The non-astronomical formulations of GMT and other correlations depend on double dates where events are recorded in both the Maya calendar and the Julian calendar. These are fairly rare. Important here is the so-called Landa Equation. It was Herbert Spinden who introduced Diego de Landa’s Relación into the calendar correlation problem in 1916. Spinden made the argument that the Year Bearer 12 Kan, since it carries the dominical letter A in Landa’s manuscript, corresponds to Sunday, July 16, 1553.
In 18th and 19th century dictionaries January 1 was always given the letter “a”. The following six days were assigned b-g. The eighth day was “a” again. The letter that falls on Sunday is called the dominical letter of the year. Sometimes the dominical letter of the year was capitalized. There are some complications since sometimes A was capitalized because it was the first day of the year, not because it was a Sunday. Hence, the dominical letters cannot determine the year of Landa’s double date and the practice of assigning dominical letters changed in New Spain. Thompson himself discredited the argument that 12 Kan 1 Pop equals July 16 1553 but he believed that the Julian year was “secure” (1553). Aldana suggests that since Thompson credited Landa’s equation as “secure” it was maintained in subsequent scholarship. This double date has become accepted and black boxed among contemporary Mayanists.
As William Hanks shows in his recent book, Colonial period manuscripts were copied and changed over the years. Landa’s manuscript is also problematic from this perspective. Matthew Restall and John Chuchiak have demonstrated that the Relación has been assembled by several copyists during the 17th and 18th century, all drawing from Landa’s manuscript. There are at least three different writers behind the manuscript we have today. The manuscript can therefore not be seen as an accurate description of Landa’s thoughts centuries before. The calendar sequence found in the manuscript may be the result of misunderstandings of later copyists. The dominical letters may not even be Landa’s, but later additions. There is therefore no anchor to the Landa equation.
However, there are other double dates. The Chronicle of Oxkutzcab states that a tun ended on 13 Ajaw 8 Xul in 1539. To be continued…