the daytime, counted from sunrise, so it moves with your sunrise. Chennai and Delhi can differ by half an hour or more on the same date, and a panchangam printed for one town is only right for that town.
2. Which calculation system. A Vakya panchangam computes the Sun and Moon from the classical Vakyakarana's own printed tables; a Drik (Thirukanitha) panchangam uses a modern astronomical ephemeris. Both are internally consistent, and they place the Moon slightly differently, so an ending time -- and occasionally the day of an observance -- can differ. Tamil almanacs are published in both systems.
3. Which ayanamsa. Nakshatras, rasis and sankrantis are measured against the fixed stars, so they depend on where the zodiac is taken to start (Lahiri is the most common choice). A tithi is the gap between the Moon and the Sun, so that starting point cancels out -- two panchangams that differ only in ayanamsa agree on tithi and disagree on nakshatra.
4. What counts as sunrise. Modern tables usually give the moment the Sun's upper edge clears the horizon, with atmospheric refraction included. Many classical almanacs use the Sun's centre on the true horizon. The two differ by a few minutes, and every time an almanac prints as 'so many nazhigai after sunrise' inherits that difference.
5. Timezone and daylight saving. The same instant reads differently on every clock, and abroad a panchangam that ignores the summer clock change is an hour out for half the year.
6. Rounding and units. Printed almanacs often give times in nazhigai (24 minutes) and vinazhigai counted from sunrise; websites give hours and minutes. Converting between the two, and rounding along the way, produces small differences that are not disagreements at all.
How to compare two panchangams fairly
- Check that both are computed for the same city -- not just the same state or country
- Check which system each uses: Vakya, or Drik / Thirukanitha (and, for Drik, which ayanamsa)
- Compare tithi first: if two Drik panchangams agree on tithi but not nakshatra, the ayanamsa is the likely cause
- Convert nazhigai to clock time from the same sunrise before calling two times different
- Calling a panchangam 'wrong' because it disagrees with one printed for another town
- Mixing systems within one decision -- for example a Drik nakshatra with a Vakya tithi
- Assuming the newest method must be the tradition your family or temple follows
- Trusting a single number with no stated city, system or method behind it
Frequently asked questions
Which panchang is the most accurate?
Accuracy only means something against a stated method. A Vakya panchangam should be judged against the Vakya tradition's own printed almanacs, and a Drik panchangam against modern astronomy. Ask which system your family or temple follows, then use a panchangam that states its system, its city and how it was checked.
Is Vakya or Thirukanitham (Drik) correct?
Both are internally consistent systems that place the Sun and Moon slightly differently. Temples and families follow one or the other by tradition. The difference matters most near a boundary, where it can change the day an observance is kept.
Why is Rahu Kalam different in my city from the newspaper?
Rahu Kalam is one of eight equal parts of the daytime, counted from sunrise. A newspaper or wall calendar prints it for one reference town, or on a fixed 6 AM to 6 PM day; computed for your own sunrise and sunset, it moves.
Does ayanamsa change the tithi?
No. A tithi is measured by the angle between the Moon and the Sun, and the ayanamsa is subtracted from both, so it cancels. Ayanamsa does change nakshatra, rasi and sankranti times.
How does Nava Panchangam calculate?
Nava offers both systems. Computed from the classical Vākyakaraṇa's own printed tables and verified against printed Tamil almanacs across 1,344 printed days. The Drik (Thirukanitha) option uses a modern ephemeris with the Lahiri ayanamsa. Sunrise, sunset and Rahu Kalam are computed for your own city; the methodology page states every convention.
Deep dives: the arithmetic behind two of the six
Why ayanamsa cancels out of the tithi, and why Rahu Kalam moves with your sunrise.
📐 Ayanamsa cancels out of the tithi
Sidereal longitudes are tropical longitudes minus the ayanamsa A. A tithi is the angle between the Moon and the Sun:
= Moontrop − Suntrop
A disappears. A nakshatra is the Moon's own sidereal longitude, Moontrop − A, so there it stays -- and a different ayanamsa moves every nakshatra boundary by the same arc.
🕰️ Rahu Kalam is a fraction of YOUR day
The daytime from sunrise to sunset is split into eight equal parts, and each weekday gives Rahu one of them (the second on Monday, the seventh on Tuesday, the eighth on Sunday, and so on).
Rahu Kalam = sunrise + (part number − 1) × part length
A longer summer day makes each part longer; a city further west starts it later. That is the whole reason a printed '7:30 to 9:00' cannot be right everywhere.