Tun

The tun is the third unit of the Maya Long Count, equal to eighteen winals, or 360 days. It is at this level that the Long Count departs once from strict base-twenty counting: eighteen winals rather than twenty are gathered into a tun, so that the tun of 360 days approximates the length of the solar year, though it falls five days short of the true 365. The tun is a fundamental unit of the count, and Maya rulers marked the completion of tuns and their multiples with monuments and ceremonies. Above the tun the scheme returns to multiplying by twenty. A Long Count date names the tally of each unit since the mythic starting point. These are the features of the Maya Long Count, the linear calendar carved on Classic-period stelae to record the passage of these day-counts.

A Tun is eighteen Winal, 360 days, the third place in the Long Count and the place where the system departs from pure base-20 counting to approximate the solar year. Twenty Tun make one Katun, 7,200 days, so the vigesimal logic resumes above the Tun place; the 360-day Tun is the single irregular step in an otherwise consistent base-20 positional system, comparable in principle, though unrelated in origin, to the way the Babylonian sexagesimal system mixed bases for practical calendrical reasons. Because a Tun of 360 days runs roughly five and a quarter days short of the true solar year, Long Count dates and the 365-day Haab drift relative to each other over long spans in a documented, predictable way that Maya astronomer-priests tracked and that modern correlation studies use as one check on proposed correlation constants between the Long Count and the Gregorian calendar.

The tun is the unit by which Classic Maya public time was actually measured out, because it is the period at which rulers commissioned monuments. Stelae were raised to mark the completion of a tun, more often a half-katun of ten tuns, and most often a katun of twenty, and the resulting series of dated stones is the backbone of Classic chronology.

That habit has a consequence historians rely on. A site with a long uninterrupted run of period-ending monuments is a site with a functioning court through that span, and the point at which the series stops is evidence about the end of that court, which is how the ninth-century collapse of the southern lowland cities is dated site by site. The last dated monument at a given city is a real datum rather than a gap in the record.

The tun is also where the count leaves strict base twenty. Eighteen winals make a tun, giving 360 days rather than 400, which brings the unit within five days of the solar year without ever being corrected to match it. Above the tun the multiplication by twenty resumes and is not broken again.

There is a second reason the tun matters to historians. Because the Long Count is linear, two dated monuments anywhere in the Maya area can be placed in order and the interval between them stated exactly in days, without any assumption about correlation with the European calendar. That property is independent of the correlation debate: even if every Maya date were shifted by some constant, their relative order and spacing would be unchanged.