appear to partially confirm this: astronomical observations are used in combination with ecological signs, such as bird migrations to determine the seasons. Ethnoastronomical studies of the Hopi of the southwestern United States indicate that they carefully observed the rising and setting positions of the Sun to determine the proper times to plant crops. However, ethnoastronomical work with the Mursi of Ethiopia shows that their luni-solar calendar was somewhat haphazard, indicating the limits of astronomical calendars in some societies. All the same, calendars appear to be an almost universal phenomenon in societies as they provide tools for the regulation of communal activities. An example of a non-agricultural calendar is the Tzolk'in calendar of the Maya civilization of pre-Columbian Mesoamerica, which is a cycle of 260 days. This count is based on an earlier calendar and is found throughout Mesoamerica. This formed part of a more comprehensive system of Maya calendars which combined a series of astronomical observations and ritual cycles. Other peculiar calendars include ancient Greek calendars. These were nominally lunar, starting with the New Moon. In reality the calendar could pause or skip days with confused citizens inscribing dates by both the civic calendar and ton theoi, by the moon. The lack of any universal calendar for ancient Greece suggests that coordination of panhellenic events such as games or rituals could be difficult and that astronomical symbolism may have been used as a politically neutral form of timekeeping. Orientation measurements in Greek temples and Byzantine churches have been associated to deity's name day, festivities, and special events. Myth and cosmology Another motive for studying the sky is to understand and explain the universe. In these cultures myth was a tool for achieving this, and the explanations, while not reflecting the standards of modern science, are cosmologies. The Incas arranged their empire to demonstrate their cosmology. The capital, Cusco, was at the centre of the empire and connected to it by means of ceques, conceptually straight lines radiating out from the centre. These ceques connected the centre of the empire to the four suyus, which were regions defined by their direction from Cusco. The notion of a quartered cosmos is common across the Andes. Gary Urton, who has conducted fieldwork in the Andean villagers of Misminay, has connected this quartering with the appearance of the Milky Way in the night sky. In one season it will bisect the sky and in