Just as the sun passes through the zenith, that is, directly overhead, twice a year as viewed from any location within the tropics, so it also passes twice a year through the point directly beneath the observer. This point is known as the antizenith or nadir. The dates of solar antizenith passage occur six months away from those of zenith passage so that, for example, the anti-zenith passage dates at a location a little way south of the Tropic of Cancer will be shortly before and after the December solstice.
The antizenith passage of the sun may seem a totally esoteric event since, unlike zenith passage, it is not directly observable. Yet some human cultures do seem to have had an interest in identifying the dates of solar an-tizenith passages and marking them with appropriate observances. It has been suggested, for example, that some of the pillars erected on the western horizon at the Inca capital city of Cusco were aligned upon sunset on the day of antizenith passage.
