Guaman Poma's Yupana and Inca Astronomy
  An explanation is provided for the Inca counting board described by Guaman Poma in 1615. Although the board could have been used in more than one way, we show that based on certain reasonable assumptions regarding non-uniform representation of numbers its most likely use was counting in multiples of 6, 24, and 72. The independent numbers represented on its five rows are 92, 31, 29, 79, and 56 that appear to be astronomically connected to sub-periods within the year and planet periods in a manner similar to Mayan astronomy. Based on these and other considerations we propose that the board fulfilled an astronomical counting function. 
Subhash Kak
physics.hist-ph physics.pop-ph