Inca calendar

Correlation of the Inca metropolitan calendar with the Julian calendar for 1500 - 1572 CE.

Introduction and explanation of use.

The present reconstruction of the Inca calendar has been described in detail in the work: Mariusz Ziółkowski, „Pacha unanchan. El Calendario Metropolitano del Estado Inca””, Editions „El Lector”, Arequipa 2015.

The calendrical correlation has been elaborated based on the following assumptions:

1. The Inca metropolitan calendar was luni-solar, with synodic lunar months counted from a new moon to the following or, more precisely, from the first appearance of the moon on the western horizon, one or two days after the astronomical new moon proper (conjunction).
   
2. The correlation of the lunar cycle with the solar one was carried out in such a way that the first month of the Inca year had to include a fixed solar event (solstices, or passages through the zenith). In order to keep the position of the lunar months relatively fixed with respect to these phenomena, it was necessary to insert a thirteenth lunar month more or less every three years. According to the proposal presented in this paper, this thirteenth month was inserted after the twelfth month of the cycle (in the tables presented in the software Converter It is marked with an "XIII"
   
3. For the purposes of this reconstruction, the versions of the Inca calendar by Cristobál de Molina “El Cuzqueño” and Juan Polo de Ondegardo have been chosen. This first starts the Inca year from May/June, the second from December. But as will be noted by comparing the columns presented in the correlation tables, this difference in terms of the beginning of the year does not influence the sequence of the months, which are largely parallel in both computations. (see figure 1).
   
4. As a basis for the correlation, the dates of the new moons have been taken, calculated by astronomers Prof. Dr. Tomasz Bulik and Dr. Marek Cieślar of the Astronomical Observatory of the University of Warsaw, by applying the procedures presented on the website: http://rhodesmill.org/pyephem/ . The software has been designed by Dr Mariusz Suchenek from the Warsaw University of Technology.
5. Regarding the dates of the Julian calendar, it is worth noting that most of the instruments available online are, unfortunately, wrong when it comes to the days of the week. The explanation of the reason for these errors is a topic for a separate post, in most cases it is probably the automated application of the proleptic Gregorian calendar.

For the purposes of this Introduction, it is enough to point out that in the Converter the last day of the official operation (in Rome) of the Julian calendar has been taken as the reference point, that is, on Thursday, October 4, 1582. Based on this date, a retroactive calculation of the days of the week has been made.

It should be noted that these new calculations confirmed the tentative correlation previously made based on the calculations of the well-remembered astronomer Robert M. Sadowski, and published as Annex II of the work cited at the beginning: Ziółkowski, 2015

Explanation of use:

El dateconverter It is currently available in the English version (names of months, days of the week and Catholic holidays):

1. To obtain a correlation with the Inca calendar, it is necessary to enter in the first line For the year of the Julian calendar:

a date between 1500 and 1572 AD, depending on the format: Year/Month/Day

2. When choosing : Check the holidays on a specific day

one will obtain the correlation for this precise day

3. When choosing: Display Inca and Christian holidays

one will obtain the corresponding correlation for the entire Julian year

4. Display the Moon dates displays all full moons and new moons for the given year
5. While Display the Julian calendar with the Inca calendar presents an abridged version of p. 2 for the entire Julian year, but without the Catholic holidays and the phases of the Moon.

In the near future, the Converter will be completed with the Inca festivities, held in months of the year, the phases of the synodic cycle of Venus and other elements of importance to scholars.

Final caveats:

This is still a „Beta version”, which is not free of imperfections. By way of example – in some cases we had problems with the thirteenth intercalary month (see for example the year 1572).

We invite Users to communicate their comments, suggestions and criticisms, which will help us to improve this instrument.

Prof. Dr. Mariusz Ziółkowski
Center for Andean Studies of the University of Warsaw in Cusco

CONVERTER >>

figure 1