Panchak

Panchak is the Moon in the last sixty degrees of the zodiac: Kumbha and Meena, from 300° to 360° of sidereal longitude.

That single sentence fixes everything else about it, and the consequences are worth spelling out because they are rarely stated.

A sixth of all time

Sixty degrees of three hundred and sixty is one sixth. The Moon is inside that arc exactly one sixth of the time.

Concretely: the Moon covers sixty degrees in about four and a half days, and returns to the same arc every sidereal month of 27.3 days. So panchak runs roughly four and a half days out of every twenty-seven, twelve or thirteen times a year.

That is the number to hold on to. A period covering one day in six is not a rare alignment; it is a recurring sixth of the calendar. Any list of things to avoid during it will collide with ordinary life about a sixth of the time by construction, and a collision is not evidence of anything.

The two definitions are the same arc, exactly

Panchak is described two ways, and sources quote them interchangeably without either noticing the other exists.

One says: the Moon in Kumbha or Meena.

The other says: the last five nakshatras, from the second half of Dhanishtha through Shatabhisha, Purva Bhadrapada, Uttara Bhadrapada and Revati. This is where the name comes from, panch is five.

They describe the identical sixty degrees, and it takes a moment to show.

Dhanishtha spans 293°20′ to 306°40′, and each of its four padas is 3°20′. So its third pada begins at 293°20′ + 6°40′ = exactly 300°00′. Kumbha also begins at exactly 300°00′. From there, Dhanishtha's second half plus four full nakshatras run 6°40′ + (13°20′ × 4) = exactly 60°00′, ending at 360°, which is exactly where Meena ends.

Not approximately the same. The same.

This page computes both rules separately anyway and would say so if they ever disagreed, which is a guard rather than an expectation: if the two ever came apart, one of the two tables would have drifted and the discrepancy would be a bug worth knowing about.

Five names, and the five is not the names

A panchak is named for the weekday it begins on:

Seven weekdays, five distinct names: Roga and Mrityu each appear twice. The five in panchak is the five nakshatras, not the five names, and the coincidence of numbers is worth separating out because it invites the wrong inference.

Why the name depends on where you are

The vara runs sunrise to sunrise, not midnight to midnight. So the weekday a panchak begins on depends on local sunrise, and a panchak opening in the small hours can carry different names in different cities.

This is the one place the city selector genuinely changes the answer here, and it is easy to get wrong in code: taking the weekday from the UTC date gives the wrong name for any panchak beginning after 18:30 UTC in India, which is about a quarter of them. A wrong-but-plausible name of that kind would never look wrong.

What the ayanamsa does

More than you might expect. Panchak boundaries are absolute sidereal longitudes (300° and 360°) so they move directly with the ayanamsa.

Lahiri and Raman differ by about 1.45°, and the Moon covers that in roughly two and a half hours. So the window opens and closes about two and a half hours apart under the two systems, which is occasionally enough to shift the weekday it begins on, and therefore the name.

The page states which ayanamsa produced the figures, as every page here does.

What is actually being computed

The Moon's sidereal longitude, and where it crosses 300° and 360°. Both crossings are found by scanning and then bisecting to the second, so the window boundaries are exact rather than rounded to the nearest day, which matters, since a panchak beginning at 23:50 and one beginning at 00:10 take different names.

The rest of what is said about panchak is interpretation, and the page does not add any.

What the window looks like in practice

A panchak runs about four and a half days, and the page gives the opening and closing instants rather than just the dates, because the boundary is a longitude crossing and it can happen at any hour.

Window length varies a little, from roughly 4.3 to 4.7 days, and the reason is the same one behind every other variation in this calendar: the Moon does not move at a constant speed. Near perigee it covers the sixty degrees faster; near apogee, slower.

The interval between windows is steadier, running 27.2 to 27.4 days. That is the sidereal month, the time the Moon takes to return to the same point against the stars, and it is a different quantity from the 29.5-day synodic month that governs tithis and lunar months. Panchak is fixed to the stars rather than to the Sun, so it follows the shorter cycle.

One consequence: panchak has no fixed relationship to the phase of the Moon. It can open at a full moon one time and near a new moon the next, and a rule of thumb tying it to a phase will be wrong most of the time.

What the page does not add

There is a considerable body of guidance about what should not be done during panchak, varying by which of the five names the window carries. None of it is computed or reproduced here.

That is a deliberate line. What this page establishes is when the Moon is in the last sixty degrees of the zodiac, which weekday that window began on, and therefore which name it takes. Those are facts about positions and sunrises, and they can be checked against any ephemeris.

What follows from them is a matter for the tradition you follow, and a calculator that issued instructions would be presenting inference as computation.

Where the sixth of the calendar figure comes from

It is worth separating the two ways of getting the same number, because they check each other.

Geometrically: panchak is sixty degrees of a three-hundred-and-sixty degree circle, and the Moon travels the whole circle at a roughly steady rate. Sixty over three hundred and sixty is one sixth.

Empirically: the windows run about 4.55 days on average and recur every 27.3 days. Divide one by the other and you get 16.7%, which is one sixth again.

If those two ever disagreed it would mean the window search was clipping something, and this site's tests assert both. The measured figure across a year comes out at 16.5%, the small shortfall being the windows partially outside the year rather than an error in the arc.

The same sixty degrees, seen two ways

The identity between the sign rule and the nakshatra rule is not a curiosity; it explains why the two descriptions have coexisted for so long without anyone reconciling them.

Signs divide the zodiac into twelve parts of thirty degrees. Nakshatras divide it into twenty-seven parts of 13°20′. Those two grids do not line up in general, and most sign boundaries fall in the middle of a nakshatra.

The 300° boundary is one of the exceptions. Because 300 is divisible by 3°20′ (it is exactly ninety padas), a pada boundary lands there precisely, and it happens to be the boundary between Dhanishtha's second and third quarters. So the arc that begins at Kumbha and the arc that begins at Dhanishtha's second half begin at the identical degree.

Both descriptions are therefore correct, both are complete, and neither is an approximation of the other. The page computes them separately as a consistency check, which is the only useful thing left to do with a coincidence of this kind.