Among the astronomical phenomena occurring only in certain parts of the globe, the Sun's zenith and nadir passages stand out prominently, exemplifying the extent to which geographic latitude determines the significance attributed to particular celestial events and influences their role in cosmological concepts. The Sun reaches the zenith and the nadir (or anti-zenith) exclusively in tropical latitudes. On the Tropics of Cancer and Capricorn, these events coincide with the solstices, whereas in the intermediate belt each of the two phenomena occurs twice a year on dates that vary as a function of geographic latitude. The idea that the ancient Mesoamericans paid attention to both zenith and nadir Sun has long been very popular and different kinds of evidence have been claimed to reflect the significance of these phenomena. But how reliable is this evidence, and to what extent does it support these hypotheses?