One of the functions of myth is to integrate the human perception and experience of the physical world, and the social and spiritual reality. It is also a way of transmitting the past to the present. How myth does this for astronomical observation and fact makes for fascinating reading. Modern interpretations of the common trends within myth, especially those relating to astronomy and cosmogeny have ranged from psychological and biological, (because we are genetically the same we interpret our experiences the same way-see Jung) to the concept that myths in preliterate cultures transmit a common tradition that goes back to the Paleolithic, especially in regards to cosmogeny, astronomy and time. While the explanation that we are all hard wired the same way and therefore necessarily interpret things the same way is no doubt correct, it is, in my opinion, insufficient to explain the specific correspondences in systems now continents apart and explanation of transmission of a common core that precedes literate history has much to recommend it. 

      One of the common themes that recurs world-wide is the division of the world into three realms: the sky, the earth and the underworld. For example, in the Egyptian Heliopolis mythic cycle, the primordial element was chaos, Nun which contained the divine consciousness, Atum. Atum alone gave birth to the first divine couple, Shu the god of the air and Tefnut, the goddess of the moisture in the atmosphere. This couple gave rise to Geb, the earth-god and Nut, the sky-goddess. Shu came between Geb and Nut and became the atmosphere; and in this process he lifted Nut's body over Geb, making her the vault of the sky. Geb and Nut, in turn, gave rise to two other couples, Osiris (associated with the underworld) and Isis; and Seth and Nephtys. 

      Atum was also associated with Re (=Ra) and Horus as embodiments of the sun. The daily path of the sun was considered to take place in two ways. One view saw the sun swallowed by Nut in the evening and reborn from her womb every morning, while another saw Re traversing the day sky in a day barge crossing onto a night barge at sunset which would take him across the underworld to again embark on the day barge at dawn. 

      In one of the Sumerian traditions, the cosmos arose from the primeval ocean. As told in the "Hymn to the Locust Tree" this three part division is quite clear: 

        After heaven had been moved far from the earth 
        After earth had been separated from heaven
        After mankind had come into being
        After the god An was established in heaven
        After Enlil was established on Earth
        After the goddess Erishkigal had received the underworld as her share.....(emphasis mine)
      Nor is this three part division restricted to the Old World. New World shamanistic traditions such as those of the Maya also conceived of a three layered universe, with the star-studded vault of the heavens, the cosmic monster, arched over the earth (the world of men), which floated in the primordial waters, with the underworld Xibalba, below. 

      In the Inca tradition, there was the starry heaven, hanaq pacha "the world above", kay pacha "this world", and ukhu pacha "the world below". This underworld was the other half of the celestial sphere as we would put it, the lower celestial hemisphere, the part of the sky we cannot see. Connecting all of three of these worlds is the Milky Way, a river bridge or road that allows travel between the three realms. (Interestingly enough the concept that the Milky Way is a road, a bridge that connects the three worlds is found in myths around the world.)