Why two Panchāṅgams can disagree

The ayanāṃśa

Nakṣatras are fixed to the stars; the equinox is not. It slides backwards against the stars by about fifty seconds of arc a year, and over two millennia the two frames have drifted more than twenty-three degrees apart. The ayanāṃśa is the correction that converts between them.

There is more than one accepted value. The Lahiri (Chitrapakṣa) value is the most widely used in South Indian practice and is the one this site applies. Others differ by fractions of a degree — which is enough to move a nakṣatra boundary by an hour or more, and occasionally enough to move a festival by a day.

The day boundary

Most almanacs take the day from sunrise. Some take particular observances from a different moment entirely: sunset for Pradosham, moonrise for Sankaṣṭī, the middle of the night for Śivarātri. An almanac that judges everything at one hour will disagree with one that does not, on precisely the days that matter most to a household.

Precision, and honesty about it

Underneath the conventions there is ordinary numerical accuracy. Solar and lunar positions are computed from series that are truncated somewhere — the question is where, and whether the almanac tells you.

This site states its own: sunrise and sunset good to about a minute, checked against an independent implementation of the NOAA solar algorithm; tithi, nakṣatra and yoga transitions good to roughly five minutes, because the lunar series is truncated to its largest terms and no ΔT correction is applied.

Five minutes matters when a transition falls near a sunrise, which is exactly when almanacs disagree. Where a date is close, ask Gurukkal rather than trusting the arithmetic.

What a family may do

• Before comparing two almanacs, check whether they state their ayanāṃśa and their location. If either does not, they are not comparable.
• If two sources differ by a day on a festival, look at what time the tithi ends. The answer is usually visible there.
• For a ceremony, let one source be the one you follow, and let the priest be it.

Often misunderstood

• That a disagreement means one almanac is wrong.
• That more decimal places means more authority. A precisely computed answer under the wrong convention is precisely wrong.

Where traditions differ

• Different sampradāyas, and different temples within one, may follow different conventions by long-standing custom. A family that has always followed its home temple has a good reason to go on doing so.

Explaining it to a child

Two people can both measure the same table correctly and get different answers if one is using inches and the other centimetres. Almanacs are like that — you have to know which ruler they used.